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Transit of Venus1639 transit

William Crabtree, as he appears in The Manchester Murals in Manchester Town Hall
Jeremiah Horrocks (1618-1641) was a largely self-taught astronomer from Toxteth Park near Liverpool. Although he attended Cambridge University, it was his enthusiasm for astronomy and background reading that commenced his interest in planetary motions. He became dissatisfied with the inaccuracies of then-available tables and started to compute his own, comparing his observations of the planets with their predicted positions.

Horrocks was stimulated by a correspondence with the Manchester astronomer William Crabtree (c.1610-1644) and by the writings of Johannes Kepler (1571-1630). By discovering an error in Kepler’s work, Horrocks realized that a Transit of Venus would occur on 24 November 1639 and both he and Crabtree were able to see the event. It was the first observation of its kind and allowed Horrocks to calculate solar parallax (triangulating using two points on the Earth and the observer’s angle to the Sun) and thus estimate the distance between the Earth and the Sun.
Portrait of Jeremiah Horrocks, held in the collection of Astley Hall Museum and Art Gallery Chorley.
Horrocks died young. His papers remained unpublished in his lifetime and many were lost. Fortunately, some of the natural philosophers that founded the Royal Society in 1660 became aware of his work and ensured that it was printed. The mathematician John Wallis FRS (1616-1703) was instrumental in publishing Horrocks’s transit observation by sending papers to Johann Hevelius (1611-1687). The eventual Mercurius in Sole (1662) incorporated Horrocks’s Venus in Sole pariter visa... 1639 (complete with a poem by the astronomer) while the entire Opera posthuma (1673) with the Horrocks-Crabtree correspondence was edited by John Wallis himself.

The combination of careful observation and precise mathematics appealed to many early Fellows, not least the best judge possible: “I must join with Mr Gregory in admiring Mr Horrox” Sir Isaac Newton declared.

By 1677, additional work on the solar parallax was being done by Edmond Halley FRS (1656-1742), who travelled to St Helena to see a transit of the planet Mercury across the Sun. Halley would later become the most eminent of scientists, a friend of Newton, the second Astronomer Royal and predictor of the comet named in his honour. But Halley made a second prediction for an event after his death and pleaded for a programme of work to take advantage of it. His paper “Methodus singularis quâ Solis Parallaxis sive distantia à Terra, ope Veneris intra Solem...” noted the next set of transits of Venus (1761 and 1769) and the opportunity they offered for science.