On Tuesday an astronomical event will happen which is so rare that nobody alive today has ever seen it. The planet Venus will spend the morning creeping across the disc of the Sun, for the first time since 1882. Full details of this once-in-a-lifetime phenomenon coming up. Today the science bit, tomorrow the history bit and on Tuesday the transit itself.
The science bit
• A transit of Venus occurs only when Venus lies exactly in line between the Earth and the Sun. This should happen every 584 days, except that the orbits of the two planets are tilted slightly so a precise alignment is very rare (happens only 4 times every 243 years). You can view a visual explanation here and read a fairly comprehensible astronomical explanation here.
• Transits occur in pairs, eight years apart. After 2004, the next transit will be in 2012.
• These pairs occur alternately 105½ years and 121½ years apart. Before 2004 the last transit was in 1882. After 2012 the next transit will be in 2117.
• Transits can only happen in June (around the 7th) or in December (around the 7th). The 2004/2012 pair are June transits, while the previous and next pairs are December transits.
• Viewed from the Earth, the diameter of Venus is approximately 1/32nd of the diameter of the Sun.
• It will take Venus six hours to cross the face of the Sun. First contact will occur around 6:20am BST and final contact around 12:23pm BST. You can watch a speeded-up animation here.
• It has to be daylight to be able to view the transit. The whole 2004 event is therefore visible from Europe and most of Africa and Asia, but nothing at all is visible from the eastern Pacific.
• Never look directly at the Sun, especially through a telescope or binoculars. Instead you could use a pinholeprojector or those goggles you bought for the 1999 eclipse, or just watch the whole thing online or on TV.
• If it's cloudy you won't see a thing but, just for once, the weather forecast is fan-bloody-tastic. Six science-y transit weblinks:onetwothreefourfivesix.
Transit of Venus
