The 2004 Transit of Venus across the Sun
General description and historyOn June 8th, 2004, starting at 3:08 in the afternoon Melbourne time, the planet Venus will pass directly between the earth and the sun. Venus, which is both the morning star and the evening star, will be visible during the transit as a black dot, tracking across the sun. You cannot look directly at the sun because it will severely damage your eyes, but it can be seen by projecting an image through a pinhole onto a white surface. It is possible that some sunspots will also be able to be seen, but these will not move. The transit of Venus is very rare. The most recent transits have occurred in December 1639 (the first transit ever seen, since it followed the invention of the telescope), in June in 1761 and 1769 , and in December in 1874 and 1882. The next one will be on June 7th, 2012 (or June 6th, depending where on earth you live!) The 1769 transit is of special importance to Australia and New Zealand, because Captain Cook was sent on his voyage to the south Pacific to make measurements of the transit, and then to look for the great south land if things were still going well. The scientific importance of the transit of Venus is that it enabled astronomers to calculate the distance of the earth to the sun. Halley (who discovered Halley's comet) was the first to propose this. The method required observers as far apart as possible, so Cook was sent to the south Pacific, whilst other measurements were made in Europe. There are also transits of Mercury across the sun. These are fairly common but Mercury is so small and far away that it is not possible to see without special equipment.
Why are the transits so rare?The length of an Earth year is 365.256 days and the length of a Venus year (also in Earth days) is 224.701 days. This means that Venus goes around the sun faster than the Earth, and they are lined up about every 19 months. So why don't we get transits every 19 months? The orbit of Venus is at an angle to the orbit of the Earth around the Sun, and so mostly when the Earth, Venus and the Sun are lined up, Venus is above or below the sun and there is no transit. There are just two points on Earth's orbit where Venus can line up exactly between the Earth and the Sun. (The plane of the Venus orbit cuts the plane of the Earth's orbit in a line, and this line crosses the Earth's orbit at two points, directly opposite each other. These points are called the nodes). As it happens the Earth goes through these nodes in June and December (half a year apart). So following one transit, when the Earth will be at one of the nodes, the future transits will occur either when both Venus and the Earth have completed a whole number of orbits around the Sun (and so get back to the same node together) or have both completed a whole number and a half of orbits around the Sun (and so get to the other node together). The spreadsheet shows when this might be. The spreadsheet also includes some near misses. The patter is something like gaps of 8, 121.5, 8, 105.5, 8, 121.5, etc going June, June, December, December, June, June, December ....
How can you use a transit of Venus to calculate the distance from the Earth to the Sun?There are some great webpages with excellent diagrams - see below. . Similar triangles and trignometry are all that are required for a basic understanding. If you want to made an approximate model, if the Sun is an orange about 8 cm diameter, the Earth is about 8 metres away and Venus is about 5 metres away. The actual diameter of the Sun is roughly 1.5 million km and its distance is roughly 150 million km. The diameter of the Earth is approximately 13000km
LinksMany interesting facts on the transit from Australian Astronomy: Some excellent animations A thorough description of the mathematics of calculating the distance
of the Earth from the Sun. Edmund Halley's original paper Some interesting Australian history and facts and pictures http://www.dsellers.demon.co.uk/venus/ven_ch_frames.htm Live images of the Sun are available on the internet from several
telescopes, including Live broadcast of the transit are available on the internet from
observatories around the world such as this one in Paris:
|
