orrery

click the image to enlarge it

During my last holiday in England I paid a visit to the Greenwich Observatory Museum, where I saw a couple of orreries. Having no camera with me, I tried to fix their design in my mind. Back home I realized I could remember very little about their construction. Wishing to built an orrery my first step was to make a drawing.

I found a visit to the local public library very helpful in finding out what really happens above our heads. After many calculations I was able to fix the number of teeth for each wheels of every train.

These numbers are shown in table 1.

The orrery is moved by four trains of wheels.

The first wheels train is part of the clock. All the wheels are 0.6 module.Their count was dictated by the pendulum length, this in turn being dictated by the size of the orrery.

The eight-day clock has an anchor escapement and the pendulum beats 140 vibration/min.

Table 1

The clock wheels train is placed in an horizontal frame, the escape wheel and pallets housed on the very edge. The second set of wheels turns the earth on its axis, making one revolution every 24 hours.

The third wheel train is made up of two separate sets: one moves the main arm which carries the earth and the moon and rotates once every 365 days; the other keeps the earth's tilted axis constantly pointing to the same direction (it is this tilt which causes the seasons to change).

The fourth and last wheel train moves the moon around the earth once every 29 days 12 hours 44 minutes and nine seconds.

This is an error of about 51 seconds: not too bad if you consider this error is approximately 20 parts in a million!

Let me digress for a moment about the wheels. When I realized the need for both 73 and 113 (prime numbers) hole circles on the dividing plate (to cut teeth with 365 and 113 teeth) I went to a professional workshop to place an order for a plate.

earth & moon gears

side view

earth & moon

pendulum & rear view

They looked at me as if I came from another planet, but when I explained what the plate was for and my moderate mechanical skill come out, I was told: '' the computer drawing is here and the XY co-ordinate milling machine is there .If you want the plate, get going!'

After one whole day on the milling machine and the payment of a very moderate sum I had my precious 73 and 113 hole dividing plate.

The main plate of the orrery has 300 teeth. On this plate there are engraved two circles: one with the number of the days and the names of the months; the other one with the signs of the Zodiac.

When I asked a professional to engrave these numbers and names on the plate I was asked for an astronomical (excuse the pun) sum, so I decided to do the work myself.

I built a simple jig and hand punched all the numbers and names, a long and very tedious job! The larger of the two rings around the earth shows where the sun rises and sets. The area between the two rings is the one illuminated before and after sunrise and sunset.