Glossary of Astronomical Terms/eclipse
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An eclipse occurs when one celestial body crosses the line between the Sun and another celestial body in such as way as to cause a shadow to appear on the other celestial body.
In the case of the Earth-Moon system, this happens in two ways:
- When the moon comes between the sun and the earth (a solar eclipse)
- When the earth comes between the sun and the moon (a lunar eclipse)
In a solar eclipse, the Moon passes between the Sun and the Earth. Because the apparent width of the Moon is approximately the same as the apparent width of the Sun, the shadow is a small area on Earth. This illustration is not to scale.
In a lunar eclipse, the Earth passes between the Sun and the Moon. Note that the Earth's shadow is much bigger than the Moon's, so the chance of a lunar eclipse is higher. This illustration is not to scale.
A solar eclipse is a relatively rarer event than a lunar eclipse, in part because the alignment of the Sun, Moon, and Earth much be in a straight line. If the Moon's orbit was in the same plane as the ecliptic, the plane formed by the Earth's orbit around the Sun, then solar eclipses would occur every new moon. However, because the Moon's orbit is inclined, a series of eclipses occurs roughly every 19 years.
Solar eclipses can be partial, where the Moon's disk only covers part of the Sun's disk, or total, where the Moon's disk covers the entire disk of the Sun. In the same way, lunar eclipses can be partial or total.
An eclipse can occur only when the apparent disk of the celestial body crossing between the Sun and the observer has a bigger diameter than the Sun's apparent disk. If the apparent disk is smaller than the Sun's disk, the phenomena is called a transit. From Earth, one can observe transits of Venus and Mercury. The last transit of Venus occurred on June 8, 2004; the next one will occur in December 2013.
In the case of solar eclipses, the Moon's diameter is 400 times smaller than the Sun, but it is 400 times closer than the Sun. So the Moon's disk just covers the Sun's disk.
