Editing Orrery (section)

== Workings ==
All orreries are ''[[planetarium]]s''. The term ''orrery'' has only existed since 1714. A ''grand orrery'' is one that includes the [[outer planet]]s known at the time of its construction. The word planetarium has shifted meaning, and now usually refers to hemispherical theatres in which images of the night sky are projected onto an overhead surface. 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 [[Astronomical transit|transits]] is called an [[astrarium]].

An orrery should properly include the Sun, the Earth and the Moon (plus optionally other planets). A model that only includes the Earth, the Moon, and the Sun is called a [[tellurion]] or tellurium, and one which only includes the Earth and the Moon is a [[lunarium]]. A [[jovilabe]] is a model of Jupiter and its moons.<ref name="OU_S100_22">{{cite book|last=Pentz|first=M.J.|title=The Earth, Its Shape, Internal Structure and Composition|publisher=[[Open University Press]]|location=Bletchley|date=1971|volume=OU_S100_22|isbn=978-0-335-02034-8}}</ref>
{{table alignment}}
{| class="wikitable col1left" style=text-align:right;
|+Key statistics of planets
!Planet!!Average distance<br>from Sun<br>([[Astronomical unit|AU]])!!Diameter<br>(in Earth diameters)!!Mass<br>(in [[Earth mass]]es)!!Density<br>(g/cm<sup>3</sup>)!!No. of<br>known<br>moons!!Orbital period<br>(Earth years)!!Inclination<br>to ecliptic<br>(degrees)!!Axial tilt<br>(degrees)!!Rotational period<br />[[Sidereal orbital period|(sidereal)]]
|-
|[[Mercury (planet)|Mercury]]||0.39||0.38||0.05||5.5||0||0.24||7.0°||0°||59 days
|-
|[[Venus]]||0.72||0.95||0.82||5.3||0||0.62||3.4°||177°||–243 days‡
|-
|[[Earth]]||1.00||1.00||1.00||5.5||1||1.00||0°||23°||23.9 hours
|-
|[[Mars]]||1.52||0.53||0.11||3.9||2||1.88||1.9°||25°||24.5 hours
|-
|[[Jupiter]]||5.20||11.21||317.9||1.3||97||11.9||1.3°||3°||10 hours
|-
|[[Saturn]]||9.54||9.45||95.2||0.7||274||29.5||2.5°||27°||11 hours
|-
|[[Uranus]]||19.2||4.01||14.5||1.3||28||84||0.8°||98°||−17 hours‡
|-
|[[Neptune]]||30.1||3.88||17.1||1.6||16||165||1.8°||28°||16 hours
|}

‡ A negative figure indicates retrograde rotation.

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 [[Barycenter|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 will show the Earth's Moon, rotating around the Earth.<ref name="Adler">{{cite web|url=http://64.107.216.64/research/collections/instruments/orreries.shtml|title=Adler Planetarium:Research Collections|date=2010|publisher=Adler Planetarium.|access-date=22 June 2011|location=1300 South Lake Shore Drive • Chicago IL 60605|url-status=dead|archive-url=https://web.archive.org/web/20120127045601/http://64.107.216.64/research/collections/instruments/orreries.shtml|archive-date=27 January 2012}}</ref> A lunarium is designed to show the [[Orbit of the Moon|complex motions of the Moon]] as it revolves around the Earth.

Orreries are usually not built to [[scale model|scale]]. Human orreries, where humans move about as the planets, have also been constructed, but most are temporary. There is a permanent human orrery at [[Armagh Observatory]] in [[Northern Ireland]], which has the six ancient planets, [[Ceres (dwarf planet)|Ceres]], and comets [[Halley's Comet|Halley]] and [[Comet Encke|Encke]]. Uranus and beyond are also shown, but in a fairly limited way.<ref name="Armagh Observatory">{{cite web |title=Human Orrery |url=https://www.armagh.space/planetarium/attractions/human-orrery |url-status=live |archive-url=https://web.archive.org/web/20230402022124/https://www.armagh.space/planetarium/attractions/human-orrery |archive-date=2023-04-02 |website=[[Armagh Observatory and Planetarium]]}}</ref> Another is at Sky's the Limit Observatory and Nature Center in [[Twentynine Palms, California]]; it is a true to scale (20 billion to one), true to position (accurate to within four days) human orrery. The first four planets are relatively close to one another, but the next four require a certain amount of hiking in order to visit them.<ref>{{Cite web|url=https://www.skysthelimit29.org/orrery.html|title=Orrery|website=Sky's The Limit Observatory & Nature Center}}</ref> A census of all permanent human orreries has been initiated by the French group F-HOU with a new effort to study their impact for education in schools.<ref name="Site des planétaires humains">{{Cite web|url=http://planetaire.over-blog.com/en|title=The Human Orrery|website=planetaire.over-blog.com}}</ref> A map of known human orreries is available.<ref name="Carte des planétaires humains">{{Cite web|url=https://www.google.com/maps/@2.5507389,-77.2359853,3z/data=!3m1!4b1!4m3!11m2!2sk4G9ui5ZqmKbfaYxNOVoKuqmMKqjow!3e3|title=Emmanuel Rollinde's "Planetaires Humains - Human Orreries" List|website=Emmanuel Rollinde's "Planetaires Humains - Human Orreries" List}}</ref>

A normal mechanical clock could be used to produce an extremely simple orrery to demonstrate the principle, 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. As Jupiter's actual year is 11.86 Earth years long, the model would lose accuracy rapidly.

===Projection===
Many [[planetarium]]s have a [[Planetarium projector|projection orrery]], which projects onto the dome of the planetarium a Sun with either dots or small images of the planets. These usually are limited to the planets from Mercury to Saturn, although some include Uranus. The light sources for the planets are projected onto mirrors which are geared to a motor which drives the images on the dome. Typically the Earth will circle the Sun in one minute, while the other planets will complete an orbit in time periods proportional to their actual motion. Thus Venus, which takes 224.7 days to orbit the Sun, will take 37 seconds to complete an orbit on an orrery, and Jupiter will take 11 minutes, 52 seconds.

Some planetariums have taken advantage of this to use orreries to simulate planets and their moons. Thus Mercury orbits the Sun in 0.24 of an Earth year, while [[Phobos (moon)|Phobos]] and [[Deimos (moon)|Deimos]] orbit Mars in a similar 4:1 time ratio. Planetarium operators wishing to show this have placed a red cap on the Sun (to make it resemble Mars) and turned off all the planets but Mercury and Earth. Similar approximations can be used to show [[Pluto]] and its five moons.