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Chapter 3
Cycles of the Moon
Guidepost
In the preceding chapter, we saw how the sun dominates
our sky and determines the seasons. The moon is not as
bright as the sun, but the moon passes through dramatic
phases and occasionally participates in eclipses. The sun
dominates the daytime sky, but the moon rules the night.
As we try to understand the appearance and motions of
the moon in the sky, we discover that what we see is a
product of light and shadow. To understand the
appearance of the universe, we must understand light.
Later chapters will show that much of astronomy hinges
on the behavior of light.
In the next chapter, we will see how Renaissance
astronomers found a new way to describe the
appearance of the sky and the motions of the sun, moon,
and planets.
Outline
I. The Changeable Moon
A. The Motion of the Moon
B. The Cycle of Phases
II. Lunar Eclipses
A. Earth's Shadow
B. Total Lunar Eclipses
C. Partial and Penumbral Lunar Eclipses
Outline (continued)
III. Solar Eclipses
A. The Angular Diameter of the Sun and Moon
B. The Moon's Shadow
C. Total Solar Eclipses
IV. Predicting Eclipses
A. Conditions for an Eclipse
B. The View From Space
C. The Saros Cycle
The Phases of the Moon (1)
• The Moon orbits Earth in a sidereal period of
27.32 days.
27.32 days
Moon
Earth
Fixed direction in space
The Phases of the Moon (2)
Fixed direction in
space
29.53 days
Earth
Moon
Earth orbits around Sun =>
Direction toward Sun
changes!
• The Moon’s synodic
period (to reach the
same position relative to
the sun, i.e., the same
lunar phase) is 29.53
days (~ 1 month).
The Phases of the Moon (3)
From Earth, we
see different
portions of the
Moon’s surface
lit by the sun,
causing the
phases of the
Moon.
The Phases of the Moon (4)
Evening Sky
New Moon  First Quarter  Full Moon
The Phases of the Moon (5)
Morning Sky
Full Moon  Third Quarter  New Moon
The Tidally-Locked Orbit of the Moon
The Earth also
exerts tidal forces
on the moon’s rocky
interior.

It is rotating with
the same period
around its axis as it
is orbiting Earth
(tidally locked).

We always see
the same side of the
moon facing Earth.
Lunar Eclipses (1)
If a light source is extended (like a large light
bulb), any object casts a shadow that consists
of a zone of partial shadow, the Penumbra,
and a zone of full shadow, the Umbra.
Lunar Eclipses (2)
Also Earth’s
shadow
consists of a
zone of partial
shadow, the
Penumbra,
and a zone of
full shadow,
the Umbra.
If the entire surface of the moon enters
the Umbra, the lunar eclipse is total.
If the moon
passes through
Earth’s full
shadow
(Umbra), we
see a lunar
eclipse.
A Total Lunar Eclipse (1)
A Total Lunar Eclipse (2)
A total lunar
eclipse can
last up to 1
hour and 40
min.
During a total
eclipse, the
moon has a
faint, red
glow,
reflecting sun
light scattered
in Earth’s
atmosphere.
Lunar Eclipses: 2006-2012
Typically, 1 or
2 lunar
eclipses per
year.
Solar Eclipses
The sun appears approx. as large in the sky
(same angular diameter ~ 0.50) as the moon.

When the moon passes in front of the sun, the moon can
cover the sun completely, causing a total solar eclipse.
Solar Eclipses: 2006-2016
Approximately 1 total solar eclipse per year
Total Solar Eclipse
Chromosphere and Corona
Prominences
Diamond Ring Effect
Earth and Moon’s Orbits
Are Slightly Elliptical
Apogee =
position furthest
away from Earth
Perihelion = position
closest to the sun
Earth
Perigee = position
closest to Earth
Moon
Sun
(Eccentricities greatly exaggerated!)
Aphelion
= position
furthest
away
from the
sun
Annular Solar Eclipses
When Earth is near perihelion, and
the moon is near apogee, we see
an annular solar eclipse.
Perigee
Apogee
Perihelion
Aphelion
The angular
sizes of the
moon and the
sun vary,
depending on
their distance
from Earth.
Annular Solar Eclipses (2)
Almost total, annular eclipse of May 30, 1984
Conditions for Eclipses (1)
The moon’s orbit is inclined against the ecliptic by ~ 50.
A solar eclipse can only
occur if the moon passes a
node near new moon.
A lunar eclipse can only
occur if the moon passes a
node near full moon.
Conditions for Eclipses (2)
Eclipses occur in a cyclic pattern.

Saros cycle: 18 years, 11 days, 8 hours
The Saros Cycle