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Lecture 3
Phases of the Moon
January 2c, 2014
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Earth’s Moon
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Phases of the Moon
• The Moon orbits the Earth about once
every month.
• The Moon appears to have different phases
because we see more or less of the lighted
side of the Moon depending on where it is
with respect to the Sun.
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Waning
Crescent
New Moon
Waxing
Crescent
Last Quarter
Full Moon
First Quarter
Phases of the Moon
Waxing
Gibbous
Waning
Gibbous
The green circle is the
Earth, looking down on
the north pole.
The Sun is MUCH
further away than
shown here. It is
drawn here to
indicate the direction
from which the Moon
and Earth are
illuminated.
The Moon is depicted in its
various positions during
one month, with an
adjacent image depicting
how it appears from Earth.
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Phases - Photos Animation 1
Animation 2 (try especially the
“both top and Earth” view)
Learning Moon Rise and Set Times
• The Earth’s rotation determines your
horizons
• Your horizons and the Sun determine local
time
• Use the local time and your horizons to
determine rise and set time for any phase
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Rotation of the Moon
• We only see one side of the moon
• Moon rotates at the same rate as it orbits the
Earth, so the period of its spin equals the
period of its orbit.
spin orbitP P
• When spin and orbital periods are correlated
it is called a synchronous orbit
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Rotation of the Moon
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Moon -- Part of Far Side
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Orbit of the Moon – Lunar
Month
• Sidereal Month = amount of time for the moon to
come to same spot with respect to the celestial
sphere. (27.321661 days)
• Solar Month = amount of time for the moon to
come to same position with respect to the sun
(29.53059 days)
– One Lunar phase cycle spans a solar month
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Sidereal and Solar Month
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About how much time does it take for the Moon
to go from its new phase to a waxing crescent?
1 d
ay
4 d
ays
18
days
26
days
0% 0%0%0%
0 of 5
A. 1 day
B. 4 days
C. 18 days
D. 26 days
120
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About how much time does it take for the Moon
to go from its new phase to a waxing crescent?
A. 1 day
B. 4 days
C. 18 days
D. 26 days
th
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The lunar cycle (synodic period)
lasts 29.53days, and the waxing
crescent phase is halfway between
new and first quarter, or about 1/8
of a cycle. The time elapsed is thus
29.53 d 3.69 d 4 days
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How much time does it take for the Moon to move
10° against the background of the fixed stars?
40
min
utes
18
hours
10
days
1.5
month
s
0% 0%0%0%
0 of 5
A. 40 minutes
B. 18 hours
C. 10 days
D. 1.5 months
120
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How much time does it take for the Moon to move
10° against the background of the fixed stars?
A. 40 minutes
B. 18 hours
C. 10 days
D. 1.5 months
The lunar orbit (sidereal period)
spans 27.32 days, so it travels 360°
against the background stars during
that time. The time elapsed is thus
27.32 d 24 h10° 0.759 d 18 h
360° d
10° is about the width o
f your palm viewed
from arm's length.
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Eclipses
• Umbra = darkest shadow of an object.
– Sun completely blocked in umbra
• Penumbra = lighter shadow
– Sun only partially blocked
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Eclipses
• Solar eclipse = when the Earth passes
through the shadow of the Moon
• Lunar eclipse = when the Moon passes
through the shadow of the Earth
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Total Solar Eclipse • Sun completely covered by the moon –
observer is standing in the umbra
View from the Earth
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Partial Solar Eclipse
• Partial Eclipse: Sun is partly blocked by the
Moon – observer is standing in the penumbra
View from the Earth
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Annular Solar Eclipse
• Moon too far from Earth to cover Sun
completely -- umbra does not reach the
Earth
View from the Earth
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Solar Eclipse
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NASA Eclipse Site Zoom into this image
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Solar Eclipse -- August 11, 1999
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Total Lunar Eclipse
• Moon passes through umbra of the Earth.
Sun E
Umbra
Penumbra
Total Eclipse
View from Earth
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Penumbral Eclipse
• Moon only passes through penumbra.
Sun E
Umbra
Penumbra
Total Eclipse
Penumbral Eclipse
View from Earth
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Partial Lunar Eclipse
• Moon partially passes through umbra.
Sun E
Umbra
Penumbra
Total Eclipse
Penumbral Eclipse
Partial Eclipse
View from Earth
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Lunar Eclipse
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Why are there few eclipses? • Orbit of moon is tilted with respect to
Earth’s orbital plane by 5°
• Line of Nodes = position where Moon and
Earth’s orbital planes intersect
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Line of Nodes
• When the moon is on the line of nodes it is
crossing the Earth’s orbital plane
• At other times moon is slightly above or
below the Earth’s orbital plane
• If moon crosses the line of nodes near a new
or full phase an eclipse will occur
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The Moon’s Orbit and Eclipses
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Phases of the Moon in-class activity
