An orrery is a mechanical device that demonstrates the relative positions and motions of the planets, moons, and sun in a solar system, usually in heliocentric fashion. The original term, dating back thousands of years, was planetarium. In 1704 the term orrery came into its own, and over time a planetarium was understood to be a darkened indoor theater on whose arched black ceiling tiny lights representing stars could be projected.
An orrery can be simple, showing only a few planets, or complex, showing an entire solar system. The intricate arrangement of gears and levers makes the orrery resemble nothing so much as an astronomical clock ~ which in fact it is.
Because they can be used to predict the motions of celestial bodies, orreries have played significant roles in at least two movies ~ the fantasy film The Dark Crystal in 1982, and the science fiction film Pitch Black in 2000.
With the advent of computers and the Internet, an orrery need no longer be mechanical. Here is a marvelous animated, interactive orrery. If you look closely, in each of the four outer corners are faint gray prompts ~
- upper left ~ speed slider, help, and music control
- upper right ~ set the date, and switch between months and the zodiac
- lower left ~ show/hide moon phase insert
- lower right ~ switch between Copernican (heliocentric) and Tycho Brahe (geocentric) views
Note that the model does not attempt to present the planets, moons, and sun in their true size relationships, nor their true distance relationships from each other. But it does faithfully show positions and motions. It is fun to play with, and presents learning possibilities for young (and not so young) minds.