First recorded transit of Venus

On this day in 1639, Jeremiah Horrocks and William Crabtree made the first known observations and recording of a transit of Venus. Horrocks was the only astronomer at the time that realized that a transit was imminent, partly because of his great interest in the recent work of Johannes Kepler, which assumed elliptical, rather than circular, planetary orbits. Kepler had suggested that there might be a near miss for a Venus transit in 1639 and, as the next full transit wouldn’t be expected for another 121 years, most astronomers at the time moved on to other areas of research. Horrocks and Crabtree on the other hand, believed that there might be errors in Kepler’s work and strove to correct them. In October 1639, Horrocks had calculated that transits of Venus occur not singly, but in pairs eight years apart, and realised that the second transit would occur in less than four weeks. He was convinced that a measurement could be made of the apparent diameter of the planet to within a fraction of a second of arc when it was seen as a dull black disk on the face of the Sun, compared to an accuracy of around one minute of arc when seen in its normal position as the bright morning star close to the Sun. He wrote to his younger brother and to Crabtree in Broughton, advising them to observe the event on Sunday, 4 December. To quote Horrocks: “The more accurate calculations of Rudolphi very much confirmed my expectations; and I rejoiced exceedingly in the prospect of seeing Venus” 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. Sir Issac Newton credits Horrocks contribution to astronomy in his famous Principia Mathematica. Horrocks can be seen in the stained glass window below, which is in St Michael’s Church, Hoole. The Latin is taken from Horrocks’s report of the 1639 transit and reads “Ecce gratissimum spectaculum et tot votorum materiem": “oh, most grateful spectacle, the realization of so many ardent desires”.

Date in History: 4 December 1639