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Jeremiah Horrocks (c.1618 - January 3, 1641), sometimes given as Jeremiah Horrox, was an English astronomer who made the first observation of a transit of Venus.
Horrocks was born in Toxteth Park, near Liverpool in Lancashire. His father was a small farmer, and his uncle was a watchmaker. He was relatively poor throughout 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. He left the University of Cambridge without formally graduating, presumably on grounds of cost, in 1635. Traditionally, he is said to have supported himself financially by holding a curacy in Much Hoole, near Preston in Lancashire, but there is little evidence for this. In Much Hoole, local tradition reports that 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 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, Lancashire, he calculated that the transit was to begin at approximately 3:00pm 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 3:15pm, 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, as well as to make an estimate of the distance between the Earth and the Sun. His figure of 59,000,000 miles was far from the 93,000,000 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 sub sole visa (Venus in transit across the Sun) was published by Johannes Hevelius at his own expense in 1662.
Horrocks returned to Toxteth Park sometime in the summer of 1640 and died suddenly and from unknown causes on the 3rd January 1641, aged about 23 years old.
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