 I'm here in Oxford's South Park, looking west over the city skyline. The sun's just sinking behind the spires over there and already visible, glistening in the twilight, is Venus. Later this year, on June 6th, Venus is going to indulge us with a twice in a lifetime spectacle. Its tiny disk is going to pass in front of the sun in what astronomers call a transit of Venus. This is a picture of Venus from the last transit in 2004 and you can see it making its way across the bright flaming disk of the sun. This image from right near the end of the transit shows the thin halo of Venus' atmosphere, backlit by sunlight. What's slightly surprising is just how unusual this is. The first problem is that the earth and Venus go around the sun in the same direction, which means that Venus only actually overtakes us on the inside once every 1.6 Earth years. The second and larger problem is that Venus' orbit is tilted with respect to the earth's by an angle of 3.4 degrees, which means there are only two special points on the orbit where Venus passes directly between the earth and the sun. The result of all this is a strange cycle where transits of Venus occur in pairs separated by almost exactly eight years and then nothing for over a century and then another pair of transits separated by eight years. So for example the last transit of Venus happened on June 8th 2004, this one is going to happen on June 6th 2012 and then nothing until 2117 and 2125. If you want to watch the transit later this year you'll need to be careful. You should never look at the sun with the unaided eye, even with sunglasses, but there are a few different ways to see what's going on safely. The simplest is to use binoculars or a pinhole to project an image onto a piece of paper or you can use extremely dark solar filters to look at the sun directly. The reason Venus is so high and bright in the evening sky at the moment is that from our perspective it's just swung out from behind the sun and currently it's right near the extreme of its orbit. Shortly it's going to dive directly back toward the disk of the sun as seen from the perspective of the earth. What I find most incredible is that as we look up at that tiny dot glistening away in the evening twilight we're actually looking at five trillion trillion tons of rock screaming through space at 35 kilometers a second. It's hurtling inevitably and very precisely towards its transit on June the 6th completely indifferent to anything happening down here on earth. UK sky watchers will only be able to catch the end of the transit as the sun rises so find a safe way to check it out about 5 a.m. on June the 6th. It's probably worth getting up before dawn for because if you miss it this is the last chance for a hundred and five and a half years. In the meantime Venus is going to be easily visible in the west after sunset for quite a few more weeks brighter than anything else in the sky other than the sun and the moon. So do take a look and try to feel a part of this entrancing cosmic clockwork.