Sunday, February 17, 2008

Eclipse!

This Wednesday night and Thursday morning, Earth will experience a total eclipse of the moon, beginning at 10:24 PM (EST). If you happen to live in a part of the world that's dark at this time, and happen to find yourself out of doors, please look up in the sky and check it out.

An eclipse is the complete or partial blocking of one celestial body by another. Although other bodies in the Solar System can eclipse each other, from Earth's perspective, eclipses occur when the Sun, Moon, and Earth are all positioned in a straight line.
4000 years ago, the Babylonians were able to predict the apparent motions of the Moon, stars, planets, and the Sun, and could eventually predict eclipses. 

Systematic observation of the heavens began in China around 3000 BC, where they were extremely sophisticated in their ability to predict eclipses.
Eclipse superstition goes back to the earliest days of sky-watching. Since these events were so important in the Chinese world, they began to chart eclipses, and soon discovered that a cycle could be used to predict them. In one possibly mythical story, two court astronomers failed to predict an eclipse in 2136 BC, and were executed for it.

Greeks borrowed extensively from the Babylonian knowledge of the heavens. When they introduced science into their model, they moved cosmology away from superstition and religion.

A solar eclipse occurs when the Moon passes between the Sun and the Earth, temporarily blocking out the Sun's light and darkening all or part of the Sun. Although the average eclipse period is two and a half minutes, the period of darkness may last as long as seven minutes.
The reason most of the human race will never see a solar eclipse is the extremely limited area in which such an eclipse is visible. The exact opposite is true of eclipses of the Moon. They actually occur less frequently than total eclipses of the Sun, but when a total lunar eclipse does occur, it is visible from slightly more than half of the Earth.

During most months, the tilt of the Moon's orbit ensures that the Full Moon passes north or south of Earth's shadow. At roughly six-month intervals, the Full Moon lies inside the shadowy cone of darkness that extends far into space from Earth.
It is during these "eclipse seasons" that lunar eclipses take place. Observers often find lunar eclipses more enjoyable than solar ones, since they can last for hours instead of minutes.

The spookiest thing about a lunar eclipse is the eerie reddish color that covers the moon. If Earth had no atmosphere, lunar eclipses would simply turn the shadowed parts of the Moon black. Since Earth's air acts like a lens or a prism, it bends part of the Sun's light into the shadow and stains this light a deep copper color.
This effect differs from one lunar eclipse to the next, depending on the weather and clouds in the Earth's atmosphere. The dark part of the Earth's shadow, or umbra, takes about two hours to cover the moon. The total part of the lunar eclipse may last for an hour or so.

If you miss this one, the next total lunar eclipse visible from the Americas won't occur until December 21, 2010. Mark your calendars so you don't forget.

There's a few outstanding books on astronomy, covering eclipses and a variety of other subjects. Two of my favorites are Astronomy: A Visual Guide, by Mark A. Garlick; and Don't Know Much About the Universe, by Kenneth C. Davis.

1 comment:

steve said...

I watched the whole thing clear as day - well, night - here in Boston. Amazing sight!