Transit of Venus, 1639

The first known observations and recording of a transit of Venus were made in 1639 by the English astronomers Jeremiah Horrocks and his friend and correspondent William Crabtree. The pair made their observations independently on 4 December that year (24 November under the Julian calendar then used in England); Horrocks from Carr House, then in the village of Much Hoole, Lancashire, and Crabtree from his home in Broughton, near Manchester.

The friends, followers of the new astronomy of Johannes Kepler, were self-taught mathematical astronomers who had worked methodically to correct and improve Kepler's Rudolphine tables by observation and measurement. In 1639, Horrocks was the only astronomer to realise that a transit of Venus was imminent; others became aware of it only after the event when Horrocks's report of it was circulated. Although the friends both died within five years of making their observations, their ground-breaking work was influential in establishing the size of the Solar System; for this and their other achievements Horrocks and Crabtree, along with their correspondent William Gascoigne, are considered to be the founding fathers of British research astronomy.

By the 17th century, two developments allowed for the transits of planets across the face of the Sun to be predicted and observed: the telescope and the new astronomy of Johannes Kepler, which assumed elliptical, rather than circular, planetary orbits.

In 1627, Kepler published his Rudolphine Tables. Two years later he published extracts from the tables in his pamphlet De raris mirisque Anni 1631 which included an admonitio ad astronomos (warning to astronomers) concerning a transit of Mercury in 1631 and transits of Venus in 1631 and 1761. The Mercury transit occurred as predicted and was observed by Johann Baptist Cysat in Innsbruck, Johannes Remus Quietanus in Rouffach and Pierre Gassendi in Paris, vindicating the Keplerian approach. But their observations threw into question previous theories about the Solar System as Mercury was shown to be much smaller than expected.

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