An eclipse occurs whenever the Sun, Earth and Moon line up exactly. If this occurrence is at the time of a full moon where the Moon passes through the Earth's shadow, it is called a lunar eclipse. The type and length of a lunar eclipse depends upon the Moon's location relative to its orbital node.

Types of lunar eclipses

A penumbral eclipse occurs when the Moon only passes through the Earth's penumbra, the outer portion of the Earth's shadow. The penumbra does not cause a noticeable darkening of the Moon's surface.

A total lunar eclipse occurs when the Moon travels completely into the Earth's umbra, the dark inner portion of the shadow. The Moon's speed through the shadow is about one kilometer per second, and the total eclipse may last up to 102 minutes. However, the time between the Moon's first contact with the umbra and last contact, when it has completely exited the umbra, may be several hours. If only part of the Moon enters the umbra, it is called a partial lunar eclipse.

The Moon doesn't completely disappear as it passes through the umbra because of the refraction of sunlight by the Earth's atmosphere. The amount of refracted light depends on the amount of clouds or dust in the atmosphere blocking the light. This causes the Moon to glow with a coppery-red hue that varies from one eclipse to the next. The following scale was devised by Andrè Danjon for rating the overall darkness of lunar eclipses:

0. Very dark eclipse; Moon almost invisible, especially in midtonality

1. Dark eclipse; gray or brownish coloration; details distinguishable only with difficulty

2. Deep red or rust-colored eclipse, with a very dark central part in the umbra and the outer rim of the umbra relatively bright

3. Brick-red eclipse, usually with a bright or yellow rim to the umbra

4. Very bright copper-red or orange eclipse, with a bluish, very bright umbral rim

Because the Moon's orbit around the Earth is inclined 5° with respect to the orbit of the Earth around the Sun, lunar eclipses do not occur at every full moon. For an eclipse to occur, the Moon must be near its orbital node—the intersection of the orbital planes. Passing through the shadow at or very close to the node results in a total or partial eclipse.

Every year there are at least two lunar eclipses. If you know the date and time of an eclipse, you can predict the occurrence of other eclipses using eclipse cycles. Unlike a solar eclipse, which can only be viewed in a certain relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of the Earth. If you were on the Moon's surface during a lunar eclipse you would witness a solar eclipse, with the Earth passing in front of the Sun.

Lunar eclipses in 2003

Two total lunar eclipses occurred in 2003. The eclipse on May 15 grazed the northern edge of the earth's shadow, and the eclipse on November 8 grazed the southern edge. These images show the eclipse in November was much brighter as the bottom rim of the Moon did not darken as much after completely entering the umbra. The color and brightness of the Moon during an eclipse varies according to the amount of light refracted by the Earth's atmosphere.

Expected lunar eclipses 2003-2005

Longest total lunar eclipse between 1900 and 2100

The longest total lunar eclipse between 1000BC and 3000AD took place on May 31st, 318. Its total phase had a duration of 1h47m14s.

History

Ancient Greek astronomers noticed that during lunar eclipses the edge of the shadow was always circular. They thus concluded that the Earth was spherical.

References

• Alan MacRobert, "October's Ideal Lunar Eclipse", Sky and Telescope (October 2004), p. 74. (Danjon numbers)

See also

• eclipse
• solar eclipse

External links

Prediction

• U.S. Navy Lunar Eclipse Computer http://aa.usno.navy.mil/data/docs/LunarEclipse.html
• NASA Eclipse home page http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html
• Lunar Eclipses for Beginners http://www.mreclipse.com/Special/LEprimer.html

Eclipse photos

• APOD 10/30/04 Total Lunar eclpse on 10/28/04 http://antwrp.gsfc.nasa.gov/apod/ap041030.html
• APOD 11/21/03 Total Lunar Eclipse on 5/16/03 http://antwrp.gsfc.nasa.gov/apod/ap031121.html
• APOD 5/22/03 Composite Lunar eclipse images from 11/9/03 http://antwrp.gsfc.nasa.gov/apod/ap030522.html
• APOD 1/18/01 Total Lunar eclipse on 1/9/01 http://antwrp.gsfc.nasa.gov/apod/ap010118.html
• APOD 7/26/00 Total Lunar eclipse photo from 7/16/00 http://antwrp.gsfc.nasa.gov/apod/ap000726.html

Fiction

• Lunar Eclipse 2105 - NASA fictional story of eclipse observed from the moon http://science.nasa.gov/headlines/y2003/04nov_lunareclipse2105.htm