What is an orrey?
A model of the solar system which is designed to show the locations of different planets is called an orrey. An orrey also tells the relationship between planets. The orrey can be moved using gearing and an internal clock. The rotational patterns of various bodies in the solar system are demonstrated by an orrey. Planetariums is the another name given to these devices. Some people may confuse these planetariums or orreys with large domes on which different images are projected.
The concept of an orrey is quite ancient according to some archaeological evidences. The large ball at the center of an orrey is sun but the planets mad moons included in it vary widely. The common planets included are Jupiter and Earth. Other plants shown are Venus, Mercury and Mars while other ones may or may not be shown. It all depends on the taste of the person and on the intended use of an orrey to show certain planets or not. Moon may be included as well in a very complex orrey. As the moons are made in a way that they must orbit other planets and planets themselves orbit the sun so the cost of this type of orrey increases significantly.
The relative size of the objects in the solar system and their comparative distances prohibits the use of orreys as a scaling instrument. However, it is recommended that there should be accurate orbits of planets so that they can complete orbits accurately which will allow people to see when and how various planets interact with each other.
For demonstrating simple topics on astronomy, there are simple orreries in some classrooms. In these classes stationary models or orreries are used which needs to be moved by hand in order to explain things like transits and eclipses. Orrey can be seen commonly at an astronomy center or a science museum. Also in some museums, antique examples of orreries can be found.