BACKGROUND: The most recent total lunar eclipse was on 3 and 4 March 2007. The next one will occur on 28 August 2007.
WHAT IS A LUNAR ECLIPSE? A lunar eclipse occurs whenever the Moon passes through some portion of the Earth's shadow. This can occur only when the Sun, Earth, and Moon are aligned exactly, or very closely so, with the Earth in the middle. Hence, the Moon is always full during a lunar eclipse. The shadow of the Earth can be divided into two distinctive parts: the umbra and penumbra. A penumbral eclipse occurs when the Moon passes through the Earth's penumbra. A special type of penumbral eclipse is a total penumbral eclipse, during which the Moon lies exclusively within the Earth's penumbra. A partial lunar eclipse occurs when only a portion of the Moon enters the umbra. When the Moon travels completely into the Earth's umbra, one observes a total lunar eclipse. A selenelion or selenehelion is a type of lunar eclipse when both the sun and the eclipsed moon can be observed at the same time. This particular arrangement has led to the phenomenon being referred to as a horizontal eclipse. It can only be observed just prior to sunset or just after sunrise. This last occurred on May 16, 2003 over Europe.
RED MOON RISING: The Moon does not completely disappear as it passes through the umbra because of the refraction of sunlight by the Earth's atmosphere into the shadow cone; if the Earth had no atmosphere, the Moon would be completely dark during an eclipse. The red coloring arises because sunlight reaching the Moon must pass through a long and dense layer of the Earth's atmosphere, where it is scattered. Shorter wavelengths are more likely to be scattered by the small particles, and so by the time the light has passed through the atmosphere, the longer wavelengths dominate. This resulting light we perceive as red. This is the same effect that causes sunsets and sunrises to turn the sky a reddish color. The amount of refracted light depends on the amount of dust or clouds in the atmosphere; this also controls how much light is scattered. In general, the dustier the atmosphere, the more that other wavelengths of light will be removed (compared to red light), leaving the resulting light a deeper red color. This causes the resulting coppery-red hue of the Moon to vary from one eclipse to the next.
OBSERVING TIP: Find a place with a clear view of the eastern horizon and station yourself there at sunset. As the sun goes down behind you, a red moon will rise before your eyes. Rising moons are often reddened by clouds or pollution, but this moon will be the deep, extraordinary red only seen during a lunar eclipse.
The American Astronomical Society and the American Meteorological Society contributed to the information contained in the TV portion of this report.
ON THE WEB: http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html