de'Dondi's dial for Mercury uses a number of intermediate wheels, including: a wheel with 146 teeth, two oval wheels with 24 irregularly shaped teeth that meshed together, a wheel with 63 internal (facing inwards) teeth that meshed with a 20 tooth pinion. The 63-tooth wheel made one rotation a year, non-uniformly because of the oval driving wheels, and made the main indicator wheel rotate through 63/20 × 12 signs of the zodiac each year.
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