(Sen) - After this week's solar eclipse which was seen from the Far East to the United States, astronomers are looking forward to the next passage of a planetary body across the Sun - the Transit of Venus.
Though less spectacular, this is a far rarer event which only comes in pairs at intervals of well over 100 years. The next, on June 5th or 6th, depending where you are in the world, will be the last that anyone alive today will see in their lifetimes.
Because the Earth's and Venus's orbits around the Sun are slightly tilted to one another, we can only see Venus in front of the Sun if we all line up at the points where the planes of the orbits cross. This can only happen in June or December. At other times, Venus will appear to pass above or below the Sun when roughly in line with Earth.
The current pairing of transits are both June events. The next will not be until 2117 and will happen in December as will the transit eight years after that in 2125.
Johannes Kepler was the first person to realise that Venus would cross the face of the Sun. He predicted the transit of December 6, 1631, but died a year before it occurred. The first known transit to be witnessed was that of 1639, by English astronomer Jeremiah Horrocks, at Much Hoole, near Preston, Lancashire, and his friend William Crabtree in Manchester.
The brilliant English astronomer and second Astronomer Royal Edmond Halley, famous for the comet that bears his name, later realised that by timing the start and end of a transit from widely spaced parts of the globe, it would be possible to calculate the distance of the Earth from the Sun, thanks to an effect called parallax. He said of the transit: "This sight . . . is by far the noblest astronomy affords."
Expeditions were sent across the world to observe the transits of 1761 and 1769 to take advantage of this opportunity to help measure the Solar System. The most famous was by Captain James Cook to Tahiti in 1769 which went on to explore and map the recently-found territories of Australia and New Zealand.
The results were not perfect, partly due to a "black drop" effect which made it tricky to tell just when Venus was at the Sun's edge and which may be due to the fact that the planet has a dense atmosphere.
The upcoming transit will last just under seven hours and Venus will appear almost one thirtieth the diameter of the Sun, and so appear in silhouette as a large black dot like a well-rounded sunspot. It will be best seen from Pacific regions - or the Arctic which then enjoys midnight Sun - but almost an hour will be visible from parts of the UK just after sunrise.
The team behind educational resource the Bradford Robotic Telescope have organised a project to recreate the experiments carried out by Captain Cook and others in 1769 at this year's transit. They want to collect timings of the event from amateur astronomers all over the world.
They have set up a site to help amateurs, plus teachers and parents, participate and share their measurements at http://transit.telescope.org/participate
Warning: Observing a Transit of Venus requires the same level of care as an eclipse of the Sun. It is dangerous to look directly at the Sun without proper protection as this could cause permanent eye damage. The only safe ways are to use a filter specifically designed for solar observing, or to project the image through binoculars or a telescope onto a screen.