Download Moon Course Section 1-5 v1.0 - One

Around the Moon in 28 Days: Lunar Observing
for Beginners
Course Notes
Section 1 - A Brief History of the Moon
Section 2 - What You Can See on the Moon
Section 3 - How The Moon Moves in the Sky
Section 4 - How To Observe the Moon
Section 5 - How To Follow The Moon Tours
Copyright © 2010 Mintaka Publishing Inc.
1
Section 1 - A Brief History of the Moon
Earth and Moon: A Dual Planet?
The Earth-Moon system is unique in our solar system. Most of the moons that revolve around
the eight planets of our solar system are a tiny fraction of the radius and mass of their host
planets. Jupiter’s biggest moons Ganymede, Callisto, and Io, which are all slightly larger in
radius than our Moon, revolve around a planet 10x larger than the Earth. It’s a similar story
with Saturn’s moon Titan. But relative to Earth, our Moon is large... a whopping 27% the radius
of Earth and 1.2% of its mass. In way, the Moon is almost a companion planet to Earth.
What’s more, the rocky planets Mercury and Venus have no moons at all, and Mars has two
puny moons that are likely captured asteroids. So why does our planet have such a large and
unique satellite? And where did it come from?
Figure 1.1 - The present-day Earth and Moon as image from the Galileo spacecraft (NASA)
Formation of the Moon: Competing Theories
One of the first potential explanations of our Moon’s origin was called the “fission hypothesis”.
As the early Earth was forming, the story goes, it rotated much faster than today. Its surface
was a spinning, seething mass of melted rock– much like a blender turned on high without the
lid – and a chunk of this early spinning material broke free. Because Earth was more massive
than the other rocky planets Mercury, Venus, and Mars, this cosmic “spatter” was captured by
our planet’s gravity and remains in orbit to this day. This theory sounds plausible enough. And
Around the Moon in 28 Days: Lunar Observing for Beginners
2
it might explain why the chemical composition of the Moon is similar to the Earth’s crust and
mantle. But fission hypothesis means the Moon should have been flung out directly above our
equator and stayed there. But it isn’t above our equator. The Moon is embedded in its own
geometrical plane roughly aligned with the ecliptic, which marks the plane in which the planets
of our solar system revolve around the Sun. What’s more, detailed mathematical studies of the
rate of spin of the Earth and the angular momentum of the Earth-Moon system suggested the
“fission hypothesis” could not explain the origin of the Moon.
Perhaps the Moon formed elsewhere in the solar system and was captured by the Earth’s
gravitational field? This “capture hypothesis” is attractive, but like fission, it doesn’t explain the
observed facts. In order for the Moon to have formed in another location in the solar system
and then been captured by our gravity, the early Earth would have needed an incredibly dense
and extended atmosphere to “put the brakes” on the Moon as it passed by to settle it down
into a stable orbit. There is no evidence the Earth had such an atmosphere. So again, it
doesn’t quite add up.
Figure 1.2 - Artist’s concept of the “solar nebula”, with the nascent sun at the center,
surrounded by a disk of gas and dust out of which the planets formed.
Perhaps the easiest explanation is that the Earth and Moon formed together out of two
coalescing lumps in the primordial accretion disk after the formation of the Sun, and came to
revolve around each other by mutual gravitation. But if this was the case, the Earth and Moon
should have very similar compositions. They don’t... unlike the Earth, the Moon has very little
iron in its crust or core. So that explanation doesn’t work either.
Around the Moon in 28 Days: Lunar Observing for Beginners
3
From all the observed data, and samples and studies on lunar formation done to date, one idea
seems to stand alone as the most plausible – the “giant impact hypothesis”. In this scenario,
a body roughly the size of Mars intersected primordial Earth's orbital path and struck a nearcatastrophic glancing blow. The collision was so violent that a huge amount vaporized debris
was ejected into space and formed a ring around Earth at a slightly inclined orbit. The ring
quickly coalesced into a molten mass, possibly within a matter of a few years, into what
became the Moon. This widely accepted explanation accounts for the Moon's lower density
and deficit of heavier elements like iron, since only the outer layers of Earth were knocked off.
If the impacting body itself had iron in its core, computer models show the iron would have
merged with the Earth’s core.
Figure 1.3 - Artist’s concept of the “giant impactor” about 1/2 hour after impact, and the newly
formed Moon coalesced out of the impact debris (credit Bill Hartmann)
Models and simulations have shown us the “giant impactor” theory is the most likely
explanation for our Earth-Moon system. Yet, only one question remains... Where did the
impactor originate? If it came from our outer solar system, the chemical composition doesn't
fit what we've learned from studying lunar rock samples returned by the Apollo astronauts. Did
the rogue body form near the early Earth? Was it in its own stable orbit, then gravitationally
perturbed and ultimately drawn towards Earth? No one knows for sure.
Structure of the Moon
As the Moon coalesced some 4.5 billion years ago, it was a ball of molten rock about one third
the size of Earth and only about 60% as dense. As the hot magma began to cool and solidify
Around the Moon in 28 Days: Lunar Observing for Beginners
4
into a crust, the heavier elements sank to form the lunar mantle. But our “new Moon” wasn't
done collecting stray objects careening around the solar system. Asteroids were continuously
being drawn toward the Moon, smashing into the surface and shattering it deep below. Some
of these interventions were so violent they may have stripped off the newly formed crust and
nearly split the forming satellite into pieces again. The seas of molten rock from these impacts
gave the Moon a thick crust, which averages about 70 kilometers (43 miles) in depth. That's
about seven times more than Earth! One of the deepest-known shattered areas where intact
material still exists went approximately 100 kilometers below the surface - a distance that
would have carried it through Earth's mantle.
The Moon's mantle, below the crust, is a thick layer of dense rock rich in heavier elements,
including some iron and magnesium. Somewhere between about half a billion to a billion years
after initial formation, the decay of radioactive materials buried beneath the surface may have
caused the Moon's mantle to melt and become volcanic. As the magma pushed upwards, it
erupted into mineral-rich lava, filling and flooding many parts of the deeply cratered surface.
As it cooled, it turned dark... hardening into a rocky material known as basalt. While we think
of the Moon's formation as being quite ancient, small scale lunar volcanic activity may have
continued until as recently as a billion years ago, only releasing about 1% of the mantle’s
material to the surface.
Figure 1.4 - The lunar crust (beige), mantle (orange), and core (grey), courtesy NASA
And then there’s the Moon’s core...
Around the Moon in 28 Days: Lunar Observing for Beginners
5
Our Earth's core consist of a rapidly rotating, molten mass consisting mainly of iron and nickel.
Scientists believe the rotational kinetic energy of this material is responsible for our magnetic
field. Even though the Moon is suspected to have a partially molten core, it doesn't produce
the same global magnetic field.
Does this mean magnetism doesn't exist on the lunar surface? No. There are areas that
display positive and negative fields, but astronomers are unsure of their origin. Perhaps the
Moon once had a more active core... or maybe these areas were created by magnetic material
that impacted the Moon and is buried beneath the surface.
Figure 1.5 – Lunar Basalt (courtesy of NASA)
By now, 4.5 billion years after its formation, the solar system has very few stray asteroids and
comets to impact the Earth and Moon and alter their geography. Here at home, our
atmosphere burns up almost all small-to-medium-sized meteoroids before they reach the
surface. But the Moon isn't quite so lucky. Large, old impacts created most of the craters we
see today on the Moon’s surface, but micro-meteoroids continue to pound into the Moon,
grinding the surface rock into a fine powder called regolith.
Thanks to sample return missions, there’s proof the lunar interior once contained hot gases.
The Moon's basalt rocks are riddled with holes called vesicles. The holes are formed by rising
gas bubbles, much like those rising from a shaken carbonated beverage. These gas pockets
were captured in the cooling magma.
The Moon’s Atmosphere
While the Moon doesn't have a dense atmosphere like Earth, it does have a surrounding
gaseous layer called the exosphere. This sheath of gas arrives on the solar wind and consists
mainly of hydrogen and helium, along with some neon and argon. The exposure to space is
Around the Moon in 28 Days: Lunar Observing for Beginners
6
very erosive. Thanks to the perpetual bombardment of micro-meteoroids heating the rock and
vaporizing the surface, other gaseous atoms are also present – sodium and potassium. Even
the Moon itself contributes a tiny amount of low-density outgassing from its interior!
Figure 1.6 - The lunar “exosphere”
These vapors collect and intensify roughly halfway between the lunar equator and poles. They
become most concentrated just before sunrise, then the solar wind chases the gases away into
space. When the sun sets on the Moon, the gases are generated again. This low density lunar
“atmosphere” is too sparse for the atoms to collide with themselves – they mostly impact the
surface. Neutral atoms can become photo-ionized, forming a sort of exo-ionosphere with a
few volts positive on the daylight side but several hundred volts negative on the lunar
nightside. Don't let the term exosphere fool you, though. It's pressure is so low that it's
stronger than a laboratory vacuum – and so thin the rocket exhaust released by the Apollo
landing craft nearly doubled its content!
Around the Moon in 28 Days: Lunar Observing for Beginners
7
Section 2 - What You Can See on the Moon
Seas and Highlands
Billions of years before the first humans looked skywards, the Moon had completed its major
formations, and left us to figure out its origins. Four centuries ago, Galileo was perhaps the
first to turn optical aid towards the Moon to discover it was another world and not a smooth
and featureless orb. He described mountains, holes, and plains – and even provided sketches
and detailed reports to his scientific peers. Yet only within the last 60 years have we come to
truly understand the origin and geology and geography of the Moon.
Figure 2.1 - Galileo's Moon Sketch - historical image]
Even without optical aid, you can see dark-grey areas on the Moon’s surface. These are maria
(MAH-ree-ah) – the Latin word for seas. These features were named by early astronomers who
thought these regions resembled bodies of water. We now know these are rugged, dry, and
desolate landscapes. Many maria are huge areas which flooded with lava during volcanic
eruptions or near-catastrophic impacts. The lava froze in place, forming a smooth rocky
surface which were later marked by meteoroid impacts.
Around the Moon in 28 Days: Lunar Observing for Beginners
8
Figure 2.2 - An image of the full Moon showing dark maria and light terrae, or
highlands
The light areas we see on the Moon are sharply-formed and heavily-cratered highlands known
as terrae - the Latin word for “lands”. These highland areas are all that is left of the original
lunar crust. With even the slightest optical aid, you can see the terrae regions are shattered
and fragmented by a continual bombardment of meteoroids, asteroids, and comets, especially
from the earliest periods of the Moon before the maria formed. Many of these craters are larger
than 25 miles (40 kilometers) in diameter or more. Of all of the early impacts in the lunar
highlands, the largest known is South Pole-Aitken Basin, which measures 1,550 miles (2,500
kilometers) in diameter.
Lunar Timescales
Lunar scientists debated for decades about how features on the Moon’s surface formed over
time. Then in the early 1960’s, astrogeologist Eugene Shoemaker used geological mapping
techniques to divide the features on the lunar surface into five age regimes – four of which are
still in use by lunar scientists. Our modern classifications for geological timescales on the
Moon are the Pre-Nectarian, Nectarian, Lower Imbrian, Upper Imbrian, Eratosthenian and
Copernican periods. While we cannot define these ages exactly, it is possible order these
events in time by using both standard stratigraphic principles and rock samples (from Apollo
missions) with a known geological context. Before we begin with the appearance of the
Moon's features, let's discuss the time frames and circumstances in which they formed.
Around the Moon in 28 Days: Lunar Observing for Beginners
9
The Pre-Nectarian period is Act One – beginning somewhere around 4,533 million years ago
and converging with the Early Imbrian approximately 3920 million years ago. During this 613
million year time span, the Moon's crust was forming. This period lasted until the time of the
so-called Nectaris impact event, which created a multi-ring impact basin visible on the Earthfacing side of the Moon. Thanks to the resulting ejected material (called ejecta) from this huge
event, we're able to use it as a stratigraphic marker, allowing us to assign time frames to 30
smaller impact basins from this period. Some of these basins will be visible during the lunar
tours later in this course.
Figure 2.3 - Nectaris Basin Region (courtesy of NASA)
The Nectarian period begins roughly 3920 million years ago and includes all events which
happened between the formation of the Nectaris basin up to the formation of the Imbrium
impact basin a billion years later. Since the solar system was still cluttered with debris from the
formation of the solar system, this was a very busy time for major lunar impacts. As many as
twelve multi-ring impact basins are recognized as belonging to the Nectarian period - including
those that caused Mare Serenitatis and Mare Crisium, which you’ll meet later in the tours. One
of the scientific missions for the crew of Apollo 16 was to collect and date the ejecta material
strewn about by the Nectaris event, but even now the age figures aren't perfectly accurate.
Around the Moon in 28 Days: Lunar Observing for Beginners
10
Figure 2.4 - The Imbrium Basin Region (courtesy of NASA)
The Imbrian period has been subdivided into two epochs- Late and Early. The Early Imbrian
age occurred between the events which formed the Imbrium and Orientale impact basins. It
was formed roughly 3850 million years ago. As far as we currently know, multi-ringed
Schrödinger basin on the far side of the Moon is the only one which formed during the Lower
Imbrian period.
Around the Moon in 28 Days: Lunar Observing for Beginners
11
Figure 2.5 - The Orientale Basin Region (courtesy of NASA)
The Late Imbrian epoch is defined as the time period between the formation of Orientale basin
and certain-sized craters that are dated due to erosion by micrometeorites. The true age of
Orientale basin hasn't been directly stated, although we can get a good idea by looking at both
the size and frequency in which later-forming craters appear on its ejecta blanket. What we do
know is about two-thirds of the volcanic rocks of the maria surfaced some 3800 to 3200 million
years ago, with a substantial portion of the lava filling the huge potholes of older impact basins.
Around the Moon in 28 Days: Lunar Observing for Beginners
12
Figure 2.6 - Visualization of impact areas with and without dark lava flow (courtesy of NASA)
Then comes the 2.2-billion-year reign of the Eratosthenian period. This lunar age is defined by
ancient craters which have almost disappeared due to erosion by impact from micrometeorites
which began some 3200 million years ago. Between impacts and seismic events, we can
measure timelines because freshly exposed surface material from younger craters appears
much brighter at first, then becomes darker over time due to space weathering from high
energy particles, solar wind implantation, and micrometeorite impacts. But this definition of the
Eratosthenian period is a little shaky, since many selenographers (ie. lunar geologists) now
believe the majority of bright ray systems get their appearance more from their composition
rather than age.
Around the Moon in 28 Days: Lunar Observing for Beginners
13
Figure 2.7 - Lunar time periods as defined by the creation of major basins by massive impact
events (courtesy of NASA)
This brings us to our final age - the Copernican period, As the youngest geological time frame,
the Copernican period began about a billion years ago (not with the formation of Copernicus
impact crater). This lunar age is defined by impact craters which display bright, optically
immature ray systems. No lava flow appears to have occurred during this time frame, so
selenographers generally agree the Moon's internal geologic activity had either decreased or
stopped altogether by this age. The Copernican period continues to the present day.
Around the Moon in 28 Days: Lunar Observing for Beginners
14
Figure 2.8 - The Full Moon. Note the rays extending out of the young crater
Tycho at the bottom of this image; Tycho is only 100 million years old
The Geography of the Moon
Now that we've had a chance to study a little lunar geology, let's have a look at its geography,
which Apollo 11 astronaut Buzz Aldrin described as “magnificent desolation”...
Figure 2.9 - False color lunar geography (courtesy of NASA)
Around the Moon in 28 Days: Lunar Observing for Beginners
15
After the dark maria and the brighter terrae, or highlands, craters are the next most visible type
of feature on the Moon’s surface. Lunar scientists believe most craters were either created by
impacts or by volcanic activity.
There are too many craters on the Moon to count. And like snowflakes, no two lunar craters are
exactly alike: they span a huge variety of depths and sizes. With so many shapes and sizes and
ages of craters, scientists looked for a way to classify them. Lunar scientist Ralph Baldwin
created the following classifications:
Class I: Young craters, from impacts after the formation of the maria, with the steepest,
sharpest walls
Class II: Intermediate craters, from before the maria - with walls showing slight degradation due
to subsequent meteoric bombardment and subsidence.
Class III: Older craters - with outlines considerably more rounded and interrupted by later
impacts.
Class IV: Ancient craters, dating back almost to the Moon's formation - with highly broken and
smoothed features.
Class V: Craters which have been partly obscured by flooding with lava at the time of the
formation of the maria.
As you examine the surface of the Moon in the upcoming tours, you’ll find it useful to
understand how to determine the approximate age of the features you see. Here are a few
rules of thumb. The dark mare materials are pretty much from the Imbrian period, and the
easily recognized, light colored lunar highlands are heavily cratered and belong to the PreImbrian period. Craters found within maria which have lava flooded floors formed after the
Imbrian event, while ones with central peaks belong to the Eratosthenean period. Last, but not
least, craters with rugged floors and bright ray systems belong to the Copernican time period.
Before we move on, let's take a look at some more surface features and characteristics that
you will encounter while looking at the Moon through a telescope...
Albedo – the amount of reflectiveness of a surface feature.
Anorthosite - granular igneous rock usually of soda-lime feldspar.
Basin – a large impact crater, with a diameter in excess of 100 kilometers.
Around the Moon in 28 Days: Lunar Observing for Beginners
16
Breccia – coarse , ore-existing rock and angular fragments.
Caldera – volcano summit depression formed by explosion or collapse.
Catena - crater chain.
Cavus – groups of hollows or irregular depressions.
Craters – indentations which are bowl or saucer shaped in configuration; a depression with
steep slopes on the surface; formed by impact or geologic activity.
Dorsum – ridge.
Ejecta – impact crater material which is thrown clear of the source and covers the surface at
least one crater diameter; streamers of material originating from a impact area.
Grabens - tectonic features that form underextension stresses composed of two normal faults,
with a down-dropped block or depression between them.
Highlands - densely cratered and elevated areas of the lunar surface.
Lacus – small plain.
Lava - volcanic rock present in mare areas; basalt flow.
Mare - the low-surface-reflectivity area filled with lava which cover the floors of older basins.
Mascon - concentrations of mass on the lunar surface.
Mensa - flat-topped ridges with cliff-like edges.
Mons – mountain.
Oceanus – a single, large dark area.
Palus – a small plain.
Patera – a disfigured crater; complex with irregular edges.
Planitia – a low plain.
Planum – a high plain.
Around the Moon in 28 Days: Lunar Observing for Beginners
17
Promontorium – a high point of land.
Ray – a bright streamer of ejecta associated with an impact crater.
Regio – large, reflective area; containing different coloration than surrounding regions.
Regolith – lunar surface material; dust and rocky debris
Rille – ditch or crack-like feature; may be straight or sinuous
Rima – fissure
Rupes – scarp or high cliff.
Scarp - sudden change in topography due to tectonic shifts, previous volcanic events, impactrelated related activity or simple erosion.
Sinus – a small plain.
Sulcus - sub-parallel furrows and ridges.
Tessera – overlapping layers of terrain.
Undae – dunes
Vallis – valley.
Around the Moon in 28 Days: Lunar Observing for Beginners
18
Figure 2.10 -The “Far Side” of the Moon (courtesy of NASA); this side of the Moon is never
visible from Earth
As you will see in the next chapter, only one side of the Moon faces Earth. This is because the
Moon rotates during the same time period that it revolves around the Earth. The side of the
Moon visible from Earth is called the “near side”. The side not visible is called the “far side”, or
sometimes, the “dark side”. Though the far side isn’t really dark... it gets the same amount of
sunlight as the near side.
Does lunar “far side” appears the same as the “near side”? The answer is no. There are far
fewer maria and many more sharp, rugged craters on the far side- including the second largest
known impact feature in the Solar System, the South Pole-Aitken basin. By comparison only
2.5% of the surface of the far side is covered by hardened lava maria while 31.2% of the
surface area on the near side is blanketed by maria. Why? Thanks the science performed by
the Lunar Prospector gamma-ray spectrometer, selenographers believe the difference is related
to a higher concentration of heat-producing radioactive elements on the near side.
Around the Moon in 28 Days: Lunar Observing for Beginners
19
Figure 2.11 - Craters of the lunar far side (courtesy of NASA)
It has only been about six decades since we've learned anything about the properties of the far
side of the Moon. On October 7, 1959 the Soviet probe, Luna 3, took the first photographs of
the lunar far side and it was indeed Soviet missions that discovered, mapped and named the
most outstanding of these features. The first humans to actually see the far side was the crew
of Apollo 8 in 1968. Astronaut William Anders described the view: “The backside looks like a
sand pile my kids have played in for some time. It's all beat up, no definition, just a lot of bumps
and holes.” The far side has been seen by all crew members of Apollo 10 through Apollo 17
and photographed by several lunar probes.
Around the Moon in 28 Days: Lunar Observing for Beginners
20
Section 3 - How The Moon Moves in the Sky
Lunar Motion and Phases
Now that we know much more about how the Earth's Moon came to be and what caused its
surface features, let's take a look at how it moves, and when and where you can see it in the
sky.
The Moon has revolved around the our planet since it formed after the the glancing “giant
impact” of a Mars-sized body with the primordial Earth some 4.5 billion years, ago. Since that
time, the two bodies have been embraced in a gravitational dance that slowly changed the
time it takes for the Moon to rotate and revolve around the Earth. Shortly after it was formed,
the Moon was much closer to the Earth, just 30,000 km away instead of the current 384,000
km. What a sight it must have been as it spanned a patch of sky as nearly as big as your hand!
In these early days of our planet, the Moon also revolved around the Earth far more quickly
than it does now.
Figure 3.1 - Visualization of Sunlight on Earth and Moon (courtesy of NASA)
But the mutual gravitation of the Earth and the Moon has slowed things down over the many
billions of years. The complex tug of the Moon on the Earth has hoisted the Moon into a more
distant orbit, where it revolves around our planet more slowly. With respect to the stars, the
Moon now takes about 27.3 days to make a full revolution around the Earth. This period of
time known as the sidereal month. As both the Earth and Moon orbit the Sun, the time it takes
Around the Moon in 28 Days: Lunar Observing for Beginners
21
to get back in the same place in the sky with respect to the Sun is longer, about 29.5 days.
This is the called the synodic month.
It turns out the continuing interaction between the Earth and Moon means the the Moon is still
receding from the Earth. But it’s a small effect. Careful measurements show the Moon gets
3.8 centimeters farther from the Earth each year.
The energy gained by moving the Moon to a higher orbit over time was paid for by a slowing of
the rotation period of the Earth. A billion year ago, a day on Earth was only 18 hours long. And
while the day is now 24 hours long, the drag from the Moon lengthens each day by 2/1000 of a
second every hundred years. The Moon has endured the same effect, though more strongly. It
rotates once each 27.3 Earth days. Since this rotation period is the same as the time it takes
for the Moon to revolve around the Earth with respect to the stars, the Moon always keeps the
same face towards Earth. We can only see half the Moon, the so-called “near side”; we never
see the other side of the Moon. No human had seen the so-called lunar “far side” until
spacecraft snapped images in the late 1950’s.
For even the most casual stargazer, the revolution of the Moon around the Earth has one
practical consequence. It means that each day during a period of revolution, as the position of
the Moon changes relative to the Sun in our sky, the Moon will appear to be lit up a little
differently. This effect is what causes the phases of the Moon we see changing from day to
day and night to night.
To help you understand how phases work, picture the Earth, Sun and Moon on a baseball field.
The Earth is the pitcher, the Sun is the catcher and the Moon is the player up to bat. When the
Moon is in the “new” phase it is at home plate. It is always in our sky, but it is either positioned
just above or just below the catcher Sun. Because the light is so overwhelmingly bright, we
cannot see it. Within 24 hours the Moon has moved slightly towards first base. From the
perspective of the Earth on the pitcher's mound, it has now separated slightly from the Sun
and appears to be a thin crescent – visible as the skies darken when the Sun sets in the west.
Around the Moon in 28 Days: Lunar Observing for Beginners
22
Figure 3.2 - The phases over the Moon over its 29.53-day period.
Each hour the Moon continues to move towards first base and becomes more illuminated.
This means that someone who is observing the Moon in eastern Europe will see the Moon's
phase at a different “hour age” than an observer on the American west coast. For example,
the European observer may see the same basic lunar phase, but the terminator (the line
between shadow and light) will have advanced by as many hours as it takes to reach the next
observer.
Around the Moon in 28 Days: Lunar Observing for Beginners
23
Figure 3.3 - Geometry of Moon phases (courtesy of NASA)
For each hour that passes, the Moon will continue to move around our imaginary baseball
diamond. When it reaches first base, half of the surface is illuminated and it is called “first
quarter Moon”. Because it is about one quarter of the way around its orbit, it becomes visible
in our sky several hours before local sunset. It will continue its hourly journey over the days –
its waxing phase - until it reaches second base. At that point, the Moon is directly opposite the
Sun in our sky and is fully illuminated – thus called “Full Moon”. When will the Full Moon
appear in the sky? As the Sun goes down in the west, the Moon will appear opposite of it in
the east. Since our Earth is also directly between the two, there are two very specific times of
the year in which the Moon can pass through our shadow – a lunar eclipse! And what times of
the year does this happen? Eclipses happen during solstice – when our Earth reaches a
specific point on its axial tilt. More about eclipses later...
The Moon does not stop at when is full, though. It continues its journey around the field rising
a bit later each night. Each successive day, the moon rises an average of about 50 minutes
later. Because it is no longer directly opposite the Sun, the terminator will move across the
surface from the lunar east to lunar west – the waning phase. It is three-quarter Moon (or lastquarter Moon) when it reaches third base and rises in the east approximately at the mid-point
of darkness. It will continue around its path, but instead of running away from the setting Sun,
Around the Moon in 28 Days: Lunar Observing for Beginners
24
it is approaching the rising Sun and becoming a slender crescent once again. This is why we
sometimes see the Moon for several hours in the morning before the skies become bright. The
Moon will continue to race towards home plate to become new again – rising just before dawn.
Just like when it reaches second base, it is either just above or just below the Sun's position.
And, just like a lunar eclipse, the axial tilt of the Earth may place it in a position for the New
Moon to obstruct the Sun – this is a solar eclipse, though it requires careful alignment of the
Earth, Moon, and Sun. The important thing to remember is this: a solar eclipse happens at
New Moon, while a lunar eclipse occurs at Full Moon.
If the Moon was spun out from the Earth, as mentioned before, it would lie on the celestial
equator, he imaginary line in the sky right above the Earth’s equator. Instead, the Moon lies
within 1.5 degrees of the ecliptic, which is the narrow band of the sky in which we find the
other members of our solar system, and which cuts through the 12 constellations of the zodiac.
As was mentioned before, this is telling, for it suggests the object that collided with Earth to
form the Moon) must have originated elsewhere in the plane of our our solar system. But the
bottom line is this: the Moon is found in the same band of sky and in the same constellations of
the zodiac as the Sun and planets.
Figure 3.4 - Where to find the Moon relative to the sun for each phase (courtesy of NASA)
Lunar Libration
As you learned above, because of the gravitational interaction of the Earth and Moon, the
Moon’s rotation period has slowed to match its revolution period, which means we can only
see 50% of the Moon’s surface from Earth. This isn’t quite accurate. It is possible to see
about 9% more of the Moon at certain times during the month because of a motion called
Around the Moon in 28 Days: Lunar Observing for Beginners
25
libration which shows itself as a slow nodding up and down and side-to-side of the Moon as
viewed from Earth.
There are three kinds of libration: (1) libration in longitude, (2) diurnal (daily) libration, and (3)
libration in latitude. Each of these effects let us get a glimpse of what’s over or around the edge
of the Moon.
Libration in longitude occurs because the moon's orbit is elliptical. As the moon orbits Earth,
its speed varies according to a law discovered in the 1600's by the German astronomer
Johannes Kepler. When the moon is relatively close to Earth in its elliptical orbit, it travels more
rapidly than its average speed. When the moon is relatively far from Earth, the moon travels
more slowly than average. But the moon always rotates about its own axis at the same rate.
So when the moon is traveling more rapidly than average, its rotation is too slow to keep all of
the near side facing Earth. And when the moon is traveling more slowly than average, its
rotation is too rapid to keep all of the near side facing Earth.
Figure 3.5 - Diurnal lunar libration (courtesy of NASA)
Diurnal libration is caused by a daily change in the position of an observer on Earth relative to
the moon. Consider an observer who is at Earth's equator when the moon is full. As Earth
rotates from west to east, the observer first sees the moon when it rises at the eastern horizon
and last sees it when it sets at the western horizon. During this time, the observer's viewpoint
Around the Moon in 28 Days: Lunar Observing for Beginners
26
moves about 7,900 miles (12,700 kilometers) -- the diameter of Earth -- relative to the moon.
As a result, the moon appears to rotate slightly to the west. So while the moon is rising in the
east and climbing to its highest point in the sky, the observer can see around the western edge
of the near side. As the moon descends to the western horizon, the observer can see around
the eastern edge of the near side.
Libration in latitude occurs because the moon's axis of rotation is tilted about 6 1/2 degrees
relative to a line perpendicular to the moon's orbit around Earth. Thus, during each lunar orbit,
the moon's north pole tilts first toward Earth, then away from Earth. When the lunar north pole
is tilted toward Earth, people on Earth can see farther than normal along the top of the moon.
When that pole is tilted away from Earth, people on Earth can see farther than normal along the
bottom of the moon.
Figure 3.6 - Libration in latitude (courtesy of NASA)
The combination of these three types of libration cause the Moon to nod “yes and no” during
the course of the month, as shown in this video of the lunar librations in 2009...
http://www.youtube.com/watch?v=6nTmOIkUoTs
Lunar Eclipses
As we know from playing “lunar baseball” to explain the phases of the Moon, during each lunar
orbit the Earth lies once between the Sun and the Moon, just as the Moon once lies between
the Sun and Earth. Thanks to slight tilts of the orbital planes, most of the time, the Moon,
Around the Moon in 28 Days: Lunar Observing for Beginners
27
Earth, and Sun aren't lined up close enough to cause one to block the other... an event we call
an eclipse.
Both lunar and solar eclipses only occur when the Earth, Sun and Moon are directly aligned.
When the Earth passes between the Sun and the Moon, which happens only at Full Moon, the
Earth’s shadow engulfs the Moon. The outer shadow cone caused by the Earth is called the
penumbra and the dark, inner shadow is called the umbra. A round body, such as a planet,
casts a shadow "cone" through space. When it's at Earth, the cone is widest at 13,000
kilometers in diameter, yet by the time it reaches the Moon it has narrowed to only 9,200
kilometers. Considering the distance to the Moon is 384,401 kilometers, that's hitting a very
narrow corridor in astronomical terms!
Figure 3.7 - Geometry of a lunar eclipse
A penumbral eclipse occurs when the Moon passes into the outer regions of Earth's shadow
cone. This produce a modest greyish shadow on the Moon. While not known to be particularly
exciting or even noticeable, some penumbral eclipses are exceptionally deep and edge of the
Moon will just graze the inner umbral shadow. This is a partial lunar eclipse.
Around the Moon in 28 Days: Lunar Observing for Beginners
28
A total lunar eclipse occurs when the full face of the Moon enters the deep shadow, or umbra.
This casts a deep shadow across the Moon. The Moon will never completely disappear as it
passes through the Earth's umbral shadow cone, however. Our atmosphere bends the sunlight
on its way to the Moon, and it scatters away blue light, passing orange-red light more.
(Exactly the same reason sunset and sunrise appears to be red). This causes the Moon to
appear copper-colored during a total lunar eclipse.
As a rule of thumb, remember that the Moon moves about its own diameter each hour, so the
very beginning of an eclipse will be difficult to notice. Slowly and steadily, the coloration will
begin to change and even inexperienced eclipse watchers will notice that something is going
on. From first to last contact of the penumbra, a lunar eclipse typically lasts a few hours.
Figure 3.8 - The copper-colored hue of a total lunar eclipse.
When you next see a lunar eclipse, try to judge the eclipse coloration on the Danjon scale. It
was was devised by Andre Danjon for rating the overall darkness of lunar eclipses:
L=0: Very dark eclipse. Moon almost invisible, especially at mid-totality.
L=1: Dark Eclipse, gray or brownish in coloration. Details distinguishable only with difficulty.
Around the Moon in 28 Days: Lunar Observing for Beginners
29
L=2: Deep red or rust-colored eclipse. Very dark central shadow, while outer edge of umbra is
relatively bright
L=3: Brick-red eclipse. Umbral shadow usually has a bright or yellow rim.
L=4: Very bright copper-red or orange eclipse. Umbral shadow is bluish and has a very bright
rim.
Lunar eclipses happen at least one or twice each year and are visible over a broad swath of the
Earth’s surface. Here is table from NASA to help you determine when the next lunar eclipse will
be visible in your area:
Around the Moon in 28 Days: Lunar Observing for Beginners
30
Solar Eclipses
During “New Moon” our nearest astronomical neighbor is positioned between the Earth and the
Sun, its lit side is turned away from us. It rises the same time the Sun rises, and sets as the
Sun sets. For each Earth “day” that passes, the Moon rises between 30 and 70 minutes later,
gradually moving away from the Sun's glare.
Figure 3-9: The alignment of the Earth, Moon, and Sun at the phase of New Moon, showing a
solar eclipse.
At new Moon, as at Full, the Earth, Moon, and Sun are lined up such that the Moon blocks the
Sun over a small patch of Earth. The perfect conjunction of our solar/lunar pair only occurs
twice a year – and sometimes as many as five – when the angled orbits match up. For the
most part, the Moon passes just above, or just below the Sun as seen from Earth, which
means no eclipse.
It turns out that our Sun's diameter is about 400 times the Moon's diameter and the Sun's
distance from the Earth is about 390 times further away. This coincidence puts the pair almost
exactly the same size as seen from our perspective – about ½ a degree of arc. Because the
Moon's orbit is elliptical, there are times when the size may vary. When the Moon is closest –
perigee – it is at its largest and produces a total solar eclipse. When at the furthest – apogee –
its apparent size is smaller and the result is an annular eclipse. On the average, a total solar
Around the Moon in 28 Days: Lunar Observing for Beginners
31
eclipse happens about every 18 months. However, from any single location on Earth, a total
eclipse occurs only once every several hundred years. It's small wonder that early cultures
considered an eclipse to be a bad omen!
But eclipses as harbinger of evil aren't the only superstitious dealings associated with New
Moon. Many cultures thought it very bad luck to dig a grave or bury a body during this 24 hour
period. Thankfully we know now the only bad luck there was for the person about to be
interred! Other commonly held beliefs are that it is best to cut the hair or nails, plant root
crops, or begin a trip, romance or business venture during New Moon. Even the weather has
been associated with solar/lunar conjunction: New Moon on Monday means skies will be fair,
Saturday produces twenty days of wind and rain, Sunday portends a flood before the month is
finished and hurricanes will blow the strongest between one and three days later. Out of all the
many myths that surround this time, one of the best belongs to the Celtic culture – New Moon
is a good time to shape fresh ideas and take a look at new projects.
Figure 3-10: A total solar eclipse; the Moon covers the Sun almost exactly, as seen from Earth
Around the Moon in 28 Days: Lunar Observing for Beginners
32
Section 4 - How To Observe the Moon
Since the Moon is so bright and close, observing it isn't difficult. All you need is your eyes,
binoculars, or a small telescope. The 28 tours that follow in this course provide challenging
objects for all three levels of optical aid.
Choosing your optics is a matter of individual taste and budget, and even the most humble
binoculars and telescopes provide a jaw-dropping view of the lunar surface compared to the
unaided eye. But there are a few key ideas and specifications to keep in mind as you choose
your optics, especially if you want to use them for general stargazing activities.
Binoculars
Binoculars are fine tools for the moon watcher. They are inexpensive and compact, and they
give a wide-field “right-side-up” view of the night sky. If you choose to use binoculars, the
traditional Z-shaped Porro-prism design is best-suited for moon watching and general
stargazing. A good pair of straight-through “roof-prism” binoculars is often too expensive for
astronomy.
All binoculars have a pair of numbers associated with them - the magnifying power times (X)
and the diameter of the objective lens in millimeters. So a pair of 8x42 binoculars, for
example, magnify by 8x and have objective lenses with diameter 42 mm.
Higher magnification makes objects look closer and large. But the lens diameter is also
important. Bigger lenses mean a brighter image, and most importantly for looking at the Moon,
higher resolution of fine detail. The tradeoff? The size, bulk, and price tag of larger-aperture
binoculars increase proportionately.
If you look through binoculars of two widely different magnifying powers at the same object,
you'll see you have the choice of a small, bright, crisp image or a big, blurry, dimmer image but why? Binoculars can only gather a fixed amount of light determined by their aperture (lens
size). When using high magnification, you're only spreading the same light over a larger area
and even the best binoculars can only deliver a certain amount of detail. Being able to steady
the view also plays a critical role. At maximum magnification, any movement will be
exaggerated in the viewing field. For example, seeing craters on the Moon with binoculars of
high magnification is a tremendous experience - if only you could hold the view still long
enough to identify which crater is which!
Around the Moon in 28 Days: Lunar Observing for Beginners
33
Figure 4.1 - Porro-prism binoculars, showing the optical path of light (right)
The so-called “exit pupil’ of binoculars is also an important number. The exit pupil is the
diameter of the pencil of light that comes out of the eyepiece of binoculars and telescopes and
enters your eye. It’s an easy number to calculate: just divide the aperture by the magnification.
So 8x42 binoculars and 6x30 and 10x50 binoculars have an exit pupil of 5 mm, roughly, while a
pair of 7x50 binoculars have an exit pupil a little over 7 mm.
If the binocular’s exit pupil is too wide, some of the light won’t enter your eye and is wasted. If
it’s too narrow, most of the light-sensitive cells on your retina will not be illuminated and the
image will appear too dim. During the daylight, the human eye has a pupil diameter of about
2mm, which makes higher magnification practical for birdwatching and sporting events. In
total darkness, the pupil increases to as much as 7 mm for younger people, and 5-6 mm for
those over the age of 40. For observing the Moon, which is fairly bright, the eye pupil will be
about 5 mm across. So choose your binoculars accordingly. If you stick with 7x35, 8x42, or
10x50, for example, you’ll be in good shape for observing the Moon.
Binoculars with a lens size of less than 30mm, such as 5X25 or 5X30, are small and very
portable. The compact models can fit easily into a pocket or backpack and are very convenient
for a quick look at well-lit situations. In this size range, low magnifications are necessary to
keep the image bright. But the small lens size limits resolution of fine detail. And such
binoculars are too small to give good views of fainter celestial objects like star clusters and
nebulae.
Binoculars with apertures of up to 40mm are a great mid-range size that can be used by
almost everyone. They give good brightness and resolution, and higher magnification shows
more detail. Yet they are not too heavy to hold steady for longer periods of time. Binoculars
up to 50-60mm in lens size are also considered mid-range, but far heavier. They are very well
suited to lunar astronomy, but the larger models may require a support (tripod, monopod, car
Around the Moon in 28 Days: Lunar Observing for Beginners
34
window mount) for extended viewing. The 50-60mm binoculars are pushing the maximum
amount of weight that can be held comfortably by the user without assistance, but don't rule
them out. Available in a wide range of magnifications, these models are for serious study and
will give crisp, bright images.
Binoculars larger than 50-60mm are serious aperture - the perfect size allowing for bright
images at high magnification. For lunar applications, binoculars with equations like 15X70 or
20X80 will open a whole new vista to your observing nights. The wide field of view allows for a
panoramic view! But you will need a tripod... such binoculars weigh 5 lbs or more and are too
heavy to hold for any length of time.
While it would be tempting to use as much magnification as possible, all binoculars (and the
human eye) have practical limits. You must consider another binocular specification-- eye relief
-- which is the amount of distance your eye must be away from the secondary (eyepiece) lens
to achieve focus. Many high "powered" binoculars do not have enough outward travel for eye
glass wearers to come to focus without your glasses. Anything less than 9mm eye relief will
make for some very uncomfortable viewing. If you wear eyeglasses to correct astigmatism, you
may wish to leave your glasses on while using binoculars, so look for models which carry
about 15mm eye relief.
Figure 4.2 - One method of mounting binoculars on a camera tripod with an optional L-shaped
adapter.
Perhaps the very best binoculars for moon watching are the so-called “image-stabilized”
binoculars offered by Canon and other companies. Inside these binoculars are tiny
piezoelectric motion sensors that detect the pitch and yaw movements caused by the slight
shaking of your hands and arms. The signal from these sensors feeds into a microprocessor,
which initiates image stabilization by controlling a vari-angle prism – a pair of glass plates
joined by flexible bellows. The space between the plates is filled with a silicon-based oil to
Around the Moon in 28 Days: Lunar Observing for Beginners
35
maximize image deflection. When you switch on the IS feature with a push button, the image
does not “freeze”, but rather wanders slowly enough for your eye to follow. And the IS works
when you sweep across a field of view, although there is a slight hesitation. The net effect: the
fast, jerky motion of the image is almost completely eliminated, which makes it easier to see
and identify features on the Moon. The IS feature also enables higher magnification than
standard binoculars. You’ll find 10x30, 12x36, 15x50, even 18x50 units that give lovely highpower views of the lunar surface.
The downside of IS binoculars? They are quite expensive. While a good pair of standard 8x42
binoculars might cost $100-$200, a pair of 12x36 IS binoculars cost $500, while 15x50 might
approach $1000.
Figure 4.3 - Canon 15x15 image-stabilized binoculars
Telescopes
If binoculars give a huge improvement in viewing enjoyment compared to the unaided eye,
telescopes are just as big a step up from binoculars. A good telescope will show thousands of
craters, mountains, valleys, rilles, and other features on the Moon. At higher magnification,
you’ll get a view of the Moon’s surface nearly as good as the Apollo astronauts who orbited the
Moon so many years ago.
But what kind of telescope do you need for lunar observing? Since the Moon is close and
large and bright compared to deep-sky objects, even a modest telescope will do nicely. And if
you already have a good telescope, there’s no need to get another one.
Around the Moon in 28 Days: Lunar Observing for Beginners
36
But if you are considering a new telescope, and you want to see the Moon well, there are a few
options to consider. There are three main types of telescopes, refractors, reflectors, and
compound telescopes. Here are a few pros and cons of each for lunar observing.
Refracting Telescopes
A simple refractor, or refracting telescope, is a hollow tube which uses a primary lens at its
opening to refract, or bend, the path of incoming light waves. This primary lens is called the
"objective lens" and is used to collect more light than the human eye. Parallel light waves that
pass through the lens are refracted over a distance called the “focal length” to form an image
at the a focal plane. Here, a second lens, or set of lenses (called the eyepiece) magnifies the
image. As with any telescope, the size of the primary or objective light gathering source is the
key to a bright image and good resolution of fine detail.
Figure 4.4 - Refracting telescopes
Because the interior of a refractor is never exposed to the outside air, the view is considered to
be far more crisp and steady - making the refractor ideal for studying lunar details. Also, unlike
reflector telescopes, there is no secondary mirror to obstruct the incoming light. This also
leads to much sharper images.
The drawback of refractors? They are expensive for their aperture. An 80-mm refractor might
cost as much as a 200 mm Newtonian reflector. Though smaller apertures aren’t a big deal for
observing the Moon, which doesn’t need as much light-gathering power as faint galaxies and
star clusters.
Around the Moon in 28 Days: Lunar Observing for Beginners
37
A more serious issue with refractors is “chromatic aberration”. Since glass lenses refract blue
and green light more than orange and red light, images of bright objects often appear with a
false color that manifests itself as a blue or purple halo. You’ll see this around the limb of the
Moon, for example. Lens designers try to minimize this effect by using compound
“achromatic” lenses. But the problem is always there, especially in short-focal length
refractors with focal ratio less than f/8. (The focal ratio is simply the focal length divided by the
diameter of the objective lens).
In the past twenty years or so, new fluorite glasses and innovations in telescope design have
led to “apochromatic” refractor lenses that show virtually no false color, even at focal ratios of
f/6 or f/7. “Apos”, as they’re called, are fantastic telescopes, with razor sharp images and
superb contrast. They are favored by astrophotographers and visual observers of the moon,
planets, and star fields of the Milky Way.
The down side? Top-notch apochromats are shockingly expensive, up to $1000 per inch of
aperture. If you have good dark sky and your main interest is the Moon and planets, you
should try out such a telescope at a star party and see if its right for you.
Figure 4.5 - An apochromatic refractor from Televue
Reflectors
Reflecting telescope (or reflectors) collect light using a curved mirror at the rear of the main
tube. Mirrors reflect all colors of visible light exactly the same, so reflecting telescopes have no
chromatic aberration. Most reflectors today are based on a design by Isaac Newton, who used
a second small diagonal mirror to direct light out the side of the telescope to an eyepiece. His
immensely practical design, the Newtonian reflector, is the main type of purely reflecting
telescope in use today by amateur astronomers.
Around the Moon in 28 Days: Lunar Observing for Beginners
38
Figure 4.6 - Light path of a Newtonian reflector
For a given aperture, Newtonian reflectors are generally the least expensive telescope because,
unlike the lenses of a refractor, only one surface of a mirror needs carefully figuring and
polishing. And since no light passes through a mirror, less expensive glass is used. Some
homemade reflectors even use glass from the discarded portholes of a ship! A top-of-the line
4-inch refractor costs as much or more as 12-inch Newtonian reflector, yet the reflector has 9
times the light gathering capability.
Newtonian reflectors occasionally require adjustment of their optical alignment– especially if
the scope gets bumped around. They are good all around telescopes, but they tend to be the
bulkiest design, with tube lengths of 4-5 feet. Newtonians also have a type of optical
aberration called “coma”. This makes stars appear wedge-shaped at the edge of the field of
view. But it’s not a big deal if the focal ratio is bigger than f/6 or so, and is not usually an issue
for lunar observing.
Newtonians came back into style in a big way in the 1980’s when telescope makers
commercialized a design by the former monk and astronomy popularizer John Dobson. The
“Dobsonian” telescope is just an inexpensive large-aperture Newtonian on a simple altazimuth
mount. Dobsonians can’t be beat for purely visual deep-space observing. With huge mirrors
(up to 36” in diameter) these new-age Newtonians are called “light buckets” because of their
immense light-collecting ability.
If you’re interested in strictly visual observing, and you want the most aperture (and lightgathering capability) for your money, a Dobsonian telescope is well worth your consideration.
An 8-inch f/6 Dobsonian with a focal length of 1200 mm is an excellent workhorse telescope
for lunar and general-purpose observing, even for beginners. It’s not too long or heavy to
Around the Moon in 28 Days: Lunar Observing for Beginners
39
move around, and still has enough aperture to show thousands of celestial sights. Yet a quality
Dobsonian scope of this size costs less than $500.
Figure 4.7 -A Newtonian reflector on a German equatorial mount
Do not forget that even a huge telescope will not make something larger just because the
telescope is bigger! For example, the size of of the Moon magnified 50X will be the size of
Moon magnified 50X in all telescopes… but a large-aperture reflector will allow you to see
small details on the Moon’s surface where a smaller aperture telescope cannot. That's
resolving power, and it’s directly proportional to the aperture of the objective lens.
Compound Telescopes
A Newtonian telescope has a big drawback: it uses a single reflection from a curved mirror to
send light to an eyepiece. That means the length of Newtonian is roughly equal to the focal
length, so you get a long and heavy telescope that needs a big and awkward mount.
Around the Moon in 28 Days: Lunar Observing for Beginners
40
But shortly after Newton developed his reflector, an obscure French Catholic priest named
Laurent Cassegrain invented a reflector that used two mirrors to fold a long optical path into a
shorter tube. Now many reflectors... and nearly all professional telescopes... use a variation of
the Cassegrain design. A new twist, added in the mid-20th century, combines a simple
spherical mirror with a specially-figured lens at the front of the tube to correct for spherical
aberration. This layout is called a Schmidt-Cassegrain telescope (SCT). A company called
Celestron built on this design and developed manufacturing techniques to produce what’s
compact but large-aperture SCT’s in large quantities.
SCT’s are great for lunar observing. Big aperture means good resolving power, much better
than smaller-lensed refractors. Yet the folded optical path means a compact package. SCT’s it
because of their longer focal length and higher magnification with a given eyepiece, have a
narrow field of view. This is a drawback if you like rich-field views of star clouds. But it’s not a
problem for lunar observing. Also, because of the secondary mirror, you won’t get the same
sharp contrast on the moon and planets with an SCT as you would with a refractor.
Figure 4.8 -A Schmidt-Cassegrain telescope on an altazimuth fork mount.
Maksutov-Cassegrains are another type of compound telescope. Maks are much like
Schmidt-Cassegrains. They have a spherical mirror to collect light and a curved lens up front to
correct for aberrations. But the corrector lens on a Mak has a simple spherical curve which is
easy to manufacture. And the secondary mirror is simply a thin layer of aluminum deposited on
the back of the lens. So unlike a Newtonian or Schmidt, a Mak requires no alignment.
Around the Moon in 28 Days: Lunar Observing for Beginners
41
The downside of the Mak’s optics? To keep aberrations small, Maks are made with a long focal
ratio… typically f/12 to f/15. That means you get a higher magnification with a particular
eyepiece and a narrower field of view than with an f/10 Schmidt or f/6 Newtonian. So Maks
aren’t great if you want wide, sweeping views of the Milky Way. They’re much better for objects
that require high magnification like planets... and the Moon!
Figure 4.9 - A Maksutov-Cassegrain telescope on an altazimuth fork mount.
Telescope Mounts
Even the finest telescope isn’t of much value if it doesn’t have a sturdy mount to point it around
and damp out vibrations. Some telescopes include a mount and a tripod; smaller scopes may
just have mounting rings or plates which allows them to be attached to a mount which you buy
separately.
The mount is as important as the optics of a telescope. It must be solid and stable. If you
have great optics on a shabby mount, you will still not see much because the image in the field
of view will bounce and shake and make it impossible for you to see any detail. If you’re
evaluating a telescope to purchase, look through it at high magnification and give the tube a
good tap on the side. If the mount takes longer than 5-7 seconds to damp out the vibrations,
then it’s unsuitable.
Around the Moon in 28 Days: Lunar Observing for Beginners
42
All telescope mounts are one of two types. An alt-azimuth mount lets the scope move up-down
(altitude) and left-right (azimuth). With these two motions, you can point a telescope to any
object in the sky. But an “alt-az” does not follow the natural motion of the sky. So to keep an
alt-az-mounted scope centered on a celestial object, you’ll have to move the scope in both
axes, which is bothersome for visual observing and completely unacceptable for photography.
A more involved mount, designed to track the motion of the stars by turning on a single axis, is
called an equatorial mount. When the “polar axis” of an equatorial mount is aligned to the
celestial pole, objects can be tracked with the movement of only the polar axis, which follows
the longitudinal (or right ascension) circles in the sky. Because only one axis needs to be
moved, equatorial mounts can be more easily motorized to track celestial objects. Of course,
to get an object in the field of view in the first place, the telescope must still be moved in both
axes.
Figure 4.10 - A Newtonian reflector on an altazimuth fork mount (left), and on a German
equatorial mount (right)
Eyepieces
Eyepieces magnify the image produced by a telescope’s objective lens or mirror. The
magnification of the telescope is simply the focal length of the objective lens divided by the
focal length of the eyepiece. So, for example, an 80-mm f/10 refractor has a focal length of
Around the Moon in 28 Days: Lunar Observing for Beginners
43
800 mm. With a 10 mm eyepiece, this telescope has a magnification of 80x. Swap in a 5 mm
eyepiece, and you get a magnification of 160x.
Most telescopes come with a couple of basic eyepieces, which are usually adequate for lunar
observing. But you might want to add to your eyepiece collection to enable a wider range of
magnification or a crisper image.
Eyepieces come in dozens of optical configurations like Plossl, Nagler, Erfle, Orthoscopic, and
so on. Each design differs two key characteristics. One is the “apparent field of view” (AFOV),
the angular diameter of the circle you see when you hold an eyepiece up to your eye. A wider
field shows you more sky, so it’s handy if you don’t have a motor drive on your telescope since
you have to move the telescope less frequently.
The other key characteristic of an eyepiece is eye relief. This is the distance you hold your eye
from the outer lens to see the full exit pupil. Short eye relief means you have to jam your eye up
close to the lens. If your eyes suffer from astigmatism, you’ll need to wear glasses when using
your telescope and you’ll need eyepieces with longer eye relief… at least 17-20 mm. Without
glasses, 10-20 mm of eye relief is fairly comfortable.
A Plossl-type eyepiece has a 50-degree AFOV. A more expensive Nagler-type has an 82degree field of view… this means you see 160% more sky than a Plossl at the same
magnification. The true field of view is simple the AFOV divided by the magnification given by
the eyepiece. For wide-field observing of extended objects like star clusters, you might
consider wide-field eyepieces with an AFOV of 65 degrees or more, like Televue Panoptics or
Nagler or Meade Ultrawide. For lunar observing, an AFOV of 60 degree or less is acceptable.
Figure 4.11 - A set of Plossl eyepieces
Around the Moon in 28 Days: Lunar Observing for Beginners
44
Not to say wide-field eyepieces won’t give you a good view of the Moon. They will. But
because of their design, wide-field eyepieces are not the best choice for viewing bright objects
like the moon and planets. To achieve a wide and flat field of view, wide-fields have 6-8 lenses,
often grouped in pairs. These lenses often bring light to focus at different points across the
field of view. This isn’t a big deal for faint objects. But it becomes a problem for brighter
objects. And the multiple lens surfaces scatter light, causing distracting “halos” around bright
objects and reduced contrast of bright and dark features.
Lower-AFOV eyepiece designs have fewer optical elements and a design that gives truer color
and high contrast at high magnification: perfect for planets and the moon, as well as deep-sky
sights like planetary nebula, small galaxies, globular clusters, and double stars. And the best
news? With fewer lenses and a narrower field of view, planetary eyepieces often cost less than
wide-field eyepieces.
Figure 4.12 - The optical layout of well-known eyepiece designs. While simpler designs like the
Plossl give a smaller field of view, they have fewer lenses to reduce the contrast of fine detail.
Here are some good design choices for planetary eyepieces:
Plössls. Your telescope may have come with one or two of these. Perfectly acceptable for
planets and the moon, with great contrast. Not a bad all-around eyepiece. But the eye relief is
not very good at short focal lengths, so beware if you must wear glasses at the telescope.
Plossls are made by dozens of manufacturers. Orion, Meade, Celestron, and Televue are all
recommended. You’ll find good short-focal length Plossls for less than $100.
Orthoscopics. An older design, but still works well. Orthos have better eye relief than Plossls,
but a narrower field of view. So these are best for the moon and planets only. Only $60 each.
Around the Moon in 28 Days: Lunar Observing for Beginners
45
Radians. A premium eyepiece from Televue, Radians give long eye relief, great contrast, and a
fairly wide field. Expensive… around $250.
Specialty Lunar Eyepieces. Companies like Williams Optics and Burgess Optical have their
own special designs optimized for the moon and planets. Good value for less than $150 or so.
To get higher magnification, eyepieces of these designs should be selected with a focal length
of 3-15 mm or so. If you have a telescope with long focal ratio, like an f/10 SchmidtCassegrain, then don’t go any shorter than 5-7 mm for an eyepiece. That gives you plenty of
magnification; any higher and the image will be too dim. With an f/5 or f/6 reflector or refractor,
you can go as low as 3 mm, but you’ll need to get your eye up close.
When you expand your eyepiece collection, don’t bother “upgrading” to a full set of 7-8 similar
eyepieces. You don’t need that many. Instead, buy 3 or 4 premium eyepieces that cover low,
medium, and high magnification. But buy the highest-quality eyepieces you can afford.
Other Accessories
Two more accessories worth considering... a Moon filter and a Barlow lens.
When looking at the Moon when it’s near full with a telescope larger than 90 mm aperture, the
image can be painfully bright. With a 200-200 mm telescope, it’s hard to look through the
eyepiece for almost any length of time. If you have a bigger scope, it makes sense get a socalled “Moon filter”, which is simply a piece of glass that reduces the intensity of light at all
visible wavelengths. The glass is mounted in a threaded sleeve that screws into the back of
your eyepiece.
Figure 4.13 - A Moon Filter and threaded sleeve
Around the Moon in 28 Days: Lunar Observing for Beginners
46
Moon filters come in a number of levels of optical density to let in 13%, 18%, or 25% of the
light from the objective lens or mirror. While it seems a shame to through away light, using
such a filter on the bright Moon actually makes it easier to see fine detail and contrasting
features. And the resolving power of the telescope remains unaffected.
Another accessory worth considering is a Barlow lens. This is a lens with a negative curvature
that’s used to effectively increase the focal length of a telescope by a factor of two or more
with a given eyepiece. That means you can double the magnification of your telescope with
each of your eyepieces, simply by inserting a Barlow lens between your telescope and
eyepiece. First invented in the mid-19th century, Barlows have multiplied in design and
configuration through the efforts of many manufacturers. Sometimes called “telenegative
amplifiers”, or by Televue, Powermates, these lenses are favored by some because they enable
higher magnification with eyepieces of medium focal length and better eye relief. They’re
especially useful for low-f-ratio telescopes. A f/7 refractor with a 100 mm objective lens, for
example, needs an eyepiece with 3.5 mm focal length to achieve a magnification of 200x.
Trouble is, a 3.5 mm eyepiece has short eye relief, and is harder to look through. But with a 2x
Barlow lens, a magnification of 200x is possible with a 7 mm eyepiece, which has better eye
relief.
Although they once came only in a 2x factor, Barlows (or whatever name they go by for a given
manufacturer) are now available in magnification factors of 2.5x, 3x, 4x, and even 5x. For your
set of eyepieces, you can double the number of magnifications available by adding a single
Barlow lens. Though you have to choose the right combination of Barlow and eyepiece focal
lengths to make sure you don’t duplicate magnifications. A 6 mm eyepiece, for example, will
give you the same magnification as an 18 mm eyepiece with a 3x Barlow.
Figure 4.14 - A 2x Barlow lens
Around the Moon in 28 Days: Lunar Observing for Beginners
47
Section 5 - How To Follow The Moon Tours
A Word About Time
In the next sections, you’ll enjoy a 28-day tour of the Moon’s surface as seen from the Earth.
But why 28 days? This is roughly how long it takes the Moon to revolve around the Earth and
move through all its phases.
As we learned earlier, with respect to the background stars, the Moon takes an average of 27
days, 7 hours, and 43 minutes to move around the Earth. But during this time, the Earth has
moved with respect to the sun, so our perspective of the Moon and sun change a little. Since
it’s the relative alignment of the sun and Moon that causes the Moon’s phases, we need to take
this into account. With respect to the sun, the Moon takes 29 days, 12 hours, and 44 minutes,
on average, to get back to the same phase relative to the sun. Which means when the Moon’s
phase is full, for example, it will take about 29.53 days to get back to full. This period is called
lunation.
We begin our tour at new Moon, when the Moon is in conjunction with the sun. Strictly
speaking, it’s not visible at this instant, but it slowly emerges in the western sky at sunset
within 24-36 hours after new. When does new Moon occur? The easiest way know is to
check online tables such as this one from NASA, which records and predicts Moon phases
from the year 1999 B.C. to 4000 A.D.
http://eclipse.gsfc.nasa.gov/phase/phasecat.html
You’ll notice in these tables, and many other resources, that Moon phases are noted in
coordinated Universal Time (UTC) or Greenwich Mean Time (GMT). For our purposes, UTC,
which is a modern form of GMT, is the time at the prime meridian in Greenwich, near London,
U.K. You can find the current GMT here:
http://wwp.greenwichmeantime.com/
You can also use this link to find the current local time in your city or in a city near you. While
there is no Summer Time or Daylight Savings Time for GMT. This calculator will take into
account summer time for each location in the world.
A couple of examples will make this clear. New York City in July observes Eastern Daylight
Time. By convention, this is 4 hours behind GMT, since New York is west of Greenwich. So
Around the Moon in 28 Days: Lunar Observing for Beginners
48
when GMT is 1400, it’s 10 a.m. in New York. In January, New York has switched to Eastern
Standard Time, which is 5 hours behind GMT. So if it’s 0800 GMT, then it’s 3 a.m. in New York.
Another example... in September, Johannesburg runs on South African Standard Time (SAST).
By convention, this is 2 hours later than GMT, since Johannesburg is east of Greenwich. So if
the Moon is in the new phase at, say, 0251 GMT, that’s 4:51 a.m. SAST.
It’s not too hard... and remember, the link above for GMT will do the calculation for you, while
the link to NASA will give you the time for the Moon phases in GMT.
Here’s a couple of sample Moon-phase tables from the NASA link for 2010 to 2013:
From this table, you can see the last New Moon for 2010 occurs on December 5 at 1736 GMT.
That’s 12:36 p.m. Eastern Standard Time.
2010 Phases of the Moon
Universal Time
New Moon
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
First Quarter
d
h
m
15
14
15
14
14
12
11
10
8
7
6
5
7
2
21
12
1
11
19
3
10
18
4
17
11
51
01
29
04
15
40
08
30
44
52
36
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
d
h
m
23
22
23
21
20
19
18
16
15
14
13
13
10
0
11
18
23
4
10
18
5
21
16
13
53
42
00
20
43
29
10
14
50
27
39
59
Full Moon
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Last Quarter
d
h
m
30
28
30
28
27
26
26
24
23
23
21
21
6
16
2
12
23
11
1
17
9
1
17
8
18
38
25
18
07
30
36
05
17
36
27
13
d h m
Jan 7 10 39
Feb 5 23 48
Mar 7 15 42
Apr 6 9 37
May 6 4 15
Jun 4 22 13
Jul 4 14 35
Aug 3 4 59
Sep 1 17 22
Oct 1 3 52
Oct 30 12 46
Nov 28 20 36
Dec 28 4 18
Around the Moon in 28 Days: Lunar Observing for Beginners
49
2011 Phases of the Moon
Universal Time
New Moon
Jan
Feb
Mar
Apr
May
Jun
Jul
Jul
Aug
Sep
Oct
Nov
Dec
d
4
3
4
3
3
1
1
30
29
27
26
25
24
First Quarter
h
9
2
20
14
6
21
8
18
3
11
19
6
18
m
03
31
46
32
51
03
54
40
04
09
56
10
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
d
12
11
12
11
10
9
8
6
4
4
2
2
h
11
7
23
12
20
2
6
11
17
3
16
9
m
31
18
45
05
33
11
29
08
39
15
38
52
Full Moon
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
d
19
18
19
18
17
15
15
13
12
12
10
10
h
21
8
18
2
11
20
6
18
9
2
20
14
Last Quarter
m
21
36
10
44
09
14
40
57
27
06
16
36
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
d
26
24
26
25
24
23
23
21
20
20
18
18
h
12
23
12
2
18
11
5
21
13
3
15
0
m
57
26
07
47
52
48
02
54
39
30
09
48
2012 Phases of the Moon
Universal Time
New Moon
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
First Quarter
d
h
m
23
21
22
21
20
19
19
17
16
15
13
13
7
22
14
7
23
15
4
15
2
12
22
8
39
35
37
18
47
02
24
54
11
02
08
42
Jan
Jan
Mar
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
d
1
31
1
30
29
28
27
26
24
22
22
20
20
h
6
4
1
19
9
20
3
8
13
19
3
14
5
m
15
10
21
41
57
16
30
56
54
41
32
31
19
Full Moon
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Aug
Sep
Oct
Nov
Dec
d
9
7
8
6
6
4
3
2
31
30
29
28
28
h
7
21
9
19
3
11
18
3
13
3
19
14
10
Last Quarter
m
30
54
39
19
35
12
52
27
58
19
49
46
21
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
d
16
14
15
13
12
11
11
9
8
8
7
6
Around the Moon in 28 Days: Lunar Observing for Beginners
h
9
17
1
10
21
10
1
18
13
7
0
15
m
08
04
25
50
47
41
48
55
15
33
36
31
50
2013 Phases of the Moon
Universal Time
New Moon
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
First Quarter
d
h
m
11
10
11
10
10
8
8
6
5
5
3
3
19
7
19
9
0
15
7
21
11
0
12
0
44
20
51
35
28
56
14
51
36
34
50
22
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
d
h
m
18
17
19
18
18
16
16
14
12
11
10
9
23
20
17
12
4
17
3
10
17
23
5
15
45
31
27
31
34
24
18
56
08
02
57
12
Full Moon
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Last Quarter
d
h
m
27
25
27
25
25
23
22
21
19
18
17
17
4
20
9
19
4
11
18
1
11
23
15
9
38
26
27
57
25
32
15
45
13
38
16
28
Jan
Feb
Mar
Apr
May
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
d
5
3
4
3
2
31
30
29
28
27
26
25
25
h
3
13
21
4
11
18
4
17
9
3
23
19
13
m
58
56
53
36
14
58
53
43
35
55
40
28
48
How To Follow The Moon Tours
As we learned in earlier sections, the Moon moves from the new phase, when it is dark as seen
from Earth, to the full phase in about 14 days. During this time, every feature on the Earthfacing side of the Moon experiences lunar sunrise and becomes visible from Earth.
So why have we done a 28-day tour instead of a 14-day tour?
The simple answer is this: after full Moon, the sun begins to set across the Moon’s face. Just
like on Earth, many features look a little different at sunset as the light comes from a different
angle. So a crater experiencing sunrise at first quarter will experience sunset at last quarter,
and depending on the topography, may present a very different view in a telescope.
Around the Moon in 28 Days: Lunar Observing for Beginners
51
Views of the crater Tycho as the sun rises (left) and sets (right)
Another difference occurs during the slender waxing and waning crescent phases just a few
days after and before new Moon, respectively. The long shadows cast during these phases
reveal a much different view of the lit portion of the Moon than when the sun rises and sets
before and after full.
So if you can, follow all 28 of these tours to observe the Moon’s surface each day in both the
waxing and waning phases throughout a full cycle. (It’s likely you’ll only be able to spot the
Moon on 26 of these days, since the Moon is lost in the sun’s glare hours before new and full).
Of course, the weather and other vicissitudes of life make it difficult to observe the Moon every
day for nearly four weeks. But try over the course of several months enjoy the many views the
Moon presents to observers on the Earth. Few people every get to do so.
Finally, a reminder about when to see the Moon during its cycle. To see the Moon each day of
the month might require a little lost sleep: an occupational hazard for amateur astronomers. To
help plan your viewing sessions throughout the 28-day cycle, here's a quick review of when
you can see the Moon...
New to First Quarter (Day 1 to 7). A day or two after new Moon, a thin crescent emerges just
east of the sun at sunset in the western sky. The Moon rises move about 12 degrees eastward
each night and rises about 50 minutes later. By first quarter, the Moon rises in the east near
mid-day and is well-placed for dark-sky viewing about 8-10 p.m. local time at mid-latitudes in
the northern and southern hemisphere. For most people, this is the easiest time to see the
Moon.
First Quarter to Full (Day 8 to 14). As the Moon moves to the waxing gibbous phase, you'll
need to stay up a little later each night. Remember the full Moon rises at sunset, so you have
Around the Moon in 28 Days: Lunar Observing for Beginners
52
to stay up at least 3-4 hours after sunset until it's high enough to view well. Though as you will
learn, the full Moon is perhaps the least interesting phase to observe in a telescope. Full to Last Quarter (Day 15 to 21). Time for some strong coffee! To see the waning gibbous
Moon in dark sky, you'll need to stay up til midnight and beyond. But the Moon won't set until
after sunrise, so if you'd rather wake up early than stay up late, take your telescope out around
4-5 a.m. to get a good view. The Moon is easier to view just before dawn (or just after sunset)
because the brightening sky gives better contrast with the brilliant lunar surface. Last-Quarter to New Moon (Day 22 to 28). Wake up early to see the last days of the lunar
cycle. The sight of a slender crescent in the eastern sky before sunrise is exceptionally
beautiful, especially if there are bright stars or planets nearby to enhance the scene. The new
Moon rises with the sun, then begins the whole cycle over again.
If you don't wish to stay up late or rise early to follow the Moon all month, there is a
solution. While we've laid out these tours over 28 days, you can get a good view of most of
the sights in the first 14 days of the lunar cycle, from Day 1-14, when the Moon is visible in the
early to mid-evening. Most of the tours focus on what comes visible along the terminator, the
boundary that separates the bright and dark areas of the Moon's face, where the shadows are
longest and detail is most striking. So, for example, you'll see the same craters along the
terminator at first quarter as you will at last quarter, although they may be shaded a little
differently, as mentioned above. As another example, sights that come into view along the
terminator two days before first quarter, on Day 5, will disappear from view along the terminator
two days before last quarter, on Day 19. So if you want to take a short-cut and pack it all into 14 days, here's how to approach it:
• On Days 1-3, use the tours as they are
• For Day 4 ... use tours for Day 4, 18
• For Day 5 ... use tours for Day 5, 19
• For Day 6 ... use tours for Day 6, 20
• For Day 7 ... use tours for Day 7, 21 (first and last quarter)
• For Day 8 ... use tours for Day 8, 22
• For Day 9 ... use tours for Day 9, 23
• For Day 10 ... use tours for Day 10, 24
Around the Moon in 28 Days: Lunar Observing for Beginners
53
• For Days 11-14... use tours as they are
The only truly unique views after Day 14 are on Day 25-28, when the Moon is a slender
crescent. There is no substitute for seeing the Moon at this time. Rise early and have a look
for yourself.
Enough preliminaries... let's get started on the tours.
Around the Moon in 28 Days: Lunar Observing for Beginners