What is an Orrery?

An orrery is nothing more than a moving mechanical model of the solar system. It demonstrates the location and how quickly various planetary bodies are moving relative to one another as it runs. A basic orrery might show the 4 inner planets orbiting the sun, with the innermost Mercury going the fastest and each subsequent planet farther out and moving more slowly. The planets are typically each held on an arm, and network of gears ensures that each one is moving at the right speed. More complicated orreries may show moons moving around planets, the planets themselves rotating, and any number of other astronomical objects and phenomena.

Terminology

There are a few different terms that are worth defining

Orrery - A model of the solar system, showing the planets moving around the sun

• Grand Orrery - An orrery that includes all planets known at the time:

  • If constructed prior to 1781, six planets (Mercury, Venus, Earth, Mars, Jupiter, & Saturn)

  • From 1781 to 1846, seven planets (following the discover of Uranus)

  • From 1846 to 1930, eight planets (following the discovery of Neptune)

  • From 1930 to 2006, nine planets (following the discovery of Pluto)

  • After 2006, a Grand Orrery describes all 8 planets following Pluto’s reclassification as a dwarf planet. A Grand Orrery may contain dwarf planets but does not need to meet the definition.

• Tellurion - An orrery showing the Earth, Moon, and Sun

• Lunarium - An orrery showing the Earth and Moon

• Jovilabe - An orrery showing the four Galilean moons of Jupiter

Astrarium - A more general term for a clockwork driven device that can be used to estimate astronomical events such as eclipses and transits. The first design by Giovanni de' Dondi has been replicated at the Museo Nazionale Scienza e Tecnologia Leonardo da Vinci.

Astronomical Clock - An even more general term for any clock that additionally shows astronomical data, such at the Prague Orloj in Prague