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Jeremiah Horrocks and the Transit of Venus

The first European scientific observation of a transit of Venus was made by Jeremiah Horrocks from his home in Much Hoole, near Preston in England, on 4 December 1639 (24 November under the Julian calendar then in use in England). His friend, William Crabtree, also observed this transit from Salford, near Manchester. Kepler had predicted transits in 1631 and 1761 and a near miss in 1639. Horrocks corrected Kepler's calculation for the orbit of Venus and realized that transits of Venus would occur in pairs 8 years apart, and so predicted the transit in 1639. Although he was uncertain of the exact time, he calculated that the transit was to begin at approximately 3:00 pm. Horrocks focused the image of the Sun through a simple telescope onto a piece of card, where the image could be safely observed. After observing for most of the day, he was lucky to see the transit as clouds obscuring the Sun cleared at about 3:15 pm, just half an hour before sunset. Horrocks' observations allowed him to make a well-informed guess as to the size of Venus, as well as to make an estimate of the distance between the Earth and the Sun. He estimated the distance of the Sun from the Earth at 59.4 million miles (95.6 Gm, 0.639 AU) – about two thirds the correct size of 93 million miles (149.6 million km), but a more accurate figure than any suggested up to that time. However, Horrocks' observations were not published until 1661, well after his death.

On the basis of his observation of the transit of Venus of 1761 from the Petersburg Observatory, Mikhail Lomonosov predicted the existence of an atmosphere on Venus. Lomonosov detected the refraction of solar rays while observing the transit and inferred that only refraction through an atmosphere could explain the appearance of a light ring around the part of Venus that had not yet come into contact with the Sun's disk during the initial phase of transit.

Stone Monument to Jeremiah Horrocks, Photograph supplied by and © of Brian Young
Stone Monument to Jeremiah Horrocks
Photograph supplied by and © of Brian Young
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