Astronomy 161: An Introduction to Solar System Astronomy Prof. Richard Pogge, MTWThF 2:30

Lecture 9: Eclipses of the Sun & Moon

Key Ideas:

• Moon passes through the Earth's shadow
• Total, Partial, & Penumbral lunar eclipses

Solar Eclipses

• Earth passes through the Moon's shadow
• Total, Partial, & Annular solar eclipses

The Eclipse Year

• How often do eclipses occur?

Umbra and Penumbra

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

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:

• About 3.7x the mean Earth-Moon distance.
• Umbra's width is 9000 km at the distance of the Moon, or ~2.6x the Moon's diameter.

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

Total Lunar Eclipse:

• Entire Moon is within the Earth's umbra.
• Can spend up to 1^h 40^m 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.

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 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:

• Just long enough for the tip to touch the Earth.
• But not large enough to cover the entire Earth.

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

Types of Solar Eclipses

Total Solar Eclipse:

• The observer is inside the Moon's umbra.
• The Moon completely covers the Sun.

Partial Solar Eclipse:

• The observer is inside the Moon's penumbra.
• Only see part of the Sun covered by the Moon.

Annular Eclipse:

• The Moon is at or near apogee, and so is too small to cover the Sun.
• The Moon's umbra does not touch the Earth, so observer's in the shadow path see the Sun as a ring ("annulus").

Total Solar Eclipses

• The Moon's umbral shadow is at most 267 km across on the Earth.
• Totality lasts at most about 7.5 minutes, with the shadow sweeping rapidly west-to-east.
• Only observers in the umbra see a total solar eclipse.
• Observers in the penumbra see a partial solar eclipse.
• Everyone else sees nothing.

Why are eclipses rare?

• A solar eclipse every New Moon
• A lunar eclipse every Full Moon

But, this clearly does not happen. Why?

• The moon's orbit is tilted ~5° from the Ecliptic.
• Where the moon's orbit crosses the Ecliptic defines the "Line of Nodes"

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

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

• a Total Lunar Eclipse every 3 years (or so).
• a Total Solar Eclipse every 360 years.

Upcoming Eclipses

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...