Lecture 9: Eclipses of the Sun & Moon

Key Ideas:

Lunar Eclipses

Solar Eclipses

The Eclipse Year

Umbra and Penumbra

Because the Sun appears as a disk ~1/2° across, Sun shadows are fuzzy rather than sharp.

This means shadows cast by the Earth & Moon are two-part shadows:

(Click on the image to view at full scale [Size: 10Kb])

Umbra: Inner core of total darkness
The disc of the Sun is completely blocked.
Penumbra: Outer, partial shadow
Sun's disc is only partly blocked, with a bit peeking over the edge.

Lunar Eclipses

Lunar Eclipses occur when the Moon passes through the shadow of the Earth.

They only occur during Full Moon when the Earth is between the Moon and the Sun.

The Earth's umbra is ~1.4 Million km long:

The Earth's umbra is not totally dark because of light scattered by the Earth's transparent atmosphere. This gives the fully eclipsed Moon a slightly ruddy appearance (think about how the Sun looks reddish at sunset or sunrise).

Three Types of Lunar Eclipses

(Click on the image to view at full scale [Size: 10Kb])

Total Lunar Eclipse:

Partial Lunar Eclipse:

Penumbral Eclipse:

Because the Moon can be completely immersed in the Earth's umbra during a total lunar eclipse, these eclipses can be seen from the entire night-time hemisphere. This is in contrast to total solar eclipses as we'll see below.

Solar Eclipses

Solar Eclipses occur when the Earth passes through the shadow of the Moon.

Solar Eclipses only occur during New Moon, when the Moon is between the Earth and the Sun.

The Moon's umbra is only 380,000 km long:

Solar Eclipses can be seen only where the shadow passes overhead.

Types of Solar Eclipses

(Click on the image to view at full scale [Size: 10Kb])

Total Solar Eclipse:

Partial Solar Eclipse:

Annular Eclipse:

Total Solar Eclipses

Total Solar Eclipses are localized and short: While we often sketch the penumbra as uniform, in reality the penumbra shades gradually from the completely dark umbra out towards the edges. The reason is simple: as you move outwards away from the edge of the umbra, you will see an increasing fraction of the Sun peeking out from behind the Moon. There is a very nice Mir image of the 1999 Aug 11 eclipse shadow showing what I mean.

Why are eclipses rare?

If the Moon's orbit were exactly aligned with the Ecliptic, we would see

But, this clearly does not happen. Why?

Eclipses only occur when the line of nodes and the Sun line up during Full Moon or New Moon.

(Click on the image to view at full scale [Size: 9Kb])

Eclipse Year

The Line of Nodes align with the Sun every 346.6 days. This is called the "Eclipse Year".

But, it must be a Full or New Moon when the nodes line up to have an eclipse. This happens only very rarely.

From a given location on the Earth you see

Upcoming Eclipses

Updated: 2010 June 3
Next Total Lunar Eclipse:
2010 December 21, visible from all of North America

Next Total Solar Eclipse:
2010 July 11, visible from the south pacific (barely touches South America)

Next Total Solar Eclipse near Columbus:
2017 August 21, totality crosses through southern Illinois and western Kentucky (Google Maps view).
Note that I define "near" to be within a day's drive of Columbus, OH.
The next Total Solar Eclipse visible from Columbus proper will be on 2099 Sept 14 (Google Maps view). Eclipse duration will be about 4 minutes in Columbus. Tell your great-grandkids...
A great source of information on upcoming eclipses (and eclipses in general) is Fred Espenak's Eclipse Home Page at the NASA Goddard Space Flight Center.
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Updated: 2010 June 3
Copyright Richard W. Pogge, All Rights Reserved.