A celestial transit

Print edition : May 07, 2004

A close look at Venus. -

IT is being billed as the rarest of rare spectacles. On June 8, planet Venus will come in between the Earth and the Sun.

The phenomenon, called Venus Transit, has taken place only six times over the past four centuries - in 1631, 1639, 1761, 1769, 1874 and 1882. It shows a definite pattern of recurrence at intervals of 8, 121.5, 8 and 105.5 years. In the present century, it will occur twice, on June 8, 2004 and on June 6, 2012 after which it will take place only in the year 2117.

India is fortunate in that the entire sequence of the Venus transit on June 8, 2004 will be visible from the country. Europe, parts of Africa, West Asia and most of Asia are the other regions from where the phenomenon can be viewed.

When the Moon comes between the Sun and the Earth, it is called an eclipse; when Venus or Mercury comes between the Sun and the Earth it is called a transit. The discs of Mercury and Venus, as seen from the Earth, are much smaller than that of the Moon. Therefore, unlike the Moon, which practically blots out the Sun, they appear no bigger than small black dots when they move across the face of the Sun. Depending on the geometry involved, the dot may traverse a different path across the face of the Sun during each transit.

According to Dr. V.B. Kamble, acting Director of Vigyan Prasar, the transits of Venus do not take place frequently because its orbit is tilted at small angles to the ecliptic and is usually either above or below the ecliptic. A transit will occur if the inferior conjunction occurs within a day or two of the date on which the planet crosses the ecliptic. The transit of Mercury occurs 13 to 14 times in a century and the transit of Venus is a rarer event.

The transits are significant because they can be used to measure the distance of the Sun from the Earth. Although the determination of the actual scale of the astronomical world dates back to the 6th century B.C., it was Edmond Halley who announced in 1691 that by observing the transit of Venus the distance of the Sun from the Earth can be determined. In 1716, he published a paper outlining a practical method of determining the dimensions of the solar system during Venus transit. However, he could not execute the plan as there was no Venus transit during his lifetime. His efforts inspired scientists to organise expeditions to measure the distance between the Sun and the Earth during transits of Venus in the 18th and the 19th centuries.

For Vigyan Prasar, there can be no better occasion to explain to the general public the basic scientific aspects related to astronomical phenomena. It has drawn up a list of activities involving students, teachers and the general public, in order to enable them to view the Venus transit through telescopes and measure the distance between the Sun and the Earth using simple geometrical instruments. Vigyan Prasar has planned a series of articles on the Venus transit in its monthly newsletter, Dream 2047.

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