Planetary models or planetaria are general terms for three-dimensional models of the solar system or of the earth, the moon and the sun. The three major types of planetaria are the tellurian (tellurium), the orrery and the armillary sphere.

Tellurians (Telluriums)
A tellurian (also called tellurium) is a mechanical astronomical demonstration device used to show seasons, eclipses and other astronomical phenomena relating to these bodies. It is generally operated by clockwork gears and chains, or string pulleys, and is comprised of a small terrestrial globe turning at the end of an arm, spherical painted wood representations of the moon and the planet Venus, and a central brass or gilt wood sphere representing the sun above the stand. Often a zodiac scale is affixed to the base. When operated, at relative representational rates, the earth spins once a day and revolves around the sun once a year, the moon revolves around the earth once a month, and Venus revolves around the sun.

Restoring Old Planetaria
The Trippensee Company used to sell replacement parts for planetaria, including their older H and S models. They since have been taken over by Science First. For more information, contact them.

Orreries
An orrery is a mechanical astronomical demonstration device used to shown the relative sizes and motions of the known planets in the solar system. It is generally operated by clockwork gears, and is comprised of spheres on wire armatures representing the planets (and their moons) that turn at relative representational rates around the central brass or gilt wood sphere representing the sun, above the stand.

Armillary Spheres
Armillary spheres have concentric rings to indicate planetary orbits, the zodiac band of constellations, and terrestrial and celestial measurement circles such as the Tropics of Cancer and Capricorn and the Equator. Sometimes they are mounted with an orrery inside. Sometimes they are mounted as garden sundials.

A Ptolemaic armillary sphere has an earth globe at the center, surrounded by celestial circle and zodiac armillary rings, demonstrating the geocentric theory of the universe developed by Ptolemy and others in ancient Greece and Rome. A Copernican armillary sphere has a sun ball at the center, with planetary and zodiac armillary rings, demonstrating the modern theory of the solar system, first popularized by Copernicus.