Grand Orrery from 1780
Date: 2008/Aug/20 Wed
Grand Orrery, c. 1780, with clock dials
Unknown maker; James Ferguson type
Yew wood, brass, silvered brass, ivory
Sun, Mercury, Venus, Earth, and Moon
Adler Planetarium purchase, 1938
When the Adler Planetarium purchased this orrery in 1938, at a coast of $400, it was shipped from England with a 12-sided glass and wood case. It arrived in Chicago with the case smashed and the works damaged. Insurance covered some of the repair cost.
Find the four dials that sit on top of the gearworks. Two are simply clocks, twelve-hour and twenty-four-hour. A third is calibrated from 0 to 291/2. Can you guess what that dial shows? (Hint: how many days are in an astronomical month, from one new moon to the next?) The fourth dial is trickier. It runs from 0 to 5"17 and back to 0, twice. This is a "lunar latitude" dial. The greatest motion of the Moon north and south of the Earth's orbit is about 5 degrees and 17 minutes, and this dial keeps track of it. Eclipses are possible only when the latitude dial is near 0.