Explanation
All orreries are planetariums or planetaria (alternative plural). The term orrery has only existed since 1714. A grand orrery is one that includes the outer planets known at the time of its construction. The word planetarium, has been captured and now usually refers to hemispherical theatres in which images of the night sky are projected onto an overhead surface. Planetariums (orreries) can range widely in size from hand-held to room-sized. An orrery is used to demonstrate the motion of the planets, while a mechanical device used to predict eclipses and transits is called an astrarium.
An orrery should properly include the sun, earth and the (earth's) moon (plus optionally other planets.) A model that only includes the earth, its moon and the sun is called a tellurion, and one which only includes the earth and moon a lunarium. A jovilabe is a model of Jupiter and its moons.
| Planet | Mean Distance from the sun |
Diameter | Mass | Density | No. of satellites | Orbital period | Inclination to ecliptic |
Inclination of equator |
Rotation rate |
|---|---|---|---|---|---|---|---|---|---|
| Mercury | 0.39 AU | 0.4 Earth diameter | 0.05 Earth mass | 5.1 kg/m³ | 0 | 0.24 | 7.0 | ? | 59 days |
| Venus | 0.72 | 1.0 | 0.9 | 5.3 | 0 | 0.61 | 3.4 | 23 | 243 days |
| Earth | 1.00 | 1.0 | 1.0 | 5.52 | 1 | 1.00 | 0 | 23 | 1 day |
| Mars | 1.52 | 0.5 | 0.11 | 3.94 | 2 | 1.88 | 1.9 | 24 | 24.5 hours |
| Jupiter | 5.20 | 11 | 318 | 1.33 | 12 | 11.86 | 1.3 | 3 | 10 hours |
| Saturn | 9.54 | 9.54 | 95 | 0.69 | 10 | 29.46 | 2.5 | 27 | 10 hours |
| Uranus | 19.18 | 4 | 15 | 1.56 | 5 | 84.01 | 0.8 | 98 | 11 hours |
| Neptune | 30.06 | 4 | 17 | 2.27 | 2 | 164.79 | 1.8 | 29 | 16 hours |
| Pluto | 39.44 | 0.5? | ?0.1 | ? | 0 | 247.69 | 17.2 | ? | 6 days |
A planetarium will show the orbital period of each planet and the rotation rate, as shown in the table above. A tellurion will show the earth with the moon revolving around the sun. It will use the angle of inclination of the equator from the table above to show how it rotates around its own axis. It was show the earths moon, rotating around the earth. A lunarium is designed to show the complex motions of the moon as it revolves around the earth.
Orreries are usually not built to scale. Some fixed Solar System scale models have been built and are often many kilometres in size. A normal mechanical clock could be used to produce an extremely simple orrery with the Sun in the centre, Earth on the minute hand and Jupiter on the hour hand; Earth would make 12 revolutions around the Sun for every 1 revolution of Jupiter. Note however that Jupiter's actual year is 11.86 Earth years long, so this particular example would lose accuracy rapidly. A real orrery would be more accurate and include more planets, and would perhaps make the planets rotate as well.
Read more about this topic: Orrery
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