A Mimic of the Sky

By Lawrence Biemiller

Rittenhouse Clock
Drevel University's astronomical clock

Philadelphia -- Sometime in the early 1770s, the renowned Pennsylvania astronomer and instrument maker David Rittenhouse built an eight-inch-wide solar system and set it in motion above the main face of a tall-case clock. Ever since -- provided the clock is kept wound -- the six tiny planets have dutifully orbited their diminutive Sun, each on a pace and path corresponding to that of its real-life counterpart (Uranus, Neptune, and Pluto had yet to be discovered). Just beyond the orbit of the clock's Saturn is a brass ring inscribed with the names of the constellations of the zodiac. Looking at it, anyone interested in peering up among the stars could tell where to search that night for a particular planet.

For most of the past 110 years, the six planets have been making their rounds in the main building at Drexel University, where the clock now stands in its Chippendale case at one end of a Victorian-era picture gallery. In addition to telling time, marking the month and day, and showing the locations of the planets, the clock tracks several esoteric astronomical phenomena -- and for good measure plays 10 different tunes on its chimes.

It is one of three known Rittenhouse astronomical clocks. Another is also in Philadelphia, at the Pennsylvania Hospital, and the third is in a private collection in York, Pa. Remarkably enough, all are still operational. Drexel's clock was given to the university in 1894 by the widow of the best friend of Anthony J. Drexel, the financier who founded the university. The clock is scheduled for a thorough cleaning and conservation this spring, after which the gallery's curator, Jacqueline M. DeGroff, plans a celebration in honor of the clock and its maker.

David Rittenhouse was born in 1732 into a family of paper manufacturers in what is now the Germantown area of Philadelphia. A friend of both Thomas Jefferson and Benjamin Franklin, he was a surveyor as well as an astronomer, an instrument maker, a legislator, and Pennsylvania's treasurer. He also served at various times as a trustee, vice provost, and professor of astronomy at the University of Pennsylvania. In 1792 George Washington appointed him as the first director of the United States Mint. The Smithsonian Institution's collection includes a telescope he built, along with two surveyor's compasses. He died in 1796.

Drexel's clock is believed to have been made about 1773, when Rittenhouse's reputation was at its height. He had recently completed two large orreries, or operating models of the known solar system (the name comes from the fourth Earl of Orrery, who commissioned one of the first such devices). Rittenhouse's orreries were wonders of their age, and both survive -- the first at Princeton University and the second at Penn, just a few blocks from Drexel. Penn's, the larger of the two, is the size of a substantial bookcase and has a star-studded solar system some three feet across. It was used in classes for decades and was at one point the central feature of the university's seal. It includes five of Saturn's moons and four of Jupiter's, and was so accurate that the university provost said of it: "It will not vary a Degree from the Truth in less than Six Thousand Years, if the present Order of Nature subsists." It no longer works, however. Princeton's has been rebuilt with an electric motor.

Unlike the orreries, the Drexel clock was intended for home use, but it is nevertheless extraordinary. "Beyond a doubt it's the most exceptional clock made in America in the 18th century," says Eric Chandlee Wilson, the clock conservator who will be doing the cleaning and making whatever other repairs are necessary. He says the clock appears to be "just about completely original" -- which is important, he says, because so many clocks have been the victims of bad repairs in the past. Like the orreries, the clock is "a testament to the range of intellectual achievement" in the colonies, he says.

It is also a testament to Rittenhouse's ambition. Miniaturizing the motions of the planets and fitting them into a clock was a considerable achievement -- it required both careful calculations and finely calibrated gears -- yet Rittenhouse was hardly content to stop there. A smaller face to the left of the main dial shows where in the zodiac the Sun and Moon appear at any given time when viewed from the Earth, the Sun being a brass ball at the end of one hand and the Moon being a smaller ball at the end of another. Ingeniously, the Moon rotates on its axis, alternately showing dark and light sides. By looking at the clock head-on, one can determine the phases of the real Moon from the miniature.

Or, of course, one could just look over to the lunarium in the clock's main face. There, a two-dimensional Moon -- painted with eyes, nose, and mouth -- revolves on a starry background in an opening that is shaped to show the Moon's phases. The main face also has hour, minute, and second hands, along with a hand pointing to the month of the year. The day of the month appears in a small window below the lunarium.

A dial below and to the left of the central face is believed to track the eccentricity of the Moon's elliptical orbit around the Earth, although Mr. Wilson says he thinks a gear may be missing from the mechanism.

He also says that part of the conservation plan for the clock is to learn more about both its functions and its history. For instance, the clock is reported to have been commissioned for the home of Joseph Potts, an ironmaker, but he is said to have refused it because it was too elaborate, and to have asked Rittenhouse for a wall clock instead. "I don't know if the entire, correct story has come down to us," Mr. Wilson says. Assuming Potts knew what kind of clock he was ordering, Mr. Wilson says, did Rittenhouse perhaps go "overboard" in adding features, just for the fun of it?

On the upper right side of the clock, a small dial labeled "Sun slower" and "Sun faster" indicates what's known as the equation of time. This is a function of Earth's being in an elliptical orbit around the Sun, as well as of Earth's tilting on its axis relative to the plane of its orbit. In practical terms, it's the difference between the identical 24-hour cycles counted out by clocks and the actual time at which the Sun is highest overhead every day -- noon as the Sun's height marks it is either running ahead of or behind noon as the clock strikes, and the two can be off by as much as 16 minutes. But like the eccentricities of the Moon's orbit, the equation of time is not something most people need to know when they glance at the hall clock.

Below the equation-of-time face is a dial for choosing among the clock's 10 tunes, which today have an ethereal quality. On either side of the face are levers for controlling how often the clock strikes, and how many tunes it plays. Three large weights power the clock's machinery. Over all, says Mr. Wilson, it is "a very complex mechanism." He expects to spend a couple of months dismantling and cleaning the works, replacing worn bearings and brushings, making other minor repairs, and then putting everything back together.

He and Ms. DeGroff, the Drexel curator, also plan to consult with astronomers to figure out how to reset the planetary machinery so that it does indeed correspond to that of the solar system. One of the challenges the clock presents is that the planets, after they are reset, remain accurate only as long as the clock is kept running constantly. While the lunarium can be reset easily enough, the planets are another matter altogether.

Some museums disapprove of running old clocks, Mr. Wilson acknowledges, but his own opinion is that if a clock is in good shape, it benefits from being run. And the Drexel astronomical clock, he says, is certainly in good enough shape. For her part, Ms. DeGroff says she looks forward to hearing the clock's chimes again in the gallery -- and to the celebration, of course. No doubt someone there will read the lines about Rittenhouse that appeared in a 1787 poem by Joel Barlow called the "The Vision of Columbus":

He marks what laws th'eccentric wand'rers bind,
Copies Creation in his forming mind,
And bids, beneath his hand, in-semblance rise,
With mimic orbs, the labours of the skies.

Copyright © 2005 by The Chronicle of Higher Education. Published February 4, 2005.