The Earth and the MoonHome

This is a simulation of the Moon rotating about the Earth as seen from the Sun.  There is a lot wrong with the simulation.  The relative sizes of the Earth and the Moon are about right, but their separation is a lot less than it should be.  Also I have slowed down the rotation of the Earth by a factor of 10.  The successive frames of the animation show the Moon at intervals of one day, but I wanted to show the rotation of the Earth as well.  There are ten frames in the animation of the Earth and to have made things right I would have had to make the whole animation ten times bigger (300 frames) and the size of the file would have been much larger too.

The simulation does, however, illustrate a few of the key factors about the Earth-Moon system.

1.  The Moon is very large relative to the Earth.  If we discount Pluto as major planet, then the Moon is the largest satellite relative to its primary of any of the major planets.

2.  The Moon revolves around the Earth in the same direction as the rotation of the Earth (anti-clockwise as seen from above the north pole) and this is the same direction of rotation of most the planets in the solar system.  (The major exceptions are Venus and Uranus.)

3.  As it revolves around the Earth, the Moon keeps the same face towards the Earth at all times (more or less - see my discussion of Libration).  Its rotation is said to be locked or synchronous.  However the Earth does not present the same face to the Moon at all times.  (Thank goodness for that or half of us would never see it.)  Over the aeons since the Moon was formed (about 4,550 million years ago) it has slowly been receding from the Earth and the Earth's rotation period has been increasing, so maybe my simulation shows the situation as it was a few thousand million years ago.  These changes are caused by the tides which produce a drag on the rotation of the Earth, slowing it down, and the energy is transferred to the Moon, driving it further away.  This effect is still occurring today and the Moon is receding from the Earth by about 3.8 centimetres per year and the Earth's rotation is slowing by about 5 milliseconds per year.

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