A Copernican Orrery

An Orrey is a mechanical representation of the motion of the planets that make up our solar system. The name comes from the 4th Earl of Orrery to whom was presented an early instrumemt.

This particular orrery was inspired by the work of Brother Bernadin of the Ploermel monastery in France. Starting in 1850 he constructed an astronomical clock that also included a complex orrery wich showed the first six planets, the earth's moon and four moons of Jupiter, all moving at their correct relative rates.This orrery can still be seen today in Ploermel. It is rather large

The version I have constructed is a Copernican Orrery because it visualises only the first five planets in the solar system. So called because Copernicus could only see 5 planets through his telescopes.

The orrery is made with hand cut wooden gears arranged that 24 gears driving the planets are linked in series with  an extra two for the earth's moon.

The gears are driven from the top down by the revolving sun which in turn is shaft driven through the central column from a reduction drive and two bevel gears in the base. This means that 4 rotations of the handle causes one sun rotation or about 58 revolutions for an earth year. Saturn requires about 1,700 revolutions of the driving wheel!

The driving handle (a gear motif) is deliberately made so that it can only be operated by fingertip in order to avoid the "how fast will it go syndrome?".

The calculation of the gear ratios was carried out by a home produced computer program and the relative orbital period of each component of the orrery is correct to 0.5% or better.

Below is a table showing the real space orbital periods  in days and the relative values achieved in my orrery. For the sun its rate of revolution is shown.

	Real	Orrery	Precision
Sun	25.417	25.417	1
Mercury	87.981	88.017	1.0004
Venus	223.618	223.535	0.9996
Earth	365.347	364.715	0.9983
Moon	27.322	27.316	0.9998
Mars	685.853	683.841	0.9971
Jupiter	4332.59	4329.319	0.9992
Saturn	10759.23	10720.217	0.9964