Thursday, April 28, 2022

Which planet rotates the "wrong" way?

I read an interesting article today explaining that, while, the Earth, the Sun, the other planets, and the Solar System as a whole, all rotate in the same counter-clockwise direction, Venus alone rotates clockwise

This extraordinary fact is likely due to an ancient collision with another (unknown) celestial body, which struck the planet Venus a glancing blow at such an angle and with such speed and force as to stop its old counter-clockwise rotation (which was established from the rotation of the original protoplanetary cloud of dust and gas) and actually set it rotating the opposite way. The extremely slow speed of Venus' rotation - it takes 243 Earth days to complete a single rotation on its own axis - lends this hypothesis credence.

Fair enough. But, part way through reading the article, I had the sudden realization that, hold on, what does it actually mean to say that the Earth and the other planets rotate anti-clockwise? Sure, if you view the Earth from "above", i.e. from above the North Pole, it seems to rotate anti-clockwise, which is why the Sun seems to rise in the east and set in the west. But, just as surely, if you view it from "below" the South Pole, it would seem to be rotating in a clockwise direction. Try it yourself with a ball, or visualize it using the spinning globe below.

So, yes the planet Venus is unusual in that it's rotation is opposite to everything else in the Solar System. But let's be a bit more careful about the language we use. Granted, the article was a "popular science" one, and not aimed at a scientific audience, but still...

Incidentally, the Earth's rotation has been showing slightly, as a result of the tidal influence of the moon, and an Earth day has been lengthening by around 2 milliseconds. Except that, more recently (i.e. in the last half century or so), it has started spinning slightly (infinitesimally) slower. This is particularly the case over the last decade or so, and our ultra-accurate atomic clocks have not needed to add a "leap second" for some time now. The Earth's slowing down has slowed down, and we're not really sure why (cue dramatic music).

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