An eclipse, also called an occultation, is an occurrence of one celestial body passing in front of another, from the point of reference of an observer. These can be viewed from a point on Earth or they can be viewed elsewhere; anywhere that an observer is able to get a viewpoint. Some of the most stunning eclipses have in fact, been observed and recorded by the Apollo astronauts and from the Hubble Space Telescope.
Most of the time, in common usage, when someone talks about an eclipse, they are usually referring to either a lunar eclipse (when the Earth passes between the sun and moon and casts a shadow on the surface of the moon) or a solar eclipse (when the moon passes in front of the sun, blocking the sun from view from an observer on Earth or in Earth’s orbit.)
However, any celestial body can be eclipsed, by definition. For instance, Venus can eclipse Saturn or Jupiter (note that Venus is closer to both the Earth and the Sun than either Saturn or Jupiter are, so it is the one that would pass between the Earth and the other two). Stars can also be eclipsed, and in fact this happens far more often than any other type of eclipse.
Though virtually all eclipses can be defined by the above, there are two main kinds of eclipse: The total eclipse and the partial eclipse. In a total eclipse, the observer falls completely within the shadow of the eclipsing body. A total eclipse can only occur if the eclipsing body has the same apparent size (relative to the observer) of the object it is eclipsing; or appears larger than the more distant body.
If the eclipsing object is smaller than the object it is eclipsing, it can never be a full eclipse. This is what makes a full solar eclipse so very interesting. Our moon is gradually moving farther away from the Earth (something on the order of an inch per year). This means that a few million years ago, the Moon was much closer than it is now, so it’s apparent size would be much larger than it is today and full solar eclipses would have been much more common as a result. In a few million years, the Moon will probably be much further away from the Earth, which will mean that it will appear to be smaller than the sun, so a full solar eclipse will not be possible then.
A partial eclipse is when only a portion of the more distant object is actually occulted by the eclipsing body. On Earth, partial eclipses of the sun and moon are much more common than full eclipses. This is because during the eclipses, the full shadow moves in a swath of a few hundred miles in diameter across the Earth as the Earth rotates.
The area just outside the total shadow is in partial shadow, called the penumbra, which is the region of the partial eclipse. The penumbra is far larger than the total shadow or umbra. Outside of the penumbra, no eclipse is visible at all. This is why, each time there is a full eclipse, it is visible from some areas on Earth but not from other places.
Eclipses in science
Eclipses can be very useful for astronomers. For example, only during a full solar eclipse can the sun’s corona or outermost layer, where the solar winds are, be observed without special equipment. In fact, it was from an eclipse of a star by Neptune that scientists first had an indication that Neptune had rings. This was because as the light of the star passed through each of the rings, it would dim slightly, then brighten again when the star was visible between the rings. Scientists quickly determined that the only phenomenon that could explain this would be the existence of rings around Neptune, which was later confirmed by pictures taken by a space probe.
Eclipses in history
Eclipses have been recorded for a very long time; by ancient Chinese, Mayan, and Mid-easterners, but also by others. In many cultures, they were considered a portent of impending doom, as either the sun or the moon appeared to be “swallowed” up and some cultures even made animal and human sacrifices to appease the gods, so that the eclipsed object could once again appear. Obviously, the sacrifices were successful, since that object did reappear. Of course, it would have also done so without the sacrifices too.
Under no circumstances should you observe a solar eclipse through a telescope or binoculars that are not fitted with special light filters. Doing so can result in blindness. There is a way this can be done safely, however. While pointing the binoculars or telescope toward the sun, hold a piece of paper or cardboard a short distance from the eyepiece. A miniature representation of the sun should appear on the paper/cardboard, allowing you to observe the eclipse. This technique is used by some solar observatories.
There are eclipses that can be observed from Earth’s surface every year. For more information on lunar and solar eclipse tables, NASA provides this information free of charge on their web site. Other astronomy sites also provide this information as well.