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In other news...

06 June 201208:54AMscience

There was a Transit of Venus today. No big deal. It's not like there's not another one for a hundred and five years. We had this whole thing planned out, we were going to watch it using some kind of jury-rigged telescope projector thingy, and then give up and play video games. That didn't happen. Because, y'know, rain. I ended up doing what normal people do and watching it on TV instead. Rare astronomical phenomena are totally something people watch on TV, right?

It took me ages to figure out the 105-8-121.5-8 pattern to the transit. It took someone on NASA TV with a paper-plate model of the inner solar system to explain it. Basically, it's because of the synchronisation of 3 different cycles: the orbits of earth and Venus, the plane of Venus' orbit, and the 'drift' of the points where Venus lines up with the earth:

  1. Venus orbits faster than earth, so for every 8 orbits earth does, Venus does 13. This means that they line up approximately every 1 5/8 earth years. So there will be five conjunctions in eight years. 2. Venus is orbiting on a slightly different plane to earth, so there is a specific spot where from earth, we can see it in front of the sun. Venus will only pass through this point, in view of Earth, once in this eight year cycle. 3. Except, that Venus is actually a little faster. It's more like 13 and a bit for every 8. So, while Venus crosses the sun's plane every Venus year, and lines up with earth every 1.6 earth years, we're only in a position to see it cross the sun from earth every 100 or so. This is when our position in our orbit, Venus' position in its orbit, and the point where Venus is in plane with the sun line up, and we stay close enough to that postition to see a transit for several years, which is why we see 2 in a row. The rest of the time, conjunctions still occur, but at the wrong part of Venus' orbit, so it doesn't actually cross the sun - it appears above or below it.

I found that really interesting, that something which seems like a weird and arbitrary set of times which shouldn't occur in a natural system are actually totally logical. It's like clockwork, literally - we get a transit when three gears moving at different speeds happen to line up.

So I guess I learned something today. Pity it won't be useful for a hundred and five years.

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