This orrery was built in 1773. It includes all the planets that were known at the time—Mercury, Venus, Earth, Mars, Jupiter, and Saturn.
What is an Orrery? An orrery (awe-uh-ree) is a mechanical model of the solar system that shows the positions and motions of the planets and their moons. Orreries became popular in the 17th and 18th centuries, a time when educated people became very interested in science. Then and now, people use orreries to demonstrate how the planets orbit around the sun. One early example was made for an English nobleman, Earl of Orrery, which is how the model got its name.
Not all orreries include every one of the planets. The sun always sits in the center, but the number of planets going around it can vary. Earth usually appears, as do Mercury, Venus, Mars and Jupiter. Some orreries include all the planets, plus some of their moons as well. Most aren't built to scale, but all the machines correctly show how the planets orbit the sun.
This painting from 1766 shows a man explaining how an orrery works.
A Modern Orrery
You can see this orrery, called the Long Now, in a small museum in San Francisco, California. The model is eight feet high and shows the positions of the planets through Saturn. The orrery moves twice a day. It will take Earth 365 days to travel around the stationary sun, the same amount of time as the actual planet.
In some orreries, the only moving parts are people. A famous human orrery takes place in Ireland's Armagh Observatory. On the observatory's grounds, people can stand on steel disks that mark the orbits of planets, a dwarf planet, and two comets. As visitors "orbit" the sun, the learn how the solar system works.