Leaving Manila

The Portuguese ship had scarcely left Manila when it began to lean alarmingly. Its cargo had been badly stowed. Even under its four lower sails, in a steady wind, it heeled so far that the voyage had to be interrupted. On 2 February 1768, Guillaume Le Gentil found himself back at the anchorage of the city he had decided to leave.

Early the following morning, a letter arrived from his friend Don Estevan Roxas y Melo. Surely this was enough to persuade him to stay. Melo had already offered lodging, meals and money; now an Italian missionary came aboard to bring the astronomer and his belongings ashore. Le Gentil proposed a different arrangement. He would watch the cargo being unloaded and restowed. If the new arrangement still looked unsafe, he would ask for a boat.

On 5 February, satisfied that the ship could at least carry him to Malacca, he sailed. Beyond the voyage lay an observation he had been preparing for since leaving France almost eight years earlier. Venus would cross the face of the Sun in June 1769. Le Gentil intended to be in Pondicherry, on the south-eastern coast of India, to watch it leave.

A distance made from timings

The dark dot crossing the Sun offered astronomers a way to measure something much larger than a planet’s silhouette. They knew the relative proportions of the planetary orbits. They needed a reliable scale: how many terrestrial miles or leagues belonged to those proportions? Establish the Earth–Sun distance, and the rest could be expressed in ordinary units.

Two people observing from widely separated places on Earth see Venus against slightly different parts of the solar disc. This apparent shift is parallax. The nearer planet changes position against the more distant Sun as the observer’s viewpoint changes. The size of that shift, combined with a known separation between observing stations and the known orbital proportions, allows the distances to be calculated.

Measuring such a tiny angle directly was difficult. Edmond Halley’s proposal converted the problem into timing. Different apparent tracks across the Sun could produce different transit durations. Observers would use telescopes and well-regulated clocks to record the moments when Venus’s edge touched the Sun’s edge. Comparing observations made far apart would supply the geometry. Halley urged several observers to attempt the work, partly because clouds might deprive any one of them of the event.

There was a demanding practical condition beneath the calculation. A station had to be in a useful place, its position understood, its timekeeping reliable and its view unobstructed at the necessary moment. Getting a telescope to India was only the beginning.

A half-minute of sand

Le Gentil sailed from France in March 1760 under royal orders. By July he had reached Isle de France, now Mauritius, where he found that the war between Britain and France made passage to India difficult. The seasonal winds added another obstruction. A fallback plan to observe from Rodrigues gave way when the frigate Sylphide offered passage towards Pondicherry. Mariners assured him that the ship could make the journey in two months, even in the adverse season.

Contrary winds and calms consumed the schedule. In May 1761, off India’s western coast, they learned that Pondicherry had fallen to the British. The ship turned back towards Mauritius. The transit came on 6 June, with Le Gentil still at sea.

He did see Venus cross the Sun. His voyage account says he observed its entrance and departure as well as he could, while leaving the resulting observation unreduced. Watching from a moving vessel was a very different achievement from supplying the carefully timed observation of a fixed station.

The difficulty produced an ingenious piece of timekeeping. Le Gentil described using the ship’s sandglass, with an assistant repeatedly turning it. Readings of the Sun’s altitude allowed him to relate those turns to astronomical time. The glass was supposed to empty in half a minute. His calculations showed that it took longer; when he later checked it ashore against a seconds pendulum, the supposed thirty seconds proved to be thirty-four and a half.

After this failure, remaining abroad became a decision. Le Gentil wanted to determine island positions and survey Madagascar’s eastern coast. Such work would occupy years. He resolved to stay in the Indian seas until the next transit, using the interval for geography, astronomy, natural history, navigation, winds and tides. He made several journeys to Madagascar rather than going home to prepare another outward passage.

By 1765 he was calculating the 1769 event for possible observing sites. Manila and the Mariana Islands had an attraction beyond their position on the map: Venus would leave the Sun when it stood high in the sky. At Pondicherry, on the Coromandel coast, the Sun would be much lower. A clear view of the critical contact mattered as much as the mathematical usefulness of a station.

The sky and the governor

Le Gentil reached Manila in August 1766. The city was under Spanish rule, and his work depended on local assistance and official permission. A magistrate, Don Manuel Galban, lent him a tower. Roxas y Melo became a close collaborator. Le Gentil had people with whom to work, instruments to use and time to learn the weather.

The weather was persuasive. Looking back over more than ninety mornings from early April to 10 July, he counted scarcely three that had been completely overcast. He admitted that he had no comparable personal knowledge of Pondicherry’s climate.

His account of the governor was much less encouraging. Le Gentil reported that Don José Raón suspected his letters of recommendation were fabricated, believing that replies from Europe could not have arrived so quickly. The accusation reached the astronomer through the father-in-law of the governor’s secretary. It is Le Gentil’s aggrieved version of the dispute, written years later, but it explains the danger he believed he faced: clear weather would not help if an official prevented him from observing.

Advice from France left the decision with him. Jérôme Lalande reported that the astronomer Pingré thought he had gone too far east and would have preferred him nearer Pondicherry. Lalande himself considered either destination acceptable. Le Gentil chose the French settlement, where he expected greater freedom to work.

His friends tried to dissuade him. Alongside Melo’s offers came financial support from Manila’s chief engineer, Don Feliciano Marqués. Before leaving, Le Gentil gave Melo and the Italian Theatine missionary instructions for observing Venus’s departure and regulating their pendulum clock. He renewed a meridian line in the house, substituting marble for its copper surface. Melo possessed two telescopes, each two feet long. One was particularly good.

The observation would still be attempted in Manila. Le Gentil would attempt his own elsewhere.

A telescope arrived from British-held Madras shortly before the transit. Le Gentil already had a long-focus objective prepared, but wanted to try the newer achromatic instrument. Its box came carried by two men and escorted by four sepoys. Law had it returned under escort after the event. The recent enemies could arrange the loan. Neither side could arrange the weather.

At Pondicherry the decisive departure of Venus fell on the local morning of 4 June 1769. The preceding mornings had been beautiful. Then the wind shifted, clouds spread and a violent squall filled the air with sand and dust. The wind weakened, but the clouds stayed. Shortly before seven, a pale brightness suggested where the Sun lay. Through the telescope, Le Gentil could distinguish nothing.

The Sun emerged after the opportunity had passed. He described more than a fortnight of dejection and the difficulty of taking up his pen to report the result to France. The observers at Madras had suffered too: wind and rain had struck their rooftop tent and overturned some of their instruments.

The observation he had left behind

Le Gentil eventually returned to France in 1771. His absence had produced genuine trouble at home. In his account, prospective heirs spread a rumour of his death and tried to obtain his revenues. His financial agent resisted them, supported by correspondence that continued across the years. The Academy had changed his status, while preserving his pension rights and privileges; a ministerial letter restored him to his associate astronomer’s place in February 1772.

His own account does not supply the familiar ending of a wife who had remarried. It places his marriage to Mlle Potier after his return. The journey had enough documented reverses without that addition.

Manila supplied a different ending. Melo sent the observation made there with the Theatine missionary. Le Gentil printed the report in French and Spanish, naming the friend he had left behind and recording the contacts of Venus. Dagelet subsequently calculated the observation, comparing it with results from other stations. Le Gentil thought it deserved consideration in the determination of the Sun’s parallax.

The apparatus Le Gentil had helped prepare in Manila had been used. On the page immediately after his account of struggling to write home, his book gives Melo’s timings: internal contact at nine hours, twenty-five minutes and forty-five seconds; complete departure at nine hours, forty-three minutes and twenty-six seconds.