Little is known about the early life of Horrocks (or Horrox) other than that he was born into a Puritan family in Toxteth, Liverpool, and was admitted to Cambridge in 1632. Even though he died ‘in his twenty second year’ he had made major contributions to astronomy and several original observations.

Horrocks noted that as the orbits of Venus and Mercury fall between the earth and the Sun, it would seem possible that at certain times the inner planets would appear to an observer on the Earth to cross the face of the Sun. The events, known as transits, are so rare that they are unlikely to be seen by chance. Only five transits of Venus have been observed, those of 1639, 1761, 1769, 1874, and 1882; the next is due in 2004.

At Cambridge, Horrocks had mastered the new astronomy of Kepler. From Kepler's recently published Rudolphine Tables (1627), he worked out that a transit of Venus was due on 24 November 1639 at 3 p.m. At this time he was probably working as a curate at Hoole near Preston in Lancashire. He prepared for the transit by directing the solar image on to a large sheet of paper in a darkened room. However, a late November afternoon in Lancashire is not the best time to observe the Sun. For Horrocks there was another problem. The predicted day was a Sunday which meant that the puritan curate could well find himself in church at the crucial moment.

Horrocks was successful in observng the transit, however, and left an account of the day in his Venus in Sole Visa (Venus in the Face of the Sun), published posthumously in 1662. The day was cloudy but at 3.15, “as if by divine interposition” the clouds dispersed. He noted a spot of unusual magnitude on the solar disc and began to trace its path; but, he added, “she was not visible to me longer than half an hour, on account of the Sun quickly setting.”

With the aid of his observations Horrocks could establish the apparent diameter of Venus as 1′ 12ʺ compared with the Sun's diameter of 30′, a figure much smaller than the 11′ assigned by Kepler. Horrocks also attempted to determine the solar parallax, and derived, although with little confidence, a figure of 15ʺ, compared with a modern value of 8ʺ.8.

Before his death Horrocks was working on an Astronomia Kepleriana (Astronomy of Kepler), and essays on comets, tides, and the Moon. Unfortunately none of this was published until long after his death. Much of his work had been lost in the chaos of the Civil War. Other material sent to a London bookseller was burnt in the Great Fire of 1666. The remainder of his papers were published by John Wallis as Opera posthuma (1678; Posthumous Works).