Jeremiah Horrocks - Transit of Venus

Transit of Venus

Horrocks was convinced that Lansberg's tables were inaccurate when Kepler predicted that a near-miss of a transit of Venus would occur in 1639. Horrocks believed that the transit would indeed occur, having made his own observations of Venus for years.

Horrocks made himself a simple helioscope by focusing the image of the Sun through a telescope onto a piece of paper, where the image could be safely observed. From his location in Much Hoole, he calculated that the transit was to begin at approximately 3:00 pm on 24 November 1639 (Julian calendar, or 4 December in the Gregorian calendar). The weather was cloudy, but he first observed the tiny black shadow of Venus crossing the Sun on the paper at about 3:15 pm, and observed for half an hour until sunset. The 1639 transit was also observed by his friend and correspondent, William Crabtree, from his home in Broughton, near Manchester.

Horrocks' observations allowed him to make a well-informed guess as to the size of Venus (previously thought to be larger and closer to Earth), as well as to make an estimate of the distance between the Earth and the Sun, now known as the astronomical unit (AU). His figure of 95 million kilometres (59 million miles, 0.63 AU) was far from the 150 million kilometers (93 million miles) that it is known to be today but it was a more accurate figure than any suggested up to that time.

A treatise by Horrocks, Venus in sole visa (Venus seen on the Sun) was published by Johannes Hevelius at his own expense in 1662. This paper, which caused great excitement when revealed to members of the Royal Society 20 years after it was written, contained much evidence of Horrocks' enthusiastic and romantic nature, including humorous comments and passages of original poetry. When speaking of the century separating Venusian transits, he rhapsodised,

" ...Thy return

Posterity shall witness; years must roll

Away, but then at length the splendid sight

Again shall greet our distant children's eyes."

At a time of great uncertainty in astronomy, when the world's astronomers couldn't agree amongst themselves and theologians argued over contradictory Scriptural passages, Horrocks, although a pious man, came down firmly on the side of scientific determinism.

It is wrong to hold the most noble Science of the Stars guilty of uncertainty on account of some people's uncertain observations. Through no fault of its own it suffers these complaints which arise from the uncertainty and error not of the celestial motions but of human observations...I do not consider that any imperfections in the motions of the stars have so far been detected, nor do I believe that they are ever to be found. Far be it from me to allow that God has created the heavenly bodies more imperfectly than man has observed them.