PHYS 1411 Introduction to Astronomy
Chapter 3
The Cycles of the Moon
Outline
Lunar Eclipses
A. Earth's Shadow
B. Total Lunar Eclipses
C. Partial and Penumbral Lunar Eclipses
Solar Eclipses
A. The Angular Diameter of the Sun and Moon
B. The Moon's Shadow
C. Total Solar Eclipses
Lunar Eclipse
• In cultures all around the world, the Moon
is a regular counter of the passing days
and months
• Sometimes unusual events of the Moon
arise
Predicting Eclipses
A. Conditions for an Eclipse
B. The View From Space
C. The Saros Cycle
Earth’s Shadow
• The shadow cast by a map tack can be
used to understand how shadows produce
eclipses
– Umbra = The region of total shadow
– Penumbra = The region of partial shadow
Types of Lunar Eclipses
• The Moon’s orbit is inclined by a bit more than 5º
relative to Earth’s orbital plane
– Moon does not always pass through center of the
Umbra
• Total lunar eclipse
– All of Moon enters the umbra
• Partial lunar eclipse
– Some part of the moon is in Umbra
– Rest of the Moon enters the penumbra
– Penumbra = The region of partial shadow
• Penumbral lunar eclipse
– Moon only enters the penumbra
– Moon is only partially dimmed
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Lunar Eclipses
A Total Lunar Eclipse (1)
Earth’s
shadow
consists of a
zone of partial
shadow, the
Penumbra,
and a zone of
full shadow,
the Umbra.
If the moon passes through Earth’s full shadow
(Umbra), we see a lunar eclipse.
If the entire surface of the moon enters
the Umbra, the lunar eclipse is total.
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.
The Angular Diameters of the
Sun & Moon
Lunar Eclipses: 2002-2012
Typically, 1 or
2 lunar
eclipses per
year.
Small Angle Formula
angular diameter linear diameter

206,265
distance
Example for Moon
angular diameter
3476 km

206,26 5
384,000 km
angular diameter  187 0 
1870
 0 .5 
60  60
Angular Diameter of Sun and Moon is important for Eclipses to occur
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Solar Eclipses
Solar Eclipses
• For millennia, cultures worldwide have
understood that the Sun is the source of
life
• People explained the terrifying sight of the
Sun gradually disappearing with stories
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.

The Moon’s Shadow: A Total Solar Eclipse
Solar Eclipses: 2002-2012
Approximately 1-2 total solar eclipse per
year
(a) The umbra of the Moon’s shadow sweeps from west to
east across Earth, and observers in the path of totality see a total solar
eclipse. Those outside the umbra but inside the penumbra see a partial
eclipse. (b) Eight photos made by a weather satellite have been combined
to show the Moon’s shadow moving across the eastern Pacific, Mexico,
Central America, and Brazil during an eclipse in 1991.
August 2017 Solar Eclipse in US
Total Solar Eclipse
Chromosphere and Corona
https://eclipse.gsfc.nasa.gov/SEgoogle/SEgoogle2001/SE2017Aug21Tgoogle.html
Prominences
NASA
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Solar Atmosphere Revealed
Earth and Moon’s Orbits Are
Slightly Elliptical
Annular Solar Eclipses
Apogee =
position furthest
away from Earth
Perihelion = position
closest to the sun
Diamond Ring Effect
When Earth is near perihelion, and
the moon is near apogee, we see
an annular solar eclipse.
Earth
Moon
Perigee = position
closest to Earth
Sun
(Eccentricities greatly exaggerated!)
Aphelion
= position
furthest
away
from the
sun
Perigee
Annular Solar Eclipses (2)
Apogee
Perihelion
Aphelion
The angular
sizes of the
moon and the
sun vary,
depending on
their distance
from Earth.
Attendance
Almost total, annular eclipse of May 30, 1984
Campus.kellerisd.net
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Predicting Eclipses
• Astronomers in early civilizations could
make educated guesses as to which full
moons and which new moons might result
in eclipses
• Making exact eclipse predictions requires
a computer and proper software
• Today we can predict precise date and
time of all eclipses
Lunar Node
The moon’s orbit is inclined against the ecliptic by ~ 50.
A points where the lunar orbit intersects the ecliptic is
called a node.
Ecliptic
Wikipedia
Conditions for Eclipses
Eclipses occur only near lunar nodes
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.
Saros (Eclipse) Cycle
• The Eclipse cycle is a
period of 18 years, 11
days and 8 hours
(6585.321 days or 223
lunar months)
• From the first eclipse of
the year the line of
nodes slips westward
19.4 degrees per year.
Causing the eclipse
season to begin 19
days earlier every year.
Conditions for Eclipses
Line of Nodes has to point towards the Sun
Eclipses occur in a cyclic pattern.
Saros (Eclipse) Cycle
• Line of node rotates once every 18.6 years causing the
moon’s orbit to precess.
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Saros (Eclipse) Cycle
Saros Cycle
• After 3 Saros cycles
(54 years 1 month).
The eclipse cycle
occurs in the same
part of the earth
www.astronomy.com
Observing an Eclipse
Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu).
Solar and Lunar Eclipse
https://www.youtube.com/watch?v=kgbK2FZFFdw
https://www.youtube.com/watch?v=is8OLhGgLAE
Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu).
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