An orrery is a mechanical model of the Solar System that illustrates or predicts the relative positions and motions of the planets and moons.  The first orrery that was a planetarium of the modern era was produced in 1704, and one was presented to Charles Boyle (who penned the Ideal Gas Law), the 4th Earl of Orrery. These models are typically driven by a clockwork mechanism with a globe representing the Sun at the center and planet rotating about that sphere on small arms.

Largely self-taught, David Rittenhouse set up his own clock shop at the age of 19.   He quickly rose to local prominence, with the steady demand for his grandfather clocks.  He also became increasingly interested in astronomy and astronomical instruments which led naturally to the creation of a series of every more sophisticated clockwork driven orreries.  In 1770 Rittenhouse completed an advanced orrery for which the College of New Jersey, now Princeton, granted Rittenhouse an honorary degree.  The college then acquired ownership of that orrery. 

Rittenhouse then made a new, more advanced model for which colony of Pennsylvania paid him £300 as a tribute.  This orrery was used as a tool for teaching astronomy. The components of the orrery, including the hundreds of gears and cranks, as well as the beautifully-detailed carved wood panels.  It was designed to be both a functional and elegant instrument.  This model was larger and more complex than earlier orreries. Once wound with the clock mechanism, the machine would rotate the planets on mechanical arms into their proper relative positions on any given date, past or future. The central panel is a “grand orrery,” showing the movements of the entire known solar system, with the sun at its center, and Jupiter, Saturn, and their satellites on the two arms. The right panel, a lunarium, depicts the movements and eclipses of the moon. A third panel, intended to show Jupiter and Saturn in more detail, was probably never completed.  This orrery can be seen today at the Kislak Center on the sixth floor of the Van Pelt library at the University of Pennsylvania. 

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