Twice in a lifetime Venus

For the first time in the Century, Venus will transit the Sun's disk on June 8, 2004. Transits are nothing but eclipses, that is, the planet's orbit happens to lie across the Sun at the time of its passage, yet the feeble 58 arcseconds wide (58/3600 of a degree) spot will not dim our star's light in any noticeable way. Asia, Africa and Europe will be privileged with ingress start at 5:13 UT to egress end at 11:26 UT, while Australia will catch the show opening before sunset and South America (except the southern tip) and North America from the Rockies eastward, will peek the last phases after sunrise.

Venus transits come in pairs 8 years apart with an about 105 year interval. Don't count on another chance after this and the June 6, 2012 one, next comes December 11, 2117. Venus's orbit is tilted 3.39 deg. with respect to the Ecliptic - the plane of Earth's orbit - while the orbital ratios of 13 turns for Venus in 8 years minus 2 days makes for the peculiar rhythm of 8 - 105 . You will find the pattern of Venus sightings as morning or evening star repeating the same months each 8 years. When the favorable position or node as it is technically known drifts once in place, it is still about right next turn 8 years latter, but wanders for 105 years until the same small drift brings about another opportunity.

Venus 8 year cycle was known as far back as the times of ancient Babylonia, where a tablet found in 1920 and dated to about 1500 BC during the reign of Ammisaduqa registered this astronomic observation, important on religious grounds as were all significant sky related events. Incidentally the Venus Tablet was for long used to controversially date the fall of Babylon in 1531, 1595 and 1651 BC until work by astronomer Vahe Gurzadyan using Moon eclipse records settled it to a more widely accepted 1499 BC (not to be confused with the final 539 BC fall).

The first observed transit

It is likely Venus transits had been observed for long though unknowingly. Chinese astronomers knew about solar spots, so it is entirely possible for them to have witnessed transits yet not associating the observed blemish with the planet.

German astronomer Johannes Kepler was the first to predict such an event for December 6, 1631. Unfortunately he died several years before and no one else was aware of his work in this respect. Neither had he predicted the next in the cycle due on December 4, 1639, which was discovered barely one month before taking place by young Jeremiah Horrocks of Lancashire, England. He and friend William Crabtree were the only humans to know of, and observe the transit. Horrocks died at 22 in 1641, but his accurate calculations confirmed by observation earned a name in the annals of astronomy.