Download Moon

Moon
Astronomy 1-1
Lecture 11-1
Vital Statistics
Mean Distance from Earth
Sidereal Period
Rotation Period
Inclination of Axis
Inclination of Orbit
Mass
Mean Density
Albedo
Astronomy 1-1
384,401 km
27 days, 7 hours, 43 minutes
27.32 days
~ 7 degrees
5 degrees
7.35 x 1022 kg
3.34 g/cm3
0.07
Lecture 11-2
Synchronous Rotation
Time it takes for the Moon to rotate about its own axis is
the same as the time to make one orbit about the Earth
The Moon always presents the same surface to the Earth
In actuality about 59% of the Moon's surface is seen from
Earth due to the Moon's wobbling which is known as
libration
Astronomy 1-1
Lecture 11-3
The Phases of the Moon
During the lunar month, the Moon's appearance changes
– phases of the moon
1) New Moon
3) First Quarter Moon
5) Full Moon
7) Last Quarter Moon
2) Waxing Crescent Moon
4) Waxing Gibbous Moon
6) Waning Gibbous Moon
8) Waning Crescent Moon
Each successive phase rises later in the evening
Astronomy 1-1
Lecture 11-4
The Phases of the Moon
Astronomy 1-1
Lecture 11-5
Lunar Month
The Moon takes longer to go through its phases than it
takes to go around the Earth
Sidereal Month
True orbital period for the Moon's motion about the
Earth. ~27.3 days
Synodic Month
Time to go from one full moon to the next. ~29.5 days
Both of these are averages
Astronomy 1-1
Lecture 11-6
Shadows
A region of darkness where light is blocked
Type of shadow produced is dependent upon whether
the source is
A Point Source
An Extended Source
A Point Source illuminating
an object produces only a one
section shadow behind the
object
The dark part of the shadow is called the umbra
Astronomy 1-1
Lecture 11-7
Shadows
Extended Source illuminating an object
Two sections to the shadow
behind the object
Penumbra
The lighter, outer portion of the shadow
Area is partially illuminated
Umbra
The dark, inner portion of the shadow
Area is not illuminated
Astronomy 1-1
Lecture 11-8
Eclipses
Caused by either
Moon coming between Earth and Sun (Solar Eclipse)
or
Earth coming between Moon and Sun (Lunar Eclipse)
Naively,
both of types of eclipses should occur every month
BUT THEY DO NOT!
Astronomy 1-1
Lecture 11-9
Eclipses
Plane of Moon's orbit
is tilted with respect to
plane of Earth's orbit
by 7 degrees
Moon can therefore be
above or below plane of
Earth's orbit and the
respective shadows will
not fall on anything
Astronomy 1-1
Lecture 11-10
Eclipses
Eclipses can only occur when a
straight line can be drawn through
the Earth, Sun, and Moon system
This can only occur when the
Moon crosses through the
Earth's orbital plane
The points were the Moon's orbital path intersects the Earth's orbital plane
are called NODES
The line joining these two points is called the LINE of NODES
Eclipses then occur when the Line of Nodes is aligned with the Sun
Astronomy 1-1
Lecture 11-11
Eclipses
By this reasoning both Solar and Lunar eclipses should
each occur twice every year
But they do NOT!
The Moon's orbit is affected by the gravitational pull of
the Sun
The orbit of the Moon precesses
This causes the line of nodes to also precess
So eclipses on average occur only 2 to 5 times a year
Astronomy 1-1
Lecture 11-12
Eclipse Types
The type of eclipse is dependent upon what part of the
shadow, the shadowed body goes through
Penumbral Eclipse
Occurs when the shadowed body only goes through
the penumbra part of the shadow.
Partial Eclipse
Occurs when the shadowed body goes partly through
the penumbra and partly through the umbra.
Total Eclipse
Occurs when the shadowed body goes totally through
the umbra.
Astronomy 1-1
Lecture 11-13
Lunar Eclipses
Occur in all three ways
Penumbral Eclipse
Partial
1/4
Astronomy 1-1
1/2
3/4
Lecture 11-14
Total Lunar Eclipse
During a total eclipse the Moon is not completely dark but has a reddish tinge
The redness that is seen across the Moon’s face is due to refraction and
filtering of sunlight through the Earth's atmosphere
Astronomy 1-1
Lecture 11-15
Solar Eclipses
Solar eclipses also occur in the same three ways as lunar eclipses
penumbral, partial, and total
There is one more type of eclipse that can only occur with respect to a
solar eclipse - an Annular Eclipse
Average distance of the Moon from the Earth is about 384,400km
Earth-Moon distance fluctuates by
about 13% from 363,300km (perigee)
to about 406,000km (apogee)
Accordingly, the apparent size of the
Moon in the sky changes by the same
amount
Astronomy 1-1
Lecture 11-16
Annular Solar Eclipse
When the Moon is at
apogee, the umbral part of
the Moon's shadow falls
short of the Earth's
surface
The Earth is in the part of
the Moon’s shadow that is
known as the antumbra
(negative shadow)
Astronomy 1-1
Lecture 11-17
The Diamond Effect and Totality
Just before totality, sunlight shines
through the lunar valleys
Diamond Effect
Last blast of sunlight before totality
Totality
Solar Corona
Astronomy 1-1
Total solar eclipse allows the viewing
of the solar corona which ordinarily is
outshined by the surface of the Sun
Lecture 11-18
Lunar Features
Features on the Moon there for at least 3 billion years
Prominent features that can be seen from the Earth are:
Craters
Highlands
Maria
Rilles
Astronomy 1-1
Lecture 11-19
Lunar Features
Differences between Near Side and Far Side
Near Side
Highlands and Maria
Astronomy 1-1
Far Side
Basically has no Maria
Lecture 11-20
Lunar Features
Astronomy 1-1
Lecture 11-21
Craters
Results of bombardment by meteorites
Surface of the moon is scarred with millions of impact craters
No erosion and little geologic activity to wear away these craters
Range in size up to many hundreds of kilometers
Most enormous craters have been flooded by lava, and only parts of the
outline are visible
Low elevation maria (seas) have fewer craters than other areas
These areas formed more recently, and have had less time to be hit
Many lines of mountainous cliffs or scarps on the lunar surface
These are remnants of the rims of ancient craters
Astronomy 1-1
Lecture 11-22
Craters
Astronomy 1-1
Lecture 11-23
Types of Impact Craters
The type of impact crater is dependent upon the size, mass, speed, and
angle of the falling object
Small, slow objects have a low energy
impact and cause small, simple craters
Moltke Crater
Large, fast objects release a lot of
energy and form large, complex
craters
Astronomy 1-1
Euler Crater
Lecture 11-24
Types of Impact Craters
Very large impacts can even cause secondary cratering, as ejected
material falls back to the ground, forming new, smaller craters, or a
series of craters
Secondary craters tend to cluster in a
ring around the main crater
The greatest number lying a bit more
than one crater diameter away
Tycho Crater
Material falling near to the main
crater tends to be moving too slowly to
form craters but instead piles up as an
ejecta blanket
Astronomy 1-1
Lecture 11-25
Highlands
Are at an elevation generally higher
than the maria
Differ chemically from the maria
Age is approximately 4 billion years
Saturated with craters
Appears lighter than the maria
Older surface than the maria
Astronomy 1-1
Lecture 11-26
Maria
Relatively smooth areas
Covers ~31% of near side surface
Vast solidified polls of basaltic lava
Estimated to be a few 100 meters thick
Lack of impact craters implies
younger than highlands
Age is approximately 3.5 billion
years
Round shapes of maria probably
formed due to giant impacts
Astronomy 1-1
Lecture 11-27
Rilles
Narrow trench like features
There are three types of rilles on the lunar surface
Straight rilles
Sinuous rilles
Arcuate rilles
Astronomy 1-1
Lecture 11-28
Straight Rilles
Long, linear paths
Section of the crust that has sunk between two
parallel faults
Readily identified when they pass through
craters or mountain ranges
Astronomy 1-1
Lecture 11-29
Sinuous Rilles
Meander in a curved path like a river
Thought to be the remains of collapsed lava tubes
or extinct lava flows
Usually begin at an extinct volcano
Astronomy 1-1
Lecture 11-30
Arcuate Rilles
Smooth curved
Found on the edges of the dark lunar maria
Believed to form when the lava flows that created
a mare cools, contracts, and sinks
Astronomy 1-1
Lecture 11-31
Lunar Origins
Various models put forth:
Fission Hypothesis
Capture Theory
Accretion Hypothesis
Giant Impact Hypothesis
Astronomy 1-1
Lecture 11-32
Fission Hypothesis
Moon broke off from the Earth while the
Earth was cooling down
Equatorial bulge was spun off from the Earth by the Earth's
high rotation rate
Problems:
1. If spin off occurred while Earth was in molten state, then
why the low iron content of Moon compared to Earth.
2. For the bulge to have been thrown off, a much higher total
angular momentum for the Earth, Moon system would have
required.
3. If the Moon was thrown off from the Earth, why is not the
Moon's orbital plane in the equatorial plane of The Earth?
Astronomy 1-1
Lecture 11-33
Capture Hypothesis
Moon was an object that wandered by the
Earth and was captured in the Earth's
gravitational field
Accounts for high angular momentum of the Moon and for
the difference in chemical composition
Problems:
1. The gravitational interaction between the Earth
and Moon would cause the Moon to speed up, not
slow down.
2. The chemical composition of the Moon is too
different from other planets or moons.
Astronomy 1-1
Lecture 11-34
Accretion Hypothesis
Earth and Moon formed at the same time
Problem:
The chemical composition of the Moon should be similar
to that of the Earth
But the chemical compositions are different
Astronomy 1-1
Lecture 11-35
Giant Impact
Hypothesis
A large planetesimal impacted Earth
at a glancing angle when the Earth
had partially solidified
Collision crushed and vaporized the surfaces of the colliding
bodies
Material forms a disk around the Earth and this material then
formed the Moon
Requires an object about the size of Mars
Astronomy 1-1
Lecture 11-36
Giant Impact Hypothesis
Astronomy 1-1
Lecture 11-37
Lunar History
Radioactive dating of lunar rock samples show that the
Moon is about 4.5 billion years old, about the same age
as the Earth.
After the initial formation, the Moon began to cool with
the attendant differentiation of the heavier materials
into the core region.
The Moon was then heavily peppered by meteorites which
caused the cratering that is now seen in the highlands
and on the far side of the Moon. In places, the impacts
probably fractured the crustal surface.
Then came the lava flows which filled in many of the
cratered areas. The magma came from depths of about
400km. Eventually the lithosphere became thick
enough to halt the magma from breaking through.
Astronomy 1-1
Lecture 11-38
Lunar History
Then some additional peppering by meteorites occurred
which produced the cratering that is seen in the maria.
Since then the Moon has been relatively quiet.
Astronomy 1-1
Lecture 11-39
Lunar Structure
Instruments left by the Apollo astronauts have yielded
valuable information about the Moon.
The instruments confirmed the fact that the Moon has no
magnetic field.
This implies that there is no molten segment to the core.
Lunar rocks have indicated that at one time the Moon did have a
weak magnetic field.
Seismometers that were left behind indicate that there is
seismic activity on the Moon but not at a rate
comparable to that on Earth.
There are on average about 3000 moonquakes per year
On Earth the number of earthquakes is in the hundreds
of thousands.
The moonquakes are far weaker than those occurring on
Earth.
Astronomy 1-1
Lecture 11-40
Internal Structure of the Moon
Analysis of the seismic waves that occur indicate the
following structure for the Moon:
A solid core which is probably rich in iron. The core extends from
the center out to a radius of about 500 km.
An asthenosphere extending from 500 km from the center to a
distance of about 1000 km from the center of the Moon. Like
on Earth the asthenosphere is plasticized.
A lithosphere extending from 1000 km to 1650 km from the
center. This is a solidified region.
The lunar crust being about 80 km thick. The lunar crust is
covered by a layer of loose fragments of rocks, crystals, and
glass. The particles range in size from fine dust particles to
large boulders. This covering layer is called regolith.
Astronomy 1-1
Lecture 11-41
Internal Structure of the Moon
Astronomy 1-1
Lecture 11-42
Quakes and Impacts
The moonquakes originate about 600 to 800km
below the surface, much deeper than on Earth.
The moonquakes are thought to occur at or near
the boundary between the Moon's lithosphere
and asthenosphere.
The seismometers also indicate that the Moon is
still being impacted by meteorites at the rate of
80 - 150 per year.
Astronomy 1-1
Lecture 11-43
Astronomy 1-1
Lecture 11-44