Longest Eclipse of the Century

One of the grandest spectacles of nature – the total solar eclipse occured on July 22, 2009. An eclipse of the Sun takes place when the Moon comes between the Earth and the Sun so that the

Moon’s shadow sweeps over the face of the Earth. This may occur on a new Moon day. This does not happen on every new Moon day due to the inclination of the Moons orbit with respect to earth’s orbit. When the Sun, the  Moon and the Earth are aligned in a straight line in the same plane, then only  an eclipse takes place. It is a remarkable coincidence that the Sun and the Moon appear very nearly of the same angular size from the Earth. However, their apparent sizes depend on their distances from the Earth. This happens because the Earth revolves in an elliptical orbit around the Sun and the Moon too revolves in an elliptical orbit around the Earth. When the Sun is closest to the Earth, and Moon the farthest, the apparent size of the Moon is smaller than that of the Sun. The Moon passing over the Sun’s disc cannot cover it completely, but leave the rim of the Sun visible. Such an eclipse is called annular. In a total solar eclipse the position of the Sun and the Moon is such that their angular size is exactly the same, so Moon completely covers the solar disc. It is an event so rare that most people do not get an opportunity to witness it even once in their lifetime. On an average, a total solar eclipse may occur at a particular place on the Earth only once in about 360 years.

The shadow of the Moon consists of two parts: the umbra, or the total shadow, a cone into which no direct sunlight penetrates; and the penumbra, or partial shadow, which is caused by light reaching from only a part of the Sun’s disc. To an observer within the umbra, the Sun’s disc appears completely covered by the disc of the Moon. For an observer within the penumbra, however, the Moon’s disc appears projected against the Sun’s discs so as to overlap it partly. The eclipse is then partial to the observer. Sometimes the Earth misses the umbra but only intercepts the penumbra. Under such circumstances, only partial solar eclipse is observed anywhere on the Earth. The umbral cone being narrow at the distance of the Earth, the total solar eclipse is observed only over a narrow strip of land or sea over which it passes. The partial solar eclipse, however, can be seen from a large number of places covered by the penumbra. Fortunately the path of totality of the 22 July eclipse  passed through a large numbers of Indian cities and densely populated regions. 

It was also the longest solar eclipse of the century, with totality lasting for 6 minutes and 39 seconds at maximum along its path making it the longest until the year 2132. Under the most favourable conditions, however, the totality can last for a maximum of 7½ minutes.

Total solar eclipse is a grand spectacle. About twenty minutes before totality, there is a sizeable decrease in the intensity of light. Some three minutes before totality the sky darkens considerably. Only a narrow crescent of the Sun can be seen.  About a minute before the totality ripples of dark and bright bands, called ‘shadow bands’, appear to move over a white plain surface such as a building facade. The intensity of light quickly drops in the next few seconds and the real drama begins. The sunlight shining through the valleys of the Moon give the appearance of a beaded necklace. The final flash of light from the Moon’s valleys produces a brilliant flare known as the 'diamond ring’. Soon after the diamond ring vanishes, the chromosphere  of the Sun that lies just above the visible photosphere blazes into view, indicating that the totality has just begun. One could then see red or orange jets of fire shooting to millions of kilometres above the surface of the Sun, called prominences. The Sun is now completely hidden behind the Moon and the magnificent pearly white corona flashes into view. Corona is the Sun’s outer atmosphere consisting of hot sparse gases that extends to millions of kilometers. It is generally quite feeble, its brightness being comparable to that of the Moon and hence not visible ordinarily due to the glare of the photosphere. The shape of the corona varies with the 11-year solar cycle giving it a different look during every total solar eclipse. And, before one realizes what has happened, the entire sequence repeats in the reverse order! During totality, all the stars and planets normally invisible during daytime also come into view. 

A total solar eclipse gives scientists the opportunity to study the atmosphere of the Sun. The corona can be studied in minute details only during such rare events. Indeed, the element Helium was discovered on the surface of the Sun during a total solar eclipse on 17 August 1868 by the French astronomer Janssen ! It was during the total solar eclipse of 29 May 1919 that the General Theory of Relativity of Einstein was first tested by observing the deflection of light coming from a distant star by the gravitational field of the Sun.

It is however advisable never to watch the Sun − eclipsed or uneclipsed − with naked eye. It could permanently damage the retina and the vision. For direct viewing of the partial phase of the eclipse only tested safe solar filters, or the dark arc welder’s glass No. 14 should be used.