## Earth and Venus

Visible to anyone in the world
Edited by Richard Walker, Wednesday, 15 Sep 2021, 01:10

This remarkable animation is from a lovely blog post by Guy Ottewell. I only offer it as a taster and strongly recommentd visiting https://www.universalworkshop.com/2016/06/07/five-petals-of-venus/ for the full story.

So what's going on? Well, we see a slightly simplified version of the motions of the Sun and Venus as seen from Earth (centre stage). It's simplified by making the objects move at uniform speed, and the orbits circular, but these are not wildy out; for exampleif you plotted the Earth's orbit on paper, it would to the human eye be industinguishable from a circle. The Earth is at the centre, the yellow circle represents the Sun and Venus is the smaller, white, circle.

Watching the animation you will see that it gradually unfolds as a pattern with five-fold symmetry. This reflects the fact that the length of a Venus year to an Earth is close to 5:13 and 13 - 8 = 5.

We are nowadays familiar with idea that both Venus and Earth, and all the other planets, revolve about the Sun, but for at least 2,000 years the world view was that the Earth stood still and everything else moved round it. This is not unreasonable, or even wrong, but it just makes the motion of the planet appear arbitrary and hard to account for, as we see from the motion of Venus in the animation.To describe it we need circles within circles within circles, 'epicycles', and although it utimately works, it gets extemely complicated, and the epicycles are like pulling a rabbit out of a hat.

Once we make that shift, to placing the Sun at the centre, it all becomes much simpler to describe. There is much still to explain; such as, the orbits are not actually circulr but ellipses, the speeds not the same all the way round the orbit, the reason for the different orbital periods is not understood; and Kepler wondered what make the planets move at all. But the heliocentric viewpoint is much simpler to deal with, and it paves the way for a better understanding of the Solar system, and then of the motion of celestial objects more widely.

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