Named after the 4^th Earl of Orrery, these models show how the planets and moons move. Below is my latest creation of an example that includes the earth, moon and sun. The "earth" used to be a pencil ornament. The "moon" is a plastic hydrogen model! The "sun" can be switched on to demonstrate, as the model operates, how lunar and solar eclipses occur.

The earth and moon drive in the model above is based upon the simple but very elegant instructions published in the June 1957 Meccano Magazine.  I used the basic framework from the April 1973 Meccano Magazine.  As can be seen I used a more modern motor.

At demonstrations I am often asked how tides effect the earth, so here goes:  As earth rotates on its axis the moon-facing tides sweep around the globe (the moon is moving too, but earth's rotation happens 30 times faster and so is more important).  This causes friction, so earth's rotation is gradually slowing.  Eventually earth's rotation will stop (with respect to the moon) and both bodies will be in lock step.  At this point one earth day will equal a lunar month.

The moon's orbit will also be affected.  For complicated reasons the moon will be pushed away from earth by 3.8cm/year in order to conserve the angular momentum lost by earth's reduced spin.  This will reduce the moon's orbital velocity, and in turn increase the length of a lunar month.

Even when the moon and earth are locked with the same face staring at each other, there will still be tides.  The sun produces tidal effects, so there would be a weak pair of tides circling the globe once a 'day'.  Over the really long term, this would slow down earth's rotation, until it was locked facing the sun.  But in practice this will never happen because the sun will blow up long before that and envelop the earth-moon system.

In case you are wondering, the background sound is from the song "Telstar".

Part 168

Next page, view other orrerys I have built.