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ASTRONOMY 161
Introduction to Solar System Astronomy
Class 4
Moon Phases & Eclipses
Wednesday, January 10
Astronomical “movies”
Lunation (Phases of the Moon)
http://antwrp.gsfc.nasa.gov/apod/ap051113.html
Shadow of the Earth (Lunar eclipse)
http://antwrp.gsfc.nasa.gov/apod/image/0311/112003lunareclipse_koehn.gif
Shadow of the Moon (Solar eclipse)
http://antwrp.gsfc.nasa.gov/apod/image/9803/moonshad_goes_big.gif
Moon Phases & Eclipses: Key Concepts
(1) Lunar phases change as we see more or less of the
Moon’s sunlit half.
(2) The Moon rotates about its axis as it revolves around the
Earth.
(3) The sidereal month=27.3 days; the synodic month=29.5
days.
(4) A lunar eclipse occurs when the Moon passes through
the Earth’s shadow.
(5) A solar eclipse occurs when the Earth passes through the
Moon’s shadow.
(1) The Moon’s phases change as we see more
or less of the Moon’s sunlit half.
Full
Waning Gibbous
Waxing Gibbous
Last Quarter
First Quarter
Waning Crescent
Waxing Crescent
New
The Moon is a sphere illuminated by the Sun.
Half closest to Sun is light.
Half furthest from Sun is dark.
Gibbous and crescent shapes
result from perspective.
Example: Crescent Moon
We see Moon in almost the
same direction as Sun.
We see only a sliver of
Moon’s sunlit side.
We see crescent Moon
close to Sun in sky.
“Horns” of crescent point
away from Sun.
(2) The Moon rotates about its axis as it
revolves around the Earth.
On Earth, we always see the same side of the Moon
(“near side”).
The “far side” is always turned away from us.
No rotation of the Moon:
Observer on distant
star always sees
same side.
Observer on Earth
sees all sides.
One rotation per revolution:
Observer on distant
star sees all sides.
Observer on Earth
always sees same
side.
(3a) The length of the
SIDEREAL month = 27.3 days
Sidereal month = time for one revolution of Moon
around Earth.
The view from Earth:
Early afternoon of Thursday, Sept 7, 2006:
full Moon in Pisces.
Late evening of Wednesday, Oct 4, 2006:
Moon in Pisces again (but now it is gibbous).
(3b) The length of the
SYNODIC month = 29.5 days
Synodic month = time between one full Moon and
the next (or one new Moon and the next).
The view from Earth:
Early afternoon of Thursday, Sept 7, 2006:
full Moon in Pisces.
Late evening of Friday, Oct 6, 2006:
full Moon again (but now it is in Aries).
Why is SYNODIC month longer than
SIDEREAL month?
Synodic – measured relative to Sun
Sidereal – measured relative to stars
Sun is a moving target; 2.3 extra days to catch up.
Total Solar Eclipse:
Total Lunar Eclipse:
The shadow of the Earth (or any other object)
has two parts:
Umbra: inner part of shadow, Sun completely hidden.
Penumbra: outer part of shadow, Sun partially hidden.
Earth’s Shadow:
Earth’s umbra stretches 1.4 million kilometers (3.7
times average Earth-Moon distance).
At the Moon’s orbit, Earth’s umbra is 9000
kilometers wide (2.6 times Moon’s diameter).
The Moon can fit inside the Earth’s umbra, with
room to spare.
(4) A lunar eclipse occurs when the Moon
passes through the Earth’s shadow.
Lunar eclipses occur when Earth is between Sun
and Moon.
Lunar eclipses occur at FULL MOON.
Three types of lunar eclipse
(1) Penumbral:
None of Moon enters umbra. Boring.
(2) Total:
All of Moon enters umbra. Totality lasts up to
1 hour 40 minutes.
(3) Partial:
Part of Moon enters umbra.
(5) A solar eclipse occurs when the Earth
passes through the Moon’s shadow.
Moon umbra stretches for 380,000 kilometers.
Minimum Earth-Moon distance =
= 363,000 kilometers.
Maximum Earth-Moon distance =
= 405,000 kilometers.
Solar eclipses occur when Moon
is between Sun and Earth.
Solar eclipses occur at NEW MOON.
Three types of solar eclipse
(1) Total: Observer is in Moon’s umbra; Moon
completely hides Sun.
(2) Annular: Moon umbra falls short of Earth;
Moon is surrounded by a ring of Sun.
(3) Partial: Observer is in penumbra, to one side of
umbra; Moon takes ‘bite’ out of Sun.
Moon’s Shadow:
The Moon’s orbital motion makes its umbra sweep
rapidly over the Earth.
Total solar eclipse is visible from a narrow path (up
to 270 km wide) for a short time (up to 7.5 min).
Total lunar eclipse is visible from half the Earth.
From Columbus, next total lunar: Mar 3, 2007;
next total solar: Sep 14, 2099
Few closing questions:
1) Why are eclipses not frequent? (medium)
2) During a total lunar eclipse, what is seen by an
observer on the surface of the Moon? (easy)
3) For an observer on the Moon, does Earth show
phases? (easy)
4) Is there more solar or lunar eclipses? (hard)