Eclipses and the Lunar Cycle
Learning Objectives
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How much of the Moon’s surface is illuminated by the
Sun at any time? Why do we see Moon phases? What
are waxing, waning, gibbous, crescent, 1st/3rd quarter?
Does the Moon have a dark side? A far side?
Can you draw the geometry of lunar and solar eclipses?
When would we see annular, penumbral, partial and
total eclipses? Why don’t eclipses occur every month?
Can you draw a diagram that shows at what time the
different Moon phases rise, cross meridian and set?
Note the [xtra] beside lectures 1-8 on the schedule.
These may be helpful to learn difficult concepts
The Cycle of Phases
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As the Moon
orbits the Earth,
we see a cycle
of phases
The Moon’s
phases depend
on how much of
the sun-lit
Moon we see
Not caused by
the shadow of
the Earth
(Not to scale)
Lunar Phases - Terms
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Waxing: Becoming Fuller
Waning: Becoming Newer (less Full)
Crescent: croissant-shaped (less than half lit)
Gibbous: “rounded” (more than half lit)
First Quarter: Moon’s orbit is one-quarter
complete (the right-half of the Moon is lit as
viewed from Earth’s Northern Hemisphere)
Third Quarter: Moon’s orbit is three-quarters
complete (the left-half of the Moon is lit as
viewed from Earth’s Northern Hemisphere)
Waxing crescent
The Face of the Moon
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From Earth, we only see one face of the Moon
Does this mean the Moon doesn’t rotate?
No, the Moon rotates so that the same face is
always pointed towards the Earth
A lunar day equals a lunar orbit
There is no dark side of the Moon!
No rotation
Rotational period=
Orbital period
Basics of Eclipses
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Lunar Eclipse
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When the Moon passes
into the Earth's shadow
 Sun – Earth – Moon
Full Moon
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Solar Eclipse
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When the Earth crosses
the Moon's shadow
 Sun – Moon – Earth
New Moon
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Why don’t eclipses
happen every Full and
New moon?
The Moon’s Orbit is Tilted!
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The Moon’s orbit is tilted to the ecliptic by 5º
The Moon appears to be about ½o in
angular size in the sky, so 5o is ~10 Moon
diameters
The Moon must also be near the ecliptic for
an eclipse to occur
Lunar Eclipses
Umbra?
Penumbra?
Latin for “complete
shadow” and “partial
shadow”
Red Moon
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During a total lunar eclipse, the Moon
turns a blood-red/burnt orange color
Red color caused by sunlight diffused
through the Earth's atmosphere
 Atmosphere scatters blue light more, so
Moon is dimly illuminated in red
Solar Eclipses
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Earth passes
into the Moon’s
shadow
Occur roughly
twice a year
The Moon’s
shadow does
not cover the
entire Earth
A solar eclipse
will only be
visible on part
of the Earth
Solar Eclipses
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Three types
 Partial
– when the
Moon only partially
blocks the Sun
 Total – when the Moon
completely blocks the
Sun
 Annular – when the
Moon appears too
small to completely
block the Sun
More on the Lunar Orbit
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The Moon's
orbit around
the Earth is not a
perfect circle
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Distance from
perigee
the Earth to the
Moon varies by 10%
This makes a noticeable difference
to the Moon’s size in the sky
This causes annular eclipses
apogee
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Important bad drawing
Moon phases, eclipses, what Moon
phase is up at what time of day
Next Time
Kepler’s Laws, or
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