Eclipse of the Sun

Source: Wikipedia, the on-line Encyclopedia

On July 22, there was an eclipse of the Sun. It was total in many parts of central and northern India (such as Benaras and Tawang) and partial in South India (such as Chennai). 
People had been talking about it for days. Many internet sites carried information about where and when to see the eclipse. How does an eclipse happen?
A solar eclipse occurs when the moon passes between the Sun and the Earth so that the Sun is fully or partially covered. This can only happen during a new moon when the Sun and Moon are in conjunction as seen from the Earth.
At least two and up to five solar eclipses can occur each year on Earth, with between zero and two of them being total eclipses. Total solar eclipses are nevertheless rare at any location because during each eclipse totality exists only along a narrow corridor in the relatively tiny area of the Moon's umbra.
There are four types of solar eclipses:
A total eclipse occurs when the Sun is completely obscured by the Moon. The intensely bright disk of the Sun is replaced by the dark silhouette of the Moon, and the much fainter solar corona is visible. During any one eclipse, totality is visible only from at most a narrow track on the surface of the Earth.
An annular eclipse occurs when the Sun and Moon are exactly in line, but the apparent size of the Moon is smaller than that of the Sun. Hence the Sun appears as a very bright ring, or annulus surrounding the outline of the Moon.
A hybrid eclipse (also called annular/total eclipse) transitions between a total and annular eclipse. At some points on the surface of the Earth it is visible as a total eclipse, whereas at others it is annular. Hybrid eclipses are comparatively rare.
A partial eclipse occurs when the Sun and Moon are not exactly in line and the Moon only partially obscures the Sun. This phenomenon can usually be seen from a large part of the Earth outside of the track of an annular or total eclipse. However, some eclipses can only be seen as a partial eclipse, because the umbra never intersects the Earth's surface.
The match between the apparent sizes of the Sun and Moon during a total eclipse is a coincidence. The Sun's distance from the Earth is about 400 times the Moon's distance, and the Sun's diameter is about 400 times the Moon's diameter. Because these ratios are approximately the same, the sizes of the Sun and the Moon as seen from Earth appear to be approximately the same: about 0.5 degree of arc in angular measure.
The Moon's orbit around the Earth is an ellipse, as is the Earth's orbit around the Sun; the apparent sizes of the Sun and Moon therefore vary.
An eclipse when the Moon is near its closest distance from the Earth (i.e., near its perigee can be a total eclipse because the Moon will appear to be large enough to cover completely the Sun's bright disk, or photosphere. 
Conversely, an eclipse when the Moon is near its farthest distance from the Earth (i.e., near its apogee) can only be an annular eclipse because the Moon will appear to be slightly smaller than the Sun. Slightly more solar eclipses are annular than total because, on average, the Moon lies too far from Earth to cover the Sun completely.
The Earth's orbit around the Sun is also elliptical, so the Earth's distance from the Sun varies throughout the year. This also affects the apparent sizes of the Sun and Moon, but not so much as the Moon's varying distance from the Earth. When the Earth approaches its farthest distance from the Sun (the aphelion) in July, this tends to favor a total eclipse, as was seen recently.
As the Earth approaches its closest distance from the Sun (the perihelion) in January, this tends to favor an annular eclipse. Such an annular eclipse will next be seen in South India in the afternoon of January 15, 2010. It will last nearly 10 minutes and will be maximum near Rameshwaram and Nagercoil in Tamil Nadu. Madurai and Chennai will see partial eclipses.
Some names
First Contact - when the moons shadow first becomes visible on the solar disk.
Second Contact - starting with Baily's Beads (caused by light shining through valleys on the moon's surface) and the Diamond Ring. Almost the entire disk is covered.
Totality - with the shadow of the moon obscuring the entire disk of the sun and only the corona visible.
Third Contact - when the first bright light becomes visible and the shadow is moving away from the sun. Again a Diamond Ring may be observed.
Final totality
Many millions of years in the past, the Moon was too close to the Earth to precisely occult the Sun as it does during eclipses today.
Due to tidal acceleration the orbit of the Moon around the Earth becomes approximately 3.8 cm more distant each year. It is estimated that in 600 million years, the moon will be too far away to allow a total eclipse: only annular eclipses will be seen.