Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 8: Eclipses

Key Ideas

Lunar Eclipses: Earth's shadow falls on the Full Moon; Total, Partial, and Penumbral lunar eclipses
Solar Eclipses: Moon's shadow falls on the Earth; Total, Partial, and Annular solar eclipses
Eclipse Year: How often do eclipses occur?

Umbra and Penumbra

Because the Sun appears as a disk 1/2 degree across, shadows are fuzzy instead of sharp
Umbra: Inner core of total darkness: Sun is completely blocked
Penumbra: Outer, partial shadow: Sun is partially blocked

Lunar Eclipses

Moon passes through the shadow of the Earth: Only occurs during Full Moon
The Earth's umbra is 1.4 million km long: 3.7 times the mean Earth-Moon distance; Umbra's width is 9000 km at the Moon and 2.6 times the Moon's diameter; Umbra is not totally dark because of scattering by Earth's atmosphere

Three Types of Lunar Eclipses

Total Lunar Eclipse: Entire Moon within the Earth's umbra; Can spend up to 1 hour 40 minutes in the umbra; Whole show can last 6 hours
Partial Lunar eclipse: Only part of the Moon enters the umbra
Penumbral Eclipse: Moon misses the umbra completely, only passes through the penumbral shadow

Solar Eclipses

Earth passes through the shadow of the Moon: Only occurs during New Moon
The Moon's umbra is only 380,000 km long: Just long enough to touch the Earth; Not wide enough to cover the Earth
Solar eclipses can be seen only where the shadow passes directly overhead.

Three types of Solar Eclipses

Partial Solar Eclipse: The observer is inside the Moon's penumbra; Only see part of the Sun covered by the Moon
Total Solar Eclipse: The observer is inside the Moon's umbra; The Moon completely covers the Sun
Annular Eclipse: Moon near apogee, too small to cover the Sun; Moon's umbra does not touch the Earth

Total Solar Eclipses

Total Solar Eclipses are localized and short: The Moon's umbra is at most 267 km across on the Earth; Totality lasts at most about 7.5 minutes; Observers in the umbra see a total eclipse
Observers in the penumbra see a partial eclipse
Everyone else sees nothing

Why are Eclipses Rare?

If the Moon's orbit were exactly aligned with the Ecliptic, we would see:: A solar eclipse every New Moon; A lunar eclipse every Full Moon
But, this does not happen. Why?: Moon's orbit is tilted 5 degrees from the Ecliptic; Intersection is the "Line of Nodes"
Eclipses only occur when the Line of Nodes lines up within 0.5 degrees of the Sun at Full or New Moon

Eclipse Year

Line of Nodes align with the Sun every 346.6 days: Called the Eclipse Year
But, it must be Full or New Moon when the nodes line up to have an eclipse
From any given location on the Earth you see: Total Lunar Eclipses every 3 years; Total Solar Eclipses every 360 years

Upcoming Eclipses

Next Total Lunar Eclipse:: 2007 March 3 (last was 2004 Oct 28)
Next Total Solar Eclipse:: 2008 August 1 (Arctic Canada, Greenland, Siberia, Mongolia, China)
Next Total Eclipse near Columbus:: 2024 April 8, totality crosses North and West of Columbus; 2099 Sept 14, next visible from Columbus

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.