# Equatorium

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Astronomical calculating instrument

For the region of South Sudan, see [Equatoria](/source/Equatoria).

Equatorium from [Johannes Schöner](/source/Johannes_Sch%C3%B6ner)

An **equatorium** (plural, **equatoria**) is an [astronomical](/source/Astronomy) [calculating instrument](/source/Mechanical_calculator). It can be used for finding the positions of the [Moon](/source/Moon), [Sun](/source/Sun), and [planets](/source/Planet) without arithmetic operations, using a geometrical model to represent the position of a given [celestial body](/source/Celestial_body).

## History

In his comment on Ptolemy's *Handy Tables*, 4th-century mathematician [Theon of Alexandria](/source/Theon_of_Alexandria) introduced some diagrams to geometrically compute the position of the planets based on Ptolemy's [epicyclical theory](/source/Deferent_and_epicycle). The first description of the construction of a **solar equatorium** (as opposed to planetary) is contained in [Proclus](/source/Proclus)'s fifth-century work *Hypotyposis*,[1] where he gives instructions on how to construct one in wood or bronze.[2]

The earliest known descriptions of **planetary equatoria** are in the Latin translation of an early eleventh-century text by [Ibn al‐Samḥ](/source/Ibn_al%E2%80%90Sam%E1%B8%A5) and a 1080/1081[3] treatise by [al-Zarqālī](/source/Ab%C5%AB_Is%E1%B8%A5%C4%81q_Ibr%C4%81h%C4%ABm_al-Zarq%C4%81l%C4%AB), contained in the *[Libros del saber de astronomia](/source/Libros_del_saber_de_astronomia)* (*Books of the knowledge of astronomy*), a [Castilian](/source/Spanish_language) compilation of astronomical works collected under the patronage of [Alfonso X of Castile](/source/Alfonso_X_of_Castile) in the thirteenth century.[2]

The *Theorica Planetarum* (c. 1261–1264) by [Campanus of Novara](/source/Campanus_of_Novara) is the earliest extant description of the construction of an equatorium in [Latin](/source/Latin) Europe.[4] Campanus' instrument resembled an astrolabe, with several interchangeable plates within a mater. The best manuscripts of Campanus' treatise contain paper and parchment equatoria with moveable parts.[2]

Early in 1393, the English [Benedictine monk](/source/Benedictines) [John Westwyk](/source/John_Westwyk)[5] completed his [Equatorie de Planetis](/source/The_Equatorie_of_the_Planetis), a manuscript containing original designs for a large equatorium, along with directions for its construction and a long series of astronomical tables calibrated for use with the device.[6] If built according to his instructions, Westwyk's equatorium would have measured 6 feet in diameter, allowing astronomers a much greater degree of precision in their calculations, but a full-scale model made to Westwyk's ideal specifications would have been prohibitively expensive during his lifetime, making it unlikely that his ideal iteration of the device was ever constructed.[7]

## Variations

A 1600 equatorium, possibly from France

The history of the equatorium does not just end after the 11th century, but it inspired a more diverse invention called “The Albion”. The Albion is an astronomical instrument invented by [Richard of Wallingford](/source/Richard_of_Wallingford) at the beginning of the 14th century.[8] It has various functional uses such as that of the equatorium for planetary and conjunction computations. It can calculate when eclipses will occur. The Albion is made up of 18 different scales which makes it extremely complex in comparison to the equatorium. The history of this instrument is still disputed to this day, as the only Albion from the past is both unnamed and unmarked.[8]

## Astrolabe compared with equatorium

The roots of the equatorium lie in the [astrolabe](/source/Astrolabe). The history of the astrolabe dates back to roughly 220 BC in the works of [Hipparchus](/source/Hipparchus).[9] The difference between the two instruments is that the astrolabe measures the time and position of the sun and stars at a specific location in time.[10] In contrast, the equatorium is used to calculate the past or future positions of the planets and celestial bodies according to the planetary theory of Ptolemy.[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*]

## Uses

The equatorium can further be specialized depending on the [epicycle](/source/Epicycle). There are three possible epicycles that can be adjusted to serve for planetary positions in three groups: the Moon, the stars, and the Sun. The Sun was considered a planet in the Ptolemaic system, hence why the equatorium could be used to determine its position.[11] Through the use of Ptolemy's model, astronomers were able to make a single instrument with various capabilities that catered to the belief that the [Solar System](/source/Solar_System) had the Earth at the center. In fact, specialized equatoria had astrological aspects of medicine, as the orientation of planets gave insight to zodiac signs which helped some doctors cater medical treatments to patients.

At least 15 minutes was needed to calculate the planetary position with the use of a table for each celestial body.[12] A [horoscope](/source/Horoscope) of that era would have required the positions of seven astronomical objects, requiring nearly two hours of manual calculation time.

## See also

Wikimedia Commons has media related to [Equatorium](https://commons.wikimedia.org/wiki/Category:Equatorium).

- [Antikythera mechanism](/source/Antikythera_mechanism)

- [Armillary sphere](/source/Armillary_sphere)

- [Astrarium](/source/Astrarium)

- [Astrolabe](/source/Astrolabe)

- [Astronomical clock](/source/Astronomical_clock)

- [Orrery](/source/Orrery)

- [Planetarium](/source/Planetarium)

- *[The Equatorie of the Planetis](/source/The_Equatorie_of_the_Planetis)*

- [Torquetum](/source/Torquetum)

## References

1. **[^](#cite_ref-1)** Proclus (1909). *Hypotyposis Astronomicarum Positionum*. Bibliotheca scriptorum Graecorum et Romanorum Teubneriana. Karl Manitius (ed.). Leipzig: [Teubner](/source/Teubner).

1. ^ [***a***](#cite_ref-Evans_2-0) [***b***](#cite_ref-Evans_2-1) [***c***](#cite_ref-Evans_2-2) Evans, James (1998). *The History and Practice of Ancient Astronomy*. Oxford & New York: [Oxford University Press](/source/Oxford_University_Press). p. 404. [ISBN](/source/ISBN_(identifier)) [978-0-19-509539-5](https://en.wikipedia.org/wiki/Special:BookSources/978-0-19-509539-5).

1. **[^](#cite_ref-Pulg2_3-0)** ["Zarqali"](https://islamsci.mcgill.ca/RASI/BEA/Zarqali_BEA.htm). *islamsci.mcgill.ca*. Retrieved 2018-05-09.

1. **[^](#cite_ref-Toomer_4-0)** [Toomer, G. J.](/source/G._J._Toomer) (1971). "Campanus of Novara". In Gillispie, Charles Coulston (ed.). *Dictionary of scientific biography*. Vol. III. New York: Scribner. pp. 23–29. [ISBN](/source/ISBN_(identifier)) [978-0-684-10114-9](https://en.wikipedia.org/wiki/Special:BookSources/978-0-684-10114-9).

1. **[^](#cite_ref-:12_5-0)** Rand, Kari Ann (2015). ["The Authorship of the Equatorie of the Planetis Revisited"](https://doi.org/10.1080/2F00393274.2014.982355). *Studia Neophilologica*. **87** (1): 15–35. [doi](/source/Doi_(identifier)):[10.1080/2F00393274.2014.982355](https://doi.org/10.1080%2F2F00393274.2014.982355).

1. **[^](#cite_ref-:11_6-0)** Falk, Seb (2020). *The Light Ages: A Medieval Journey of Discovery*. London: Penguin. [ISBN](/source/ISBN_(identifier)) [978-0241374252](https://en.wikipedia.org/wiki/Special:BookSources/978-0241374252).

1. **[^](#cite_ref-:10_7-0)** Price, Derek de Solla (1955). *The Equatorie of the Planetis*. Cambridge: Cambridge University Press. p. 67. [ISBN](/source/ISBN_(identifier)) [9781107404274](https://en.wikipedia.org/wiki/Special:BookSources/9781107404274). {{[cite book](https://en.wikipedia.org/wiki/Template:Cite_book)}}: ISBN / Date incompatibility ([help](https://en.wikipedia.org/wiki/Help:CS1_errors#invalid_isbn_date))

1. ^ [***a***](#cite_ref-:3_8-0) [***b***](#cite_ref-:3_8-1) Truffa, Giancarlo. ["The Albion of Rome. A unique example of Medieval Equatorium"](https://www.academia.edu/17412951).

1. **[^](#cite_ref-9)** ["Third Solution: The Equant Point - SliderBase"](http://www.sliderbase.com/spitem-1163-5.html). *www.sliderbase.com*. Retrieved 2018-05-09.

1. **[^](#cite_ref-:4_10-0)** ["the definition of astrolabe"](http://www.dictionary.com/browse/astrolabe). *Dictionary.com*. Retrieved 2018-05-09.

1. **[^](#cite_ref-:0_11-0)** Colledge, Eric (1955). "The Equatorium of the Planets". *Blackfriars*. **36** (424–5): 276–284. [doi](/source/Doi_(identifier)):[10.1111/j.1741-2005.1955.tb00671.x](https://doi.org/10.1111%2Fj.1741-2005.1955.tb00671.x). [JSTOR](/source/JSTOR_(identifier)) [43816789](https://www.jstor.org/stable/43816789).

1. **[^](#cite_ref-:2_12-0)** Fosmire, Michael (2014). "Richard of Wallingford". *Biographical Encyclopedia of Astronomers*. Springer, New York, NY. pp. 1831–1832. [doi](/source/Doi_(identifier)):[10.1007/978-1-4419-9917-7_1167](https://doi.org/10.1007%2F978-1-4419-9917-7_1167). [ISBN](/source/ISBN_(identifier)) [978-1-4419-9916-0](https://en.wikipedia.org/wiki/Special:BookSources/978-1-4419-9916-0).

## Further reading

- [Seb Falk's blog: making a planetary equatorium](http://astrolabesandstuff.blogspot.co.uk/2012/10/my-weekend-as-medieval-craftsman.html)

[Portals](https://en.wikipedia.org/wiki/Wikipedia:Contents/Portals):
- [Astronomy](https://en.wikipedia.org/wiki/Portal:Astronomy)
- [Outer space](https://en.wikipedia.org/wiki/Portal:Outer_space)
- [Solar System](https://en.wikipedia.org/wiki/Portal:Solar_System)
- [Spaceflight](https://en.wikipedia.org/wiki/Portal:Spaceflight)
- [Stars](https://en.wikipedia.org/wiki/Portal:Stars)

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Adapted from the Wikipedia article [Equatorium](https://en.wikipedia.org/wiki/Equatorium) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Equatorium?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.