Put the Moon in orbit

A few weeks ago I came across a page asking 'Can you put the moon in orbit?' (you may need a modern web browser to see this). The page uses scaleable vector graphics (SVG) and Javascript to make quite a nice, basic simulation of an elliptical orbit, but I was slightly worried by the political/theological conclusion drawn from such a simple simulation. In this simulation, if you are to get the Moon in a circular orbit you will need it to start on a circular orbit. The website concludes that "it [the Moon] began its journey in a circle on the circle" and therefore was always like that - a static, unchanging universe apparently 'proved' by a Javascript animation. Unsurprisingly, this argument has made rather a lot of assumptions.

First of all the simulation assumes only two objects and side-steps the issue of more objects (such as would be expected in standard solar system formation theories) by stating that "introducing more particles does not solve the problem it just gives you more problems". Yes it does. Mainly, it makes perfectly circular, unperturbed orbits such as in the animation very difficult to sustain. It also makes life more difficult for the coder who made the animation as they actually have to think a bit more.

Secondly, the simulation simplifies things again by treating the Earth and the Moon as points, even though they get represented by circles and a 'bounce' has been included when these circles touch. This makes for a nice looking simulation but it ignores an important effect; that of tides. If you have ever been to the coast you will have experienced the tides caused by the Moon on the water, but the Moon also creates a smaller effect in the rock on the Earth (and vice versa but that is another story). Constantly stretching and squashing the Earth (even slightly) converts some of the energy from the orbit into heat in the Earth. The results is that the Moon is moving away from the Earth at about 3.8 cm per year. This has been measured by people with telescopes and lasers.

So this simulation is waaaay too simple to be taken as an exact copy of reality. If you were to do a more complicated simulation, including more physics, you could see that a giant impact can generate a Moon like ours with many of the same orbital properties.

As ever, you can't take everything on the internet at face value. It is useful to be a bit sceptical, especially in these days when certain political or religious views try to masquerade as science.

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Posted in astro blog by Stuart on Sunday 05th Feb 2006 (04:36 UTC) | Permalink
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