Yesterday’s birthday boy has got to be a serious candidate for the most unfortunate astronomer of all times. Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil de la Galaisière (September 12, 1725 – October 22, 1792) was a French astronomer whose life as a scientist started auspiciously in that he was personally trained in his profession by the Cassini’s at the observatory in Paris. In order to understand when and why it all turned pear shaped we have to go back to the 16th and 17th centuries.
One of the consequences of the new cosmological models proffered by Copernicus in 1543 (heliocentrism) and Tycho in 1588 (geo-heliocentrism) was that the inner planets, Mercury and Venus, would or should cross, or as the astronomers say transit, the face of the sun. Now this does not occur by every orbit as the orbits of the planets are tilted slightly with respect to the apparent path of the sun around the earth and such transits are only visible from the earth when the planet, earth and sun are in the correct respective positions. For Venus this currently occurs in a pattern that repeats every 243 years, with pairs of transits eight years apart separated by long gaps of 121.5 years and 105.5 years.
Kepler the great astronomical calculator of the Early Modern period made the first accurate calculation for a transit of Venus for the year 1631 but unfortunately this was not observable from Europe as it took place at night. Even Kepler made mistakes and his calculations missed the next transit in 1639 but the brilliant young English astronomer Jeremiah Horrocks corrected Kepler’s tables and correctly predicted and observed this transit. Horrocks used one method of parallax based on the respective sizes of the sun and Venus to calculate the distance of the sun from the earth. His figures were wildly inaccurate but he did produce a figure for the distance that was many factors better than all previous estimates.
In 1663 in his Optica Promota James Gregory (1638 – 1675) sketched another method of parallax that could be used to calculate the earth sun distance that involved timing the beginning and end of a transit from widely spaced positions on the earth and then applying trigonometry to the figures obtained. This method was worked out in detail by Edmond Halley (1656 – 1742) who had first considered it whilst observing a transit of Mercury on St Helena in 1677; he published his mature reflection in 1716 suggesting an international effort to carry out the necessary observations. Even the long lived Halley did not live long enough for the next transit in 1761 but the French astronomer Joseph-Nicolas Delise (1688 – 1768) took up the idea and in 1760 expeditions of astronomers equipped with the best available instruments set off to all corners of the globe to observe the forthcoming transit. Among others the first sea trials of John Harrison’s timepiece to determine longitude was taken on the English expedition to Jamaica. Later James Cook would make another trial of a Harrison watch on his expedition to observe the 1769 transit in the South Seas. In 1761 Mason and Dixon, of the famous Line, were supposed to observe the transit from Sumatra but there ship was attacked by the French forcing them to return to England from whence the set out again only reaching South Africa from where they successfully observed the transit.
But what of Le Gentil? He had been chosen by the French Academy of Science to observe the transit from Pondicherry in India. He sailed from France in 1760 and made station on Mauritius, then known as Isle de France. As he was preparing to sail to Pondicherry he learned that it had been attacked and captured by the British so he changed his plans and shipped with a French frigate heading for Coromandel with only three months to go before the transit. However the captain of the ship hearing of the developments of the war with the British turned back to Mauritius forcing Le Gentil to observe the transit from a ship at sea making his observations scientifically worthless.
Back in Mauritius Le Gentil decided to stay in Asia for eight years and to observe the 1769 transit. He occupied his time by undertaking expedition around the Indian Ocean collection natural history data and specimens. In 1766 he decide to make his observations from Manila in the Philippines and set off by ship to his new destination. In the Philippines he changed his mind again and decided to return to Pondicherry. His decision was based on two factors a hostile attitude from the Spanish Governor of the Philippines and a request from the Academy of Science back in France to make his observations in India. After various adventures to sea he arrived in time for the transit in Pondicherry. He was treated with respect by the British authorities who even supplied him with a new high quality telescope and he set up his equipment for the great day. The days leading up to the transit and the days thereafter were perfect for observing however on the day of the transit the heavens were covered by clouds making all attempts at observing impossible. Le Gentil’s disappointment was only deepened as he discovered that the weather in Manila had been perfect for observing on the day.
Tired, ill and demoralised Le Gentil decided to return to France arriving in Mauritius in 1770 too ill to travel further. Finally late in the year he recovered sufficiently to set sail for France only to run into a hurricane at the Cape of Good Hope that drove his ship back to Mauritius where he landed again in 1771. Le Gentil had difficulties finding another ship prepared to take him back to Europe, finally a Spanish captain agreed to take him and after more sea adventures he finally arrived in Cadiz from where he proceeded overland back to France crossing the Pyrenees on October 8th 1771 however his trials and tribulations were long not over.
During his absence Le Gentil had been assumed dead and his heirs and creditors had already divided up his estate and the Academy of Science had awarded his position to somebody else. With the help of the King Le Gentil was able to get his position restored and in fact he married happily and had a daughter to comfort him in his old age. The last disappointment was the loss of his natural history specimens that had disappeared on the ship that had been driven back to Mauritius by the storm in his first attempt to return home. If ever an astronomer was born under a bad sign then it was Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil de la Galaisière.