transit of Venus
A transit of Venus is the passage of Venus across the face of the Sun as seen from Earth. Transits of Venus are quite rare and only occur when Venus is at or near one of its nodes when at inferior conjunction. For this to happen, the date of inferior conjunction must be within a few days of June 7 or December 8. The most recent transit of Venus began on June 8, 2004.
Five synodic periods of Venus are almost equal to 8 years, and so transits normally occur in pairs eight years apart; as a much more precise relation, 152 synodic periods are approximately equal to 243 years. After a pair of transits 8 years apart, no further transit will occur at the same node for at least 235 years; a pair will occur at the other node in the meantime, however. The intervals between pairs of transits are alternately 105.5 years and 121.5 years; thus there were or will be a series of December transits in 1631 and 1639; 1874 and 1882; 2117 and 2125, and a series of June transits in 1761 and 1769; 2004 and 2012, etc.
The transit of December 6, 1631 was predicted by Kepler, and Gassendi (who had made the first observation of a transit of Mercury) observed the planet at every possible moment on December 4, 5, 6, and 7, but did not see a transit. This is now known to be due to the transit having occurred during the night of December 6/7, when the Sun was invisible from Europe. Kepler had foretold only a "near miss" for 1639, but the remarkable calculations of Jeremiah Horrocks, a young but very competent and enthusiastic English amateur astronomer, who was curate of Hoole in Lancashire, suggested that this was wrong, and that a transit would in fact occur on December 4, 1639. In the moments when his clerical duties would allow, Horrocks watched for the event, and in mid-afternoon he was rewarded with the first recorded observation of a transit of Venus. Horrocks had informed a friend – William Crabtree of Manchester – of his prediction, and shortly before sunset he too saw the transit in progress. No other observations of this transit are known.
In 1716, Edmond Halley pointed out that accurate observations of the times of the contacts of transits of Venus could be used to obtain a more accurate determination of the solar parallax; for this reason the next four transits (June 5, 1761, June 3, 1769, December 8, 1874, and December 6, 1882) were widely observed. Future transits won't be used to determine solar parallax, as newer methods are now available. In any case, accurate measurements are difficult for several reasons. One of these is the so-called black drop – an optical illusion whereby the gap between the limit of Venus and the limit of the Sun, just after second and before third contact, appears to be bridged momentarily by a black spot; although this effect lasts for only a few seconds it renders accurate determination of the time of these contacts very difficult.
Transits of Venus will still be widely observed, however, not only for their rarity but also for the additional information they give about the orbit of Venus. After the one on June 8, 2004, the next transits will take place on June 5, 2012, December 10, 2117, and December 8, 2125.