Motion of the Moon

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Motion of the Moon
Diurnal motion from east to west Monthly
motion along ecliptic from west to east against
the stars
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The Moon
As we pass through the month, the appearance of
the Moon changes each night. We say the Moon
goes through a series of phases.
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Discussion
Many people believe incorrectly that the phases
of the Moon are caused by the shadow of the Earth
on the Moon. What simple observation of the Moon
could you make to prove that they are wrong?
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First quarter Moon
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The real reason
The Moons phases are actually cause by the
changing position of the Moon with respect to the
Sun. The side of the Moon facing the Sun is
always fully illuminated (except during a lunar
eclipse). But because the angle we view the Moon
differs from that of the Sun, we see varying
degrees of the Moons surface facing us
illuminated.
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New Moon
Occurs when the Moon is in the same part of the
sky as the Sun. Thus the Moon is up during the
day, and the illuminated side of the Moon faces
away from the Earth. The Moon is not visible
when it is new.
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Waxing crescent
When less than ½ of the Moons face is
illuminated. It is the western side of the
Moons face that is illuminated and the Moon is
visible in the sky before Sunset. Waxing means
increasing.
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First Quarter
When exactly ½ of the Moons face is illuminated.
It is the western side of the Moons face that
is illuminated and the Moon is up before sunset.
Although ½ of the Moons face is illuminated it
is called a quarter because the Moon is ¼ of the
way through its cycle and occurs about one week
after the new phase. The Moon is 90 degrees away
from the Sun in the sky.
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Waxing Gibbous
When more than ½ of the Moons western face is
illuminated.
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Full Moon
when the Moons face is fully illuminated. At
this time the Moon is directly opposite the Sun.
Occurs about 2 weeks after the new phase.
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Waning Gibbous
when more than ½ of the Moons face is
illuminated. But now it is the eastern face that
is illuminated and waning means decreasing.
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Last Quarter
when ½ of the Moons face is illuminated. Again
it is the eastern half that is illuminated and
the Moon is visible after sunrise. Occurs about
3 weeks after the new phase.
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Waning Crescent
When less than ½ of the eastern half of the
Moons face is illuminated.
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Discussion
If the Moon is in its full phase on March 21st
and the Sun sets at 600 P.M. what time does the
Moon rise? What time does the Moon reach the
meridian? What time does the Moon set?
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Discussion
If the Moon is in its last quarter phase on March
21st and the Sun sets at 600 P.M. what time does
the Moon rise? What time does the Moon reach the
meridian? What time does the Moon set?
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Discussion
Suppose it is the first day of autumn in the
northern hemisphere. What phase is the Moon if
the Moon is located at the position of the vernal
equinox?
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Discussion
Suppose it is the first day of autumn in the
northern hemisphere. What phase is the Moon if
the Moon is located at the position of the winter
solstice? Explain.
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Discussion
The Moon goes through its phases in 29.53 days.
This is called the synodic month, i.e. the month
relative to the Sun. The actual orbital period
of the Moon is 27.32 days, the sidereal month.
Why is there a difference in the synodic and
sidereal months?
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Discussion
The Moon always keeps the same side facing the
Earth. Does this mean the Moon does not spin on
its axis? Explain?
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Synchronous Rotation
If the Moon did not spin on its axis we would see
the Moons entire surface over the month. The
Moon therefore does rotate but the rotation
period is exactly equal to its orbital period and
rotates such that we always see the same face.
Thus, the Moons rotation period is synchronous
with its orbital period.
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Synodic and sidereal months
The synodic month, the month relative to the Sun,
the time the Moon takes to go through its phases
is 29.53 days. The sidereal month, the actual
orbital period of the Moon is 27.32 days.
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Thus the lunar day is equal to 29.5 Earth days.
For observers on the Moon the Sun would rise and
be in the sky for 14.25 days, followed by 14.25
days of night. Note that this is the Moons
mean solar day. Its sidereal day is its actual
rotation (and revolution) period of 27.32 days.
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Eclipses
If the Sun, Earth and the Moon line up so that
the Moons shadow falls on the Earth or the
Earths shadow falls on the Moon, we see and
eclipse.
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Solar eclipse
Earth passes through the Moons shadow.
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Lunar eclipse
Moon passes through Earths shadow.
The eclipse is named for the object that should
normally be visible to Earth bound observers, but
isnt.
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Discussion
What phase must the Moon be in for a solar
eclipse to occur? What phase must the Moon be
in for a lunar eclipse to occur?
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The Moons orbit
The Moons orbit is tilted 5 degrees relative to
the ecliptic. Thus the Moon spends ½ of its
orbit above the ecliptic and ½ below. Because
the ecliptic is the path of the Sun in the sky,
eclipses can only occur when the Moon lies in the
ecliptic. Hence, the name ecliptic.
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Discussion
From a latitude of 42 degrees, how high above the
horizon can the Moon get in the sky?
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Discussion
What time of the year can the full Moon be its
highest above the horizon? Explain.
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Discussion
The Moon crosses the ecliptic twice each orbit or
twice a month. Why dont we get an eclipse twice
a month?
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Types of Solar eclipses
Total the Moon completely covers the surface of
the Sun.   Partial the Moon covers only part
of the Sun, i.e. the Moon and Sun are not
perfectly aligned.
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The parts of a shadow
Because the Sun has a finite size, the Moon and
Earth cast a shadow with two parts, called the
penumbra and the umbra. The penumbra is the
outer, lighter part of the shadow and corresponds
to the Suns surface being only partially
covered. The umbra is the central, darkest part
of the shadow and corresponds to the entire disk
of the Sun being covered.
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Types of Solar eclipses
Annular the Moon and Sun are perfectly aligned
but the Moon is furthest away from Earth in its
orbit and appears too small to completely cover
the Sun, leaving the rim of the Sun exposed.
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Types of Lunar Eclipses
Penumbral the Moon passes through the lighter
part of Earths shadow. Thus in a penumbral
eclipse the Moon only passes through the lighter
outer part of the Earths shadow and observers on
the Moon would only view a partial solar eclipse.

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Types of Lunar Eclipses
Partial only part of the Moon passes through
the umbra of Earths shadow. Thus some observers
on the Moon would see a total solar eclipse,
those passing through the umbra, while others
would see only a partial solar eclipse.
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Types of Lunar Eclipses
Total the Moon passes completely through the
umbra of Earths shadow.
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Discussion
During a total lunar eclipse the Moon does not
get completely dark. Instead it glows an
orangish-red color. Why?
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Total Lunar eclipse July 1982
Why do you think the bottom of the Moon is
lighter than the top?
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Total Lunar eclipse of July 16, 2000
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The motions of the Planets
Each planet follows the same diurnal motion as
the Sun, Moon and stars, rising in the east and
setting in the west each day. Like the Sun and
Moon, each planet moves west to east with respect
to the stars. This is called direct motion. In
addition, all the planets stay close to the
ecliptic.
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Retrograde Motion
Occasionally each of the planets will also move
east to west with respect to the stars for a
short time. This is called retrograde motion,
motion opposite that of direct motion.
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Path of Mars
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Two types of planets
Superior planets Mars, Jupiter, and
Saturn Superior planets can appear in the sky at
any time of the night. Inferior planets
Mercury and Venus Inferior planets always stay
close to the Sun.