Jeremiah Horrocks (1618 – January 3, 1641), sometimes given as Jeremiah Horrox (the Latinised version that he used on the Emmanuel College register and in his Latin manuscripts), was an English astronomer who was the only person to predict, and one of only two people to observe and record, the transit of Venus of 1639.
Horrocks was born in Lower Lodge, Otterspool in Toxteth Park, near Liverpool, Lancashire. His father was a small farmer; his uncle was a watchmaker; he was relatively poor during his entire brief life. He joined Emmanuel College on May 11, 1632 and matriculated as a member of the University of Cambridge on July 5, 1632 as a sizar. In 1635 he left without formally graduating, presumably due to the cost of continuing his studies. The traditional view is that he supported himself financially by holding a curacy in Much Hoole, near Preston in Lancashire, but there is little evidence for this. According to local tradition in Much Hoole, he lived at Carr House, a substantial property owned by the Stones family who were prosperous farmers and merchants, and was a tutor for the Stones children. He may have been a Calvinist and, through his connection with Emmanuel College, a Puritan, although there is little evidence of his religious convictions.
At Cambridge, he became familiar with the works of Johannes Kepler, Tycho Brahe, and others. Horrocks read most of the astronomical treatises of his day, found the weaknesses in them and was suggesting new lines of research by the age of seventeen. He was the first to demonstrate that the Moon moved in an elliptical path around the Earth, he wrote a treatise on Keplerian astronomy and began to explore mathematically the properties of the force that later became known as gravity. Sir Isaac Newton acknowledged Horrocks's work as the bridge which connected him with Copernicus, Galileo, Brahe and Kepler. 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 focused the image of the Sun through a simple telescope onto a piece of card, 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 November 24, 1639 (Julian calendar, or December 4 in the Gregorian calendar). The weather was cloudy, but he first observed the tiny black shadow of Venus crossing the Sun on the card 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 Salford.
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. His figure of 59 million miles (95 million kilometres, 0.63 AU) was far from the 93 million miles (150 million kilometers) 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 sub sole visa (Venus in transit across 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 Venetian 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."
Horrocks also put his energies into the highly complex task of determining the Moon's orbit. He correctly hypothesised that the moon's orbit was elliptical rather than circular, and he anticipated Isaac Newton in suggesting an influence on the moon's orbit from the sun as well as the earth. In the final months of his life he also made detailed study of tides, in an attempt to explain the nature of lunar causation of tidal movements.
Horrocks returned to Toxteth Park sometime in the summer of 1640 and died suddenly and from unknown causes on the 3 January 1641, aged only 22. As expressed by his astronomer friend William Crabtree, "What an incalculable loss!" (Opera Posthuma of Jeremiah Horrocks, ed. John Wallis, London, 1672.)