Eclipse Details: Lunar Eclipse

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Let's explore a little more about lunar and solar eclipses. Remember that an eclipse happens when an object passes through another object's shadow. Any shadow consists of two parts: an umbra which is the region of total shadow and the penumbra which is the outer region of partial shadow. If the Moon were to pass through the Earth's umbra, an observer on the Moon would not be able to see the Sun at all---she would observe a solar eclipse! An observer on the Earth looking at the Moon would see a total lunar eclipse. The Earth's shadow is pretty big compared to the Moon so a total lunar eclipse lasts about 1 hour 45 minutes.

If the Moon only passed through the outer part of the shadow (the penumbra), then the observer on the Moon would see the Sun only partially covered up---a partial solar eclipse. The observer on the Earth would see the Moon only partially dimmed---a partial lunar eclipse.

Select the image to get information about this image taken by Gordon Garradd (will display in another window).

During a total lunar eclipse you see another interesting effect---the Moon turns a coppery (or bloody) red. The reason why some sunlight reaches the Moon despite the fact that the Moon is in the Earth's umbra is that the sunlight refracts or bends as it passes through the Earth's atmosphere. Dust particles in the Earth's atmosphere remove much of the bluer colors in the sunlight so only the redder colors make it to the Moon. The amount of dust determines the deepness of the red colors. The dust in the air is also why the Sun appears redder at sunset on Earth. The observer on the Moon would see a reddish ring around the Earth even at mid-eclipse!

Eclipse Details: Solar Eclipse

Select the image to get information about this image of the July 11, 1991 solar eclipse taken by Fred Espenak (will display in another window).

In a total solar eclipse the bright disk of the Sun is completely covered up by the Moon and you can see the other parts of the Sun like the corona, chromosphere, and prominences. Unfortunately, only the tip of the Moon's umbra reaches the Earth (the tip hitting the Earth is at most 270 kilometers [168 miles] in diameter) and it zips along the Earth's surface at over 1600 kph (1000 mph) as the Moon moves around the rotating Earth. This means that a total solar eclipse can last a maximum of only 7.5 minutes. Usually total solar eclipses last only 3-4 minutes. Because of the orbital motion of the Moon and the rotation of the Earth, the umbra makes a long, narrow path of totality.

Sometimes the umbra does not reach the Earth at all (only the penumbra) even though the Moon is on the ecliptic and it is exactly in New Moon phase. A bright ring will be visible around the Moon when it is lined up with the Sun---an annular eclipse (because of the annulus or ring of light around the Moon).

Eclipse Web Sites

1. NASA's Eclipse Bulletins.
2. Fred Espenak's eclipse site. A less technical version of the NASA site. Fred Espenak is also the main contributer to the NASA site.
3. Sky & Telescope's eclipse site.

Vocabulary

Review Questions

1. What are the two parts of a shadow and in which part is the Sun partially visible?
2. Why does a new phase occur every month but a lunar eclipse occurs only twice a year?
3. Why is the path of totality of a solar eclipse different each time? Why does the latitude of the path vary?
4. Why does the Moon turn orange-red during a total lunar eclipse?
5. Would a person on the Moon ever experience an annular solar eclipse? Explain your answer.
6. Why do we not have just annular solar eclipses or just total solar eclipses when the Moon and Sun are exactly lined up?

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last updated: 28 May 2001