The Scientific Use Of Transits

Leaving aside the transits of Mercury in 1661, 1664, and 1674, the next important occurrence in this field of endeavor was in November 1677 when another was observed by Edmond Halley, who at the time was on the island of St Helena, engaged in a survey of the South Atlantic.

This episode is significant because Halley later expounded a technique for using complete transit timings to determine the distance between the Earth and the Sun (that is, the astronomical unit or AU), a measurement that was sorely wanted. It was contemporary ignorance of the scale of the Solar System that led to the inaccuracy of Halley's computed track for the 1715 eclipse (see Chapter 7). An even more important consideration was that navigational accuracy at sea required precise knowledge of the future positions of the Sun, Moon, and planets.

Many authors credit Halley with inventing the transit technique. Actually it had been a Scottish mathematician, James Gregory, who first propounded the idea, in 1663, but it was Halley's later description of the concept that led to attempts to put it into action. He read a paper on this topic to the Royal Society in 1691, but did not publish his analysis until 1716. Halley realized that only transits of Venus, not Mercury, would afford a feasible avenue for determining a better value for the mean Earth—Sun distance. Knowing that those transits would not occur until 1761 and 1769, Halley recognized he would not live to put the technique to the test (he died in 1742). Nevertheless, he was happy to leave his reputation to posterity, just as he knew that he would not be around to see the return of the comet that bears his name, in 1758.

The significance of these transits we should put in the context of the era. The quest for a practical method for determining the longitude of a ship at sea was an overbearing desideratum at the time, as we discussed in Chapter 3. Astronomers were engaged in a race to develop some accurate method that would allow the position of a vessel far from the sight of land to be easily and accurately measured. Reckoning the distance to the Sun was not some abstract piece of scientific curiosity: it bore the promise of more accurate celestial tables and thus improved navigation. In consequence the British (and other) governments were strongly interested in having the transits utilized by their astronomers to ascertain that quantity, from which the distances and motions of the Moon and planets could be computed using Kepler's Laws. This explains the expense and effort put into this endeavor, as exemplified by the voyage of Lieutenant James Cook detailed in the next section.

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