Transits of Mercury and Venus hold an interesting place in astronomical history. The main reason is the slightly different times that the events occur as seen from different locations on the surface of the Earth. The diameter of our planet is appreciable when compared to the distance to Mercury and Venus, allowing astronomers to "triangulate" the planets from various points on Earth when they are seen against the bright background of the Sun's limb.
In this way the difference between the distances of the planet and the Sun can be found. Because of its considerably larger size, Venus is better for this purpose, and in fact, during the 18th and 19th centuries transits of Venus provided astronomers with the best opportunity for measuring the absolute scale of the solar system from Keplers third law. Such a value was poorly known at that time.
Sir Edmund Halley, of comet fame, was the first to realize that transits could be used to measure the Earths distance from the Sun.
Today this method has been completely superceded, chiefly because those moments when either Mercury or Venus begin moving onto or off of the Sun required precise accuracy which more than two centuries ago were always difficult to attain. Nonetheless, elaborate expeditions which were mounted to observe the transits of Venus in 1761 and 1769 provided astronomers with their first good value for the distance of the Earth to the Sun (called an astronomical unit, or AU).
Johannes Kepler (1571-1630) made the surprising discovery that in the year 1631 Mercury and Venus would both transit the Sun within less than a month of each other. Mercury would do so on Nov. 7, followed by Venus on Dec. 6. The sight of a planet passing in front of the solar disk had never been seen before, so Kepler and his soon-to-be son-in-law, Jacob Bartsch, issued an "admonition" to all astronomers to be on the watch for these events.
Since Kepler himself was uncertain about the exact circumstances (as he was concerned about the accuracy of his own tables), he urged prospective observers to carefully watch the Sun a day early and, should nothing be seen, not give up until the day after.
Unfortunately, early November 1631 brought a very stormy and unsettled period of weather to much of Europe.
So far as we know, only three individuals actually observed the transit of Mercury and only one Pierre Gassendi (1592-1655) left a detailed account. According to Gassendis writings, he observed from Paris and saw the transit by means of projecting an 8-inch-wide image of the Sun from his telescopes eyepiece onto a white screen. At around 9 a.m. local time on Nov. 7, through a scattered-to-broken layer of cloud cover, Gassendi anxiously watched the black dot of Mercury much smaller than he had expected -- as it slowly moved across the Sun.
Kepler did not witness this event. He died on Nov. 15, 1630, almost exactly a ear before his predicted transit. Despite his fears that his calculations might be off by a day or two, Keplers predicted the transit within five hours of it actually taking place an astonishing feat for that era.
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