A letter about the Orrery
September 13, 2001
I was thrilled to see the gorgeous photo of the Rittenhouse orrery in the September 12 PAW (Letter from the Editor). As an undergraduate astrophysics major during the early 70s, I used to pass this wondrous blue dial every day on the way to my basement cubicle in Peyton Hall. In researching my new book Parallax: The Race to Measure the Cosmos, I discovered that the orrery is even more of a mechanical marvel than PAW's brief description allowed. Its scaled-down "planets" move in elliptical orbits, shaped and oriented to one another, just like the real ones. Surrounding the orrery's face is a driven (now motorized) brass ring engraved with celestial coordinates, such that the progress of each planet against the heavens can be read directly. The ring rotates one degree every 72 years to correct the coordinate readings for the slow gyration of the Earth's axis. The Rittenhouse orrery is a mechanical time machine: Turn a winch and you will see how the planets were arrayed up to 5,000 years in the past or the future. Historical eclipses and eclipses yet to come can be observed and their dates recorded.
David Rittenhouse had intended the orrery for the University of Pennsylvania (then the College of Philadelphia), but sold it instead to Princeton (College of New Jersey) when Princeton president John Witherspoon arrived with cash in hand. A second orrery was built and delivered in short order to Philadelphia, where it now resides in the University of Pennsylvania's Van Pelt Library. The genius of David Rittenhouse is alluded to in Joel Barlow's 1787 epic poem "The Vision of Columbus":
He marks what
laws th'eccentric wand'rers bind,
Alan W. Hirshfeld 73
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