On those very rare occasions when Venus is passing in front of the Sun, the jet-black circular outline of Venus has a diameter that is about thirty times smaller than the visible solar disc. A spot this size is just detectable to the unaided eye. Interestingly, however, the ancient astronomical histories record no such observations. Transits of Venus were unsuspected. Things changed in the seventeenth century. The German astronomer Johannes Kepler (1571 – 1630) used Tycho Brahe’s accurate planetary observations not only to work out the exact shape of planetary orbits but also to produce a reliable ephemeris of the ever-changing planetary positions in future years named the Rudolphine Tables in honour of the German Emperor Rudolf II (published in 1627). Kepler predicted that Venus would cross the Sun on 6^th December 1631. Unfortunately it was night-time in Europe and no observations are known from other places on Earth.

Jeremiah Horrocks (1618 – 1641) a young Liverpudlian astronomer had a similar fascination with astronomical tables, and was annoyed about their inconsistencies. He made his own evaluations of the position of Venus, and especially the times of its inferior conjunction (when the planet passes between the Earth and the Sun). Horrocks realised that Venus would actually transit the Sun on Sunday 24^th November 1639. (This date is ‘old style’. When England adopted the Gregorian Calendar in 1752 it became 4^th December). Not only did Horrocks confirm Kepler’s prediction that transits were spaced by about 120 years, he also discovered that they usually occurred in pairs, early December 1631 and 1639; early June 1761 and 1769; early December 1874 and 1882; early June 2004 and 2012 and so on. Horrocks was living in Much Hoole, Lancashire (some ten miles west of Preston), probably as the tutor to the children of the Stones family of Carr House. This obscure twenty-year-old predicted the 1639 transit less than a month before the event. He set up his simple refracting telescope on the third floor of Carr House so that it projected a six-inch diameter image of the solar disc onto a graduated circle. This circle was marked out in degrees so that Horrocks could measure the size of the disc of Venus, the direction of its path across the Sun and the speed at which it moved. For five hours on that cloudy, wintry Sunday Horrocks stayed with his telescope. At 3.15 p.m., half an hour before sunset, the western clouds broke. There was the large black disc of Venus on the edge of the Sun, and in the remaining thirty minutes he made three accurate measurements of its movement. Not only did this transit observation provide a key reference point on the orbit of Venus, it was also a major breakthrough in planetary astronomy, leading to an accurate estimation of the diameter of Venus, the Earth’s twin. Horrocks wrote up his observations and they were published as Venus in Sole Visa (Venus visible on the Sun) by the great Danzig astronomer Johannes Hevelius in 1662, twenty-one years after Horrocks’s untimely death when aged about 22.