Had it not been for the Anglo-French war and a cloudy sky, astronomical history might have been made in 18th century Pondicherry. But Kodaikanal can lay claim to some part of it.

On June 6, Indians will wake up to a unique astronomical phenomenon: the Transit of Venus. The second planet of our solar system will be seen as a tiny black dot traversing the face of the sun between sunrise and 10.30 a.m. IST

Venus is always visible from Earth — it appears either before sunrise (the morning star) or right after sunset (the evening star). But during transits, Venus, Earth and the sun line up in space, causing Venus to appear as a black dot against the disc of the sun.

Transits of Venus are truly rare. A pair of such events occurs every 105.5 or 121.5 years. The last event occurred in June 2004, and the next will occur 105 years from now, in December 2117.

In the 18th century, one such event almost helped India create history. In March 1760, French astronomer, Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil de la Galaisière or simply Guillaume Le Gentil set sail for Pondicherry on a unique mission: to study the Venus transit of June 6, 1761.

He was part of an international collaborative, headed by Russian astronomer Mikhail Lomonosov, whose ultimate goal was to measure the distance between Earth and the sun. Astronomers of the time thought that this would, in turn, help them gauge the size of the solar system.

Lomonosov and Le Gentil's ambitions rested on the shoulders of several giants. In 1631, Johannes Kepler discovered the shape of the solar system, and predicted that all planets revolved around the sun in the same geometric plane. Using Kepler's calculations, British astronomer Jeremiah Horrocks recorded a Venus transit for the first time in 1639.

Then in the early 18th century, Edmond Halley, discoverer of the famous Halley's Comet, identified that a transit presented the unique opportunity to measure the distance between Earth and the sun.

But Halley died before Venus transited in 1761. That transit was best seen across Asia, and Le Gentil hoped to make his historical contribution from Pondicherry, then a French territory.

At sea

But sadly for Le Gentil, Europe was at war between 1756 and 1763. Collectively called the Seven Years' War, these conflicts between the British and the bourbons of France and Spain; and the Prussians and Austrians, affected not just Europe, but also its colonies. In India, conflicts broke out around the French colonies in Chandranagore in West Bengal, and Pondicherry, in the south.

The war in the south is better known as the Third Carnatic War. Irish Commander Sir Eyre Coote took charge of the British forces in Madras, and led his army to victory in the Battle of Wandiwash in January 1760. The French army under Comte de Lally retreated to Pondicherry. Sir Coote besieged the settlement, and refused to relent until de Lally surrendered in January 1761, a few months before the transit.

Le Gentil was already well on his way to India when Pondicherry fell, and in spite of his many attempts to make it to the Coromandel coast, he was forced to retreat to Mauritius. On June 6, 1761, when Venus transited, Le Gentil was at sea, and his rocking vessel confounded his observations though the skies were clear. Incidentally, Lomonosov's observations were marred by optical illusions and the astronomical unit remained elusive.

Missed again

But Venus was set to transit again in June 1769, and Le Gentil waited in Madagascar for the war to come to an end. In 1763, the British and the French signed the treaty of Paris, and the French regained control of Pondicherry. Le Gentil arrived at Pondicherry in 1768, and waited eagerly for the event. But as his misfortune would have it, cloudy skies blanketed the southern coast of India, destroying any chance of timing the entire event.

In 1769, however, Captain James Cook of Britain successfully moored in Tahiti to record the event from start to finish. His observations helped European astronomers calculate the astronomical unit, and subsequently the size of the entire solar system. The more advanced techniques of today, like radar, show that their calculations were a little off: the solar system is 1.008 times smaller than their assumptions. But Cook's contributions, which could have been Le Gentil's, have a unique place in astronomical history.

Cook went on to discover Australia and New Zealand and was a much-celebrated hero in Britain. By a strange turn of destiny, William Petrie, a civil servant for the East India Company, brought one of Cook's clocks to the Madras Observatory in the late 18th century. This instrument which helped create history now stands — and still ticks — at the Kodaikanal Solar Observatory.

(Shweta Krishnan is a science writer, and has contributed articles to Sky and Telescope Magazine, Boston.)

Keywords: Venus transit

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