The first predicted sighting

The first predicted sighting

THE 7TH of November, 1631, dawned a cloudy day in Paris. Pierre Gassendi, an astronomer, had been patiently waiting with his telescope to catch a glimpse of Mercury, which had been predicted to appear by Johannes Kepler in 1629.

Finally, at 9 am, he had a fleeting glimpse, but since the planet was unexpectedly small, he assumed it was a sunspot. After several rounds of hide-and-seek with Mercury and the clouds, he was convinced he'd captured the planet. He wrote, "I could hardly persuade myself that it was Mercury, as my expectation of a larger magnitude bothered me, and I wondered if I had been deceived in the previous measurement. But the Sun shone again, I discovered further movement, and only then did I conclude that Mercury had come in on his splendid wings." Wide publicity followed, and Gassendi's tributes to Kepler led to the acceptance of Kepler's somewhat complex ephemeris, the Rudolphine Tables. Kepler had found planets travelled in elliptical orbits with the Sun at one of the two focusses of the ellipse.

Transits of Mercury take place about 13 times in a century.By 1677, astronomers realised that accurate observations of a Mercury transit would provide an important test for the ephemerides that had by then superseded those made by Kepler, One of these was prepared by mathematician Thomas Streete, who had observed the 1661 transit from London and later wrote Astronomica Carolina, planetary tables named in honour of the new king, Charles II.

Between 1682 and 1684, Edmund Halley, who subsequently became the Royal Astronomer of Britain, made a series of observations with a 1.67 m sextant, which reduced the importance of Mercury in planetary theory, but gave astronomers the crucial missing observations of Mercury near its most distant point from the Sun. Halley added these observations, "which had never before been seen in these parts of Europe", to a reprint of Streete's tables in 1710.

Although Halley modestly said they "abundantly evince the certainty of Streete's Mercurial astronomy", they were also his personal triumph in solving the two millenium-old observational problem of where Mercury really was at all points in its orbit around the sun. The fleet messenger of the Gods had been captured at last.