An orrery is a mechanical model of the solar system that illustrates or predicts the relative positions and motions of the planets and moons, usually according to the heliocentric model. It may also represent the relative sizes of these bodies; but since accurate scaling is often not practical due to the actual large ratio differences, a subdued approximation may be used instead. Though the Greeks had working planetaria, the first orrery that was a planetarium of the modern era was produced in 1704, and one was presented to Charles Boyle, 4th Earl of Orrery — whence came the name. They are typically driven by a clockwork mechanism with a globe representing the Sun at the centre, and with a planet at the end of each of the arms.
is a rare old word, designating a mechanism similar to the
orrery, but with only the planet Earth is rotating around the
The word "Lunarium" is sometimes used to speak
about a mechanism where only the Moon rotates around the Earth,
with no Sun. It is equivalent to a globe equiped with a little
sphere representing the Moon.
For more simplicity, we can rank these different words in a
frame, presenting the different parts of the solar system used
in the mechanism :
Moon <Earth ; Moon < Earth <Sun ; Planets < Sun