Jeremiah was born at Lower Lodge Farm in Toxteth Park. He became an astronomer and was the first person to demonstrate that the moon moved around the Earth in an elliptical orbit. He was also the only person to predict the transit of Venus of 1639, an event which he and his friend William Crabtree observed and recorded – and which was influential in establishing the size of the Solar System.
Jeremiah made his own specialist instruments for the study of astronomy. His father and uncles were watchmakers – he helped with the family business by day and, in return, the watchmakers in his family assisted in the design and construction of instruments to study the stars at night. His observations, combined with those of his friend and correspondent William Crabtree, had convinced him that Johannes Kepler’s Rudolphine tables (published in 1627), were in need of some correction – although more accurate than the commonly used tables produced by Philip Van Lansberg. Kepler’s tables had predicted a near-miss of a transit of Venus in 1639 but, having made his own observations of Venus for years, Horrocks predicted a transit would indeed occur.
He focused the image of the sun through a telescope onto a plane surface, where it could safely be observed. He calculated that from his location in Much Hoole the transit would begin at approximately 3.00pm 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 at about 3.15pm; and he continued to observe for half an hour until the sun set. The 1639 transit was also observed by 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 – and to 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, 0.63AU) was far from the 150 million kilometres (93 million miles) known today, but it was more accurate than any suggested up to that time.
He was also the first to demonstrate that the moon moved in an elliptical path around the Earth. He anticipated Isaac Newton in suggesting the influence of the sun as well as the Earth on the moon’s orbit. In his Principia Newton acknowledged Horrocks’ work in relation to his theory of lunar motion. He made detailed studies of tides in attempting to explain the nature of lunar causation of tidal movements. Horrocks is remembered on a plaque in Westminster Abbey, and the lunar crater Horrocks is named after him.