[BRIEF NOTE] A quoi ça sert?

[rfmcdonald]

In reading Venus, written by British public figure and astronomer Patrick Moore, I discovered the central importance of the transits of Venus against the face of the Sun in the 18th and 19th centuries. Following the calculations of Edmond Halley, it was discovered that--in theory--one could use the transits of Venus to measure the distance between the Earth and the Sun with a high degree of accuracy. Kepler's laws had allowed for the construction of a complete scale map of Sol's planetary system, that is of the relative distances between planets and their primary, but absolute distances weren't known. Venus, located inside Earth's orbit and with a larger apparent disk than that of Mercury, could provide the perfect target for astronomers. The only provisos were that the observations not only had to be timed precisely, forcing astronomers to determine the exact duration of the transit from the moment it appeared to enter the Sun's disk to the moment it exited, but there had to be paired observations from widely-separated areas so as to provide a sufficiently wide baseline for accurate measurement.

This brings us to the French effort to study the 1761 transit. In Moore's words:

No account of the 1761 transit would be complete without relating the sad story of the French astronomer Legentil--or, to give him his full name, Guillaume Joseph Hyacinthe Jean Baptiste Le Gentil de la Galasière, whose expedition must have been the unluckiest in the whole history of science. In March 1760 he set sail for Pondicherry in India, where conditions for the transit were expected to be very good. The first part of his journey, to the Isle de France [Mauritius], was reasonably uneventful apart from a near-encounter with British warships off the Cape of Good Hope. Then, to his consternation, he learned that Pondicherry had fallen to the British, and all Legentil could do was to head back to the Isle de France. He had a perfect view of the transit--but from the deck of a ship, and could make no useful observations of any kind.

What next? Rather than return home empty-handed, he elected to remain in the East until 1769, and observe the second transit instead. He was not idle, and in fact he made agreat many varied and valuable scientific observations, but the transit was always uppermost in his mind, and he gave careful consideration to the choice of site. He finally decided upon Manila, in the Philippines, and arrived there in August 1766, which certainly gave him plenty of time to prepare.

Then, in July 1767, Legentil received a letter from France. The French authorities were very anxious for him to go back to Pondicherry rather than stay in the Philippines. Finally Legentil agreed, possibly because he had run foul of the corrupt Governor of Manila, and by March 1768 he was back in Pondicherry. This time he had full co-operatuion from the British, who even loaned him an excellent 3-foot telescope, and he set up his equipment.

On 2 June 1769, the day before the transit, the weather was perfect. Then the cloud rolled in; Legentil had to wait under an overcast sky, praying for a miracle which did not happen. Half an hour after the end of the transit, the sky was crystal clear again ... and then it transpired that at Manila there had been no cloud cover at all.

One can well imagine Legentil's feelings. It was rather too long to wait for the next transit (that of 1874), so at last he decided to go home. After two minor shipwrecks he reached Paris in October 1771, to find that he had been presumed dead and that his heirs were preparing to distribute his property; he took legal action to reclaim it at something later than the eleventh hour. It seems likely that by then his enthusiasm for transits of Venus was somewhat dampened (97-99)

After the 19th century transits of 1874 and 1882, astronomers gave up. It turned out that "transits of Venus simply were not able to yield an accurate value for the length of the astronomical unit" (103) of distance between the Earth and the Sun. The astronomical unit was only determined with precision in the 20th century, ironically enough because of Venus, after a radar pulse was bounced off the planet, the echo was received, and the time multiplied by the speed of light to produce an accurate figure.