## 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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## Meteorites near you

Visible to anyone in the world

I've always assumed that meteorites are quite rare, and as far as ones of any size are concerned that's true. But I was surprised recently to find that thousands of very small "micrometeorites" land on the Earth's surface constantly. Typically they are less than a millimetre in size and weight only a tiny fraction of a gram. If they enter the atmosphere at the right angle and velocity they may suffer some heating but still survive to ground level.

How common are they? Very. A back-of-an envelope calculation suggests that one falls on each square meter about once a fortnight on average. Think of that! You have probably added one to your stock of meteorites during the course of today.

They have been collected in places such as polar regions and in certain geological strata. But finding them in inhabited parts of the world is challenging because of the swarms of other dust-sized particles, many of them of human origin. However recent painstaking research has found undisputed micrometeorites in urban gutters. These are promising sites, because they collect run-off, probably separate out heavier particles (think of panning for gold) and probably trap less contamination from road dust and the like.

If you wanted to find your own, how would you start? First get a neodymium magnet (like the one I mentioned in a previous post about iron in cornflakes, and easily obtained). Put this in a plastic bag and swirl it around in gutter gloop.

Then enclose this in another plastic bag, and pull the magnet out of the inner bag. Voila! Candidate particles will have been collected between the bags.

Now comes the hard part. Almost all will be metallic grains created by human activity. So you will have to examine your finds under a microscope and carefully pick out the stardust. Not at all easy, but luckily there is a book by a dedicated enthusiast.

Images from here

Permalink 3 comments (latest comment by Jon Hirst, Saturday, 28 Jan 2017, 19:00)
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