transit of venus...
hi guys! Eyes to the skies early next month for a twice-in-a-lifetime-and-a-bit planet pile-up: The Transit of Venus, coming to the Heavens near you on 8th June 2004.
so, what is it?
Well, basically there are three decent-sized rocks up there that can come between us and the Sun. One is our own Moon; the other two, as everyone should know, are Mercury and Venus.
When the Moon's orbit places it between us and the Sun, we call this either a Solar Ecplise: Total or Partial depending on whether all, or just some, of the Sun is blocked out.
When the orbits of either Mercury or Venus place themselves between us and the Sun, we call these Transits. The planets are much too far away to block out the Sun, but they can be seen travelling (or 'transitting') across the face of the Sun - like a small black dot making its way across a yellow circle. The last Transit of Mercury took place on 7th May 2003.
The last Transit of Venus took place in 1882, so only 122-year-olds could say, 'I've been there, done that, and got the t-shirt.'
This will make it the biggest must-see Heavenly event in Britain since the last Total Solar Eclipse in 1999.
The 'sun-block factor' with the Transit of Venus is, of course, much slighter than with an Eclipse. This is because, although Venus is about four times as big as Earth's Moon, it is much, much further away, and so appears tiny in comparison.
Transits of Venus, it turns out, are a bit like your bus to school: you wait ages for one, and then two come along at once. The next Transit is, in fact, due in only eight years time, on 5th/6th June 2012.
so how does that work?
Being closer to the Sun, Venus has a shorter orbit (and hence a shorter year) than Earth. Whereas Earth's orbit takes about 365 days to complete, Venus has a year of about 225 days.
Think of Venus as a track athlete running along the inside lane of an oval circuit, with its Earth competitor running along in the outside lane.
Well, if you're anything like us, your intuition is now telling you that, if Earth orbits the Sun regularly every 365 days, and Venus orbits the Sun regularly every 225 days, then there should be Transits much more often, and much more regularly. None of this 'nothing for 120 years then suddenly twice together' sort of thing. And you'd be right. In fact, the actual maths work out that a Transit should happen exactly once every 584 days (about every year and a half - this event is called an Inferior Conjunction).
But that's because you're intuiting. In fact, in the third dimension, the orbits of Earth and Venus are at angles to each other. So most of times, when the orbit of venus 'crosses' ours, the planet is actually hanging, so to speak, above or below us, failing to block out the Sun. So no Transit!
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