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“What’s Up”
“The Moon: Astronomy for Everyone”
Today it takes 29.5 days for the Moon to complete a cycle of phases around the
Earth, and appears in the same position in the sky relative to the background stars
upon completion of its orbit around the Earth. This period is known as the synodic
month or lunar month. The difference between the sidereal month and the longer
synodic/lunar month stems from the fact that the Moon orbits slightly past 360
degrees and lines up with the background stars to compensate for movement of the
Earth as it in-turn orbits the Sun. It’s something of a celestial ballet.
Even though the Moon continuously shows the same face to earth it has a slight
wobble or libration allowing earth based observers to see approximately 59% of
the Moon’s surface. You can simulate the process of libration by looking in a
mirror and then observing yourself looking slightly up, down, left, and right.
The Lunar surface consists mainly of mountainous highlands, smooth lowlands
and craters. The light colored and mountainous highlands are the oldest lunar
terrain, billions of years old, while the smooth dark colored lowland called Maria,
or “seas”, are volcanic lava flows that occurred many millions of years ago,
flooding parts of the Moon and have long since cooled. Today there is no volcanic
activity on the Moon. Large and small craters pepper the lunar surface and were
the result of meteor impacts, most of which occurred many billions of years ago,
and continue today but to a much lesser extent than today.
Origin of the Moon: It is generally accepted that the Moon formed about 50
million years after our solar system came into being some 4.6 billion years ago.
There are four theories of how the Moon was formed: coformation, capture,
fission, and impact.
Coformation theory asserts that the Earth and Moon coalesced as distinct
bodies in proximity to each other from the same debris. Differences in the
geologic composition of the Earth and Moon tend to dispel this theory.
Capture theory assumes that the Moon originated away from Earth and was
captured into orbit by the Earth’s gravity as it floated nearby. This theory is
mathematically unlikely and there is enough similarity in the composition of the
Earth and Moon to suggest that they weren’t formed independently of
each other.
Fission theory would have a hot young Earth spinning so rapidly that
a molten blob spurted off to become the Moon. There is no evidence to
suggest that Earth ever spun that rapidly and such a blob is not likely to
have remained in orbit.
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Phases of the Moon: The orbital motion of the Moon relative to the Earth
and Sun cause the Moon to appear in phases. The Moon goes through four main
phases as it travels from west to east in its monthly orbit of the earth. Try
observing the moon at the same time each night and its eastward movement will
be revealed. The phases are caused by the angle of sunlight reflected on the lunar
surface as the Moon changes position relative to the sun and us as observers here
on Earth.
A new Moon occurs when the moon and sun are
together in the daytime sky. It rises at sunrise, is up all
day, but its lit far side is away from us facing the Sun
and therefore the near side facing us is not normally
visible. It sets with the Sun resulting in a Moonless
night. The first quarter or half Moon is lit on its
eastern side by the sun, rises about noon, and sets near
midnight. A full Moon occurs when the Moon and Sun
are opposite each other in the sky relative to earth, so
that the near side of the Moon that faces us is brightly
lit by the Sun as it rises at sunset and is visible all
night until it sets at dawn. The last quarter Moon is
also a half Moon that is lit on its western side by the
sun. It rises about midnight and sets around noon.
This phenomena in astronomy is known as captured
rotation, or tidal locking. Millions of years ago in the
Earth Moon history the Moon was closer to the Earth
and it rotated on its axis at a different speed than that of
the Earth’s. Back then if you were around to witness it,
the moon would appear much larger in the sky and its
“Man in the Moon” face would be seen to change as it
rotated on its axis in its journey around the Earth.
The reason for the change in the Moon’s rotational
speed is due to the Gravity of the Earth tugging so
greatly on the moon, and because the moon’s rotation
on its axis was different than the Earth’s rotation, the
Moon’s crust would bulge towards the Earth. It’s just
like the oceans on the Earth that bulge today as the
result of the same gravitational influence between the
Earth and Moon. This rotating bulge of the Moons crust
robs energy from the Moon’s rotational inertia and very slowly caused the Moon
to slow down in its rotation until after millions of years the moon finally gave up
the fight to remain tidally locked to the rotation of the larger Earth.
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Illustration by: Shawn Martin
In between the four main phases there are also
crescent and gibbous phases. Crescent and gibbous
phases occurring before the full Moon are called
waxing, while crescent and gibbous phases occurring
after full Moon are called Waning.
Observing the Moon: Moon phases and lunar terrain such as Maria,
mountains, and larger craters all can be easily observed with the unaided eye.
Books and maps naming all the lunar features are readily available in the
astronomy section of most book stores and can enhance your observing
experience. Binoculars are well suited for observing the moon and provide more
detailed views of the lunar features. Steady your binoculars on a tripod or rest
them on a bean bag atop a post or fence as you observe.
Back in 1609 Galileo used the first astronomical telescope to observe lunar
mountains and craters and thereby prove their existence. Telescopes can provide
incredibly detailed views of these and many other features. The best telescopic
views of lunar terrain occur close to the terminator line which marks the
boundary of the lit and unlit portions of the Moon during a phase. The long
shadows cast near the terminator show depth and detail to the lunar surface that
is not obvious when it’s fully lit as during a full Moon.
At times the Moon can appear very bright in the eyepiece of a telescope
especially at full Moon. Special moon filters can be purchased at astronomy
stores to dim and increase the contrast of the image. If you don’t have such a
filter, try wearing sunglasses especially the polarized kind, to achieve the same
effect with your telescope, binoculars, and the unaided eye. Light or hazy cloud
cover can also act as filter to tone down the brightness of the Moon.
Remember never point binoculars or a telescope at the Sun! Doing so can
cause permanant damage to your vision, including loss of eyesight. Wait until
the Sun has set to be completely safe from accidentally doing so.
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Point this
astronomical chart
toward North and
match the stars with
those in the real sky.
Planets
S
Viewable in a pair of Binoculars or small telescope
Mercury - is lost in the sunset.
Venus - (magnitude –4.7) shines brightly low in the east during dawn. 5:00 am
local time.
Mars - (only magnitude +1.2) remains 6° lower left of Venus this week. 5:00
am local time.
Jupiter - (magnitude –2.3, in Capricornus) shines brightly in the southeast
before and during dawn. 5:00 am local time.
Saturn - (magnitude +0.8, in Leo) is highest in the south at dusk and moves to
the southwest later. In a telescope, Saturn's rings appear 4° from edge on, their
widest this year.
Uranus - (6th magnitude) is low in the sunrise glow. It's to the upper right of
Venus. 5:00 am local time.
Neptune - (8th magnitude, in Capricornus) shines brightly in the southeast
before and during dawn. 5:00 am local time.
Pluto (dwarf planet) - (14th magnitude, in northwestern Sagittarius) is highest
in the south before the first light of dawn. 5:00 am local time.
Moon
Waxing
First Quarter
Full
Last Quarter
Waning
New
May 28
May 31
June 7
May 17
June 15
May 19
May 24
Doug Grouchy RASC, St. John’s Centre
ACTIVITIES
1. Look in the weather section of The Telegram to find the
phases of the Moon. Observe the moon during these phases and
see the difference in the lunar terrain throughout the month.
2. Are there any other articles or information published in The
Telegram that relate to the phases of the moon in any way?
For more activities go to
www.thetelegram.com and click on
Brought to you by
Moon Cycle (or Lunar Cycle) is referring to the moon's continuous orbit
around the earth. As the moon orbits the earth, it's appearance (the phase)
changes and thus gives us an indication of the moon's progress in the cycle
(the age).
You can contact the Royal Astronomical Society of Canada, St. John’s
Centre at www.rasc.ca/stjohns/
WA R N I N G ! “ W h e n u s i n g a t e l e s c o p e o r b i n o c u l a r s , a l w a y s b e s u r e
NEVER TO LOOK AT THE SUN! This can cause serious and permanent eye
damage. To be safe, always make sure the Sun is fully set below the horizon
before going outside with your telescope or binoculars.”
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Newspaper In Education
THE TELEGRAM
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The Moon rotates on its axis every 27.3 days and simultaneously orbits the
Earth in the same 27.3 day period. This orbital period is known as the sidereal
month and also explains how the Moon continuously shows only one side to Earth,
a process known as captured rotation. That is why the
Moon continuously shows the same familiar “Man in
the Moon” face and never seems to change?
Shawn Martin Observing Director RASC, St. John’s Centre
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Lunar Characteristics: The Moon is about
1/4 the size of Earth and its rocky crust is similar
in composition to the Earth’s mantle. It orbits the
Earth from west to east and rises and sets every day
with the eastward rotation of the Earth in the same
manner as the Sun and stars.
May 15 - Mid June
Impact is the most widely accepted theory of lunar formation. It details how a
Mars-sized object traveling through space at an approximate speed of 40,000
kph sideswiped the young Earth in a celestial hit and run. As the impactor
continued on its way a tremendous amount of rubble and debris was blown into
Earth orbit where it formed into a ring (like the rings around the planet Saturn)
and then coalesced over time into the Moon. This theory explains similarities in
the composition of the Moon to Earth’s mantle, however, it must be said that
origin of the Moon is not yet completely understood.
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The Moon is our nearest celestial neighbor
and the easiest and most accessible of all
astronomical objects to observe. It has an
interesting origin, travels in phases across the
night sky, and can be observed by the unaided
eye without any astronomical equipment.