Eric Watson, Saddleworth, England, 1986

I have taken photos of the orrery off its stand so as to have a white background to better expose the machine.

This photo shows the mechanism that Watson used to derive the zodiac precession which has a period of 25,800 years. The blue carriage next to the calendar indication pointer has a four lobed cam mounted on the underside. This cam is moved one quarter turn each year as it passes over a pin mounted to the orrery frame below and between the calendar and zodiac chapter rings (this was broken off). That cam is connected to a worm gear below the blue carriage and that worm is connected to the pinion seen which is then connected a second worm gear that meshes with the toothed perimeter on the calendar ring. With this mechanism Watson was able to derive a 25,800 year period for the zodiac ring from the one year rotation of the calendar ring. In my opinion a brilliant design.