An orrery is, simply, a 3-D model of the solar system with each planet on an arm that circles the sun. More interesting, and useful, orreries contain a clockwork mechanism that spins the planets around the sun to show their respective positions throughout their orbits. Some of the most advanced orreries contain not just the planets and the sun, but the different moons as well. More basic ones contain just the sun and a few planets, usually Mercury, Venus, Earth, Mars, and Jupiter. Another simplification is that the planets’ orbits are usually shown as circles, rather than the actual complex ellipses. Despite the elaborate care used to regulate the mechanics running the motion of these orreries, and the amazing artwork decorating them, they are rarely constructed to scale. Proper sizing would make them so inconveniently large as to not be practical.

Orreries have an interesting, and ancient, history. Cicero mentions an orrery made by a Greek philosopher named Posidonius in the first century B.C. He describes the orrery as having the sun, moon, and the five planets known at the time all moving in orbits showing their diurnal motion, which is the daily movement of the planets as seen from Earth. Posidonius probably uses the Antikythera mechanism for his orrery, though most likely simplifying it. On a sidenote, the Antikythera mechanism is a highly accurate astrological calculator dating from about 100 BC. No artifacts of similar complexity appear until nearly a thousand years later. (See page Antikythera Mechanism)

George Graham constructed the first modern orrery in 1704 and gave it, or its plans, to the famous instrument maker John Rowley. In 1712, Rowley made a copy for Prince Eugene of Savoy and later, another for his patron, Charles Boyle, fourth Earl of Orrery. That is where the device’s rather odd name comes from.
Graham’s first orrery only showed the relationships, minus scale, between the Earth, sun, and moon. However, with the Earl’s patronage and Rowley’s expertise, the model was eventually expanded to show all the planets and moons known at the time.

One particularly interesting orrery is the human model, located in the Armagh Observatory. In this orrery, people take the places of the various planets, moons, and comets by hopping from one large metal tile to another. The tiles are labeled with each planet’s information and the time, in Earth days, that the planet reaches that particular point in its orbit.


Another interesting orrery is the Long Now Foundation orrery. It is eight feet tall, and shows the planets Mercury through Saturn. Each planet is made from stone that most closely resembles it. For example, Jupiter is a banded sandstone. The planets are attached to gears, which are run by a binary calculation engine. To keep this orrery completely accurate, the machine runs through and updates the calculations once every twelve hours.

Orreries are still made today, but in this world of computer calculators, they are more for beauty that actual study. There are only a handful of instrument makers left who can even make these delicate models and the intricate machinery that runs them.

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