page_title: Transit of Venus
section_title: 1639 – first scientific observation
hierarchical_section_title: Transit of Venus / History of observation / 1639 – first scientific observation
caption_attribution_description: English: The first observed Transit of the Planet Venus predicted &amp; observed by Jeremiah Horrocks, 24th November 1639. Portrait held in the collection of Astley Hall Museum and Art Gallery Chorley and the property of Chorley Council.
context_page_description: A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and a superior planet, becoming visible against the solar disk. During a transit, Venus can be seen from Earth as a small black dot moving across the face of the Sun. The duration of such transits is usually several hours. A transit is similar to a solar eclipse by the Moon. While the diameter of Venus is more than three times that of the Moon, Venus appears smaller, and travels more slowly across the face of the Sun, because it is much farther away from Earth.
  Transits of Venus are among the rarest of predictable astronomical phenomena. They occur in a pattern that generally repeats every 243 years, with pairs of transits eight years apart separated by long gaps of 121.5 years and 105.5 years. The periodicity is a reflection of the fact that the orbital periods of Earth and Venus are close to 8:13 and 243:395 commensurabilities.
  The last transit of Venus was on 5 and 6 June 2012, and was the last Venus transit of the 21st century; the prior transit took place on 8 June 2004. The previous pair of transits were in December 1874 and December 1882.
context_section_description: In 1627, Johannes Kepler became the first person to predict a transit of Venus, by predicting the 1631 event. His methods were not sufficiently accurate to predict that the transit would not be visible in most of Europe, and as a consequence, nobody was able to use his prediction to observe the phenomenon.
  The first recorded observation of a transit of Venus was made by Jeremiah Horrocks from his home at Carr House in Much Hoole, near Preston in England, on 4 December 1639 (24 November under the Julian calendar then in use in England). His friend, William Crabtree, also observed this transit from Broughton, near Manchester. Kepler had predicted transits in 1631 and 1761 and a near miss in 1639. Horrocks corrected Kepler's calculation for the orbit of Venus, realized that transits of Venus would occur in pairs 8 years apart, and so predicted the transit of 1639. Although he was uncertain of the exact time, he calculated that the transit was to begin at approximately 15:00. Horrocks focused the image of the Sun through a simple telescope onto a piece of paper, where the image could be safely observed. After observing for most of the day, he was lucky to see the transit as clouds obscuring the Sun cleared at about 15:15, just half an hour before sunset. Horrocks's observations allowed him to make a well-informed guess as to the size of Venus, as well as to make an estimate of the mean distance between the Earth and the Sun – the astronomical unit (AU). He estimated that distance to be 59.4 million mi (95.6 million km; 0.639 AU) – about two thirds of the actual distance of 93 million mi (150 million km), but a more accurate figure than any suggested up to that time. The observations were not published until 1661, well after Horrocks's death. Horrocks based his calculation on the (false) presumption that each planet's size was proportional to its rank from the Sun, not on the parallax effect as used by the 1761 and 1769 and following experiments.
caption: Jeremiah Horrocks makes the first observation of the transit of Venus in 1639, as imagined by the artist W. R. Lavender in 1903
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