Six Planet Orrery by Fred Jaggi of Cranston, RI (firstname.lastname@example.org)
An orrery is not, of course, an astrolabe, but Fred’s accomplishment in making a working model deserves exposure. Following is Fred’s description.
Peter Grimwood, a present-day orrery maker, gave me a diagram and wheel layout for a planetarium representing the six planets known since antiquity. The design is in the style of Benjamin Martin of Fleet St. ca 1750. It contains 14 gears, which allow the planets to orbit the sun in a steady circular path with an orbital period within 1/10th of one percent of the actual mean period. In those days, when calculations were done by hand, makers strived to calculate gear trains that would accurately represent natural orbits and the more advanced orreries produced elliptical paths and showed more planets and their satellites.
Last winter’s project was to build Grimwood’s planetarium. Unlike a clock, most of the gears are arraigned in two stacks set at a fixed distance. A third shaft is fitted to accommodate the high gear ratio needed to represent Saturn’s orbit. Six different modules, diametral pitch (DP), were used to allow the gears to mesh correctly. I had to make a fly cutter for each of the modules. The gears were cut using a homemade ball bearing spindle made up from pipe fittings and a dividing head. The tooth numbers on two of the gears are prime. For these, I pasted Excel spreadsheet grids on wooden disks attached to the dividing head. James E. Morrison adapted one of his astrolabe programs for my dial. I first thought about having the dial laser engraved locally, but the engravers said they were not able to laser engrave brass. I then turned to Hockerill Engraving in Devon, England who produced an inexpensive brass dial by chemical etching, which I then silvered. The dial has deep, sharp indentations, equivalent to mechanical engraving.
One turn of the dial represents two weeks motion and it’s fun to watch Mercury race around in 87 days while Saturn moves at the almost imperceptible rate of nearly 30 years.