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An equatorium (plural, equatoria) is an astronomical calculating instrument. It can be used for finding the positions of the Moon, Sun, and planets without calculation, using a geometrical model to represent the position of a given celestial body.


The earliest extant record of a solar equatorium, that is, one to find the position of the sun, is found in Proclus's fifth-century work Hypostasis,[1] where he gives instructions on how to construct one in wood or bronze.[2] Although planetary equatoria were also probably made by the ancient Greeks,[2] the first surviving description of one is from the Libros del saber de astronomia (Books of the knowledge of astronomy), a Castilian compilation of astronomical works collected under the patronage of Alfonso X of Castile in the thirteenth century, which includes translations of two eleventh century Arabic texts on equatoria by Ibn al‐Samḥ and al-Zarqālī.[2] Theorica Planetarum (c. 1261-1264) by Campanus of Novara describes the construction of an equatorium, the earliest known description in Latin Europe.[3]

Richard of Wallingford (1292–1336) is known to have built a sophisticated equatorium named Albion in 1326. It could calculate lunar, solar and planetary longitudes. Unlike most equatoria, the Albion could also predict eclipses.[4] The device is described in a manuscript and in drawings by the Abbot. It consisted of several rotating disks, showing the courses of the sun, moon and stars. These disks were operated manually. It was not a clockwork mechanism.


Al-Zarqali, was an Arab Muslim instrument maker, mathematician, and astronomer. He wrote a treatise on the construction of his improved equatorium in around 1080/1081 CE.[5] He was the first to show clearly that the motion of the solar apogee - when the sun is furthest from earth - is 12.0 seconds per year (the actual value is 11.8 seconds per year).[6] Al-Zarqali also corrected Ptolemy's work, calculating the length of the Mediterranean Sea as 42°, while Ptolemy said it was 62°.[7] His work was quoted multiple times in Nicolaus Copernicus's “De Revolutionibus Orbium Celestium” where he discussed the revolution of the celestial orbs.[5] Although Al-Zaqali's work was based on the belief that the earth was at the center of the universe, many of his primary concepts still apply to the modern world of astronomy.[8]

Al-Zarqali designed his equatorium using the Ptolomaic astronomical model, which held that the earth was at the center of the universe with the stars fixed to a large sphere surrounding it.[9] Ptolemy theorized that all of the stars moved around the earth every single day, and the motion of the sun, moon, and planets surrounded it. He believed that each planet moved on a small sphere or circle, called an epicycle, that moved on a larger sphere or circle, called a “deferent” which contains the earth at the center.[10] The equatorium was designed on the principles of epicycle and deferent.


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 at the beginning of the 14th Century.[11] 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.[11]

Astrolabe compared with equatorium[edit]

To understand where the roots of the Equatorium began, look back to what inspired it: the Astrolabe. The history of the Astrolabe dates back to roughly 220 BC and was invented by Hipparchus.[12] 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. There are also specialized astrolabes for land and for boats. The Astrolabe works by adjusting the moveable components (labels) to a specific date and time.[13] Afterward, line up the astrolabe with the horizon by holding it up and setting one end of the label to where one end is at eye level and the opposite end is pointed towards whichever celestial structure you want to view.[13] In contrast, the equatorium is a rarer astronomical instrument. It is used to calculate the past or future positions of the planets and celestial bodies according to the planetary theory of Ptolemy. What is similar about these tools is that they are both calculating devices that simplify and efficiently make geometrical calculations.


The equatorium can further be specialized depending on the 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.[7] 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 had the earth at the center. In fact, specialized equatoriums 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.[14] A horoscope of that era would have required the positions of seven astronomical objects, requiring nearly two hours of manual calculation time.

See also[edit]


  1. Proclus (1909). Hypotyposis Astronomicarum Positionum. Bibliotheca scriptorum Graecorum et Romanorum Teubneriana. Karl Manitius (ed.). Leipzig: Teubner. 
  2. 2.0 2.1 2.2 Evans, James (1998). The History and Practice of Ancient Astronomy. Oxford & New York: Oxford University Press. p. 404. ISBN 978-0-19-509539-5. 
  3. Toomer, G. J. (1971). "Campanus of Novara". in Gillispie, Charles Coulston. Dictionary of scientific biography. III. New York: Scribner. pp. 23–29. ISBN 978-0-684-10114-9. 
  4. Morrison, James E.. "Richard of Wallingford". History of Astronomy. 
  5. 5.0 5.1 "Zarqali". 
  6. "Abu Ishaq Ibrahim Ibn Yahya Al-Zarqali | Muslim Heritage" (in en). 
  7. 7.0 7.1 Colledge, Eric (1955). "THE EQUATORIUM OF THE PLANETS". Blackfriars 36 (424–5): 276–284. 
  8. "Al-Zarqālī | Spanish Muslim scholar" (in en). Encyclopedia Britannica. 
  9. "Equatorium" (in en-US). 
  10. Price, D. J. (2012). Equatorie of Planetis. Cambridge University Press. p. 128. ISBN 978-1-107-40427-4. 
  11. 11.0 11.1 Truffa, Giancarlo (in en). The Albion of Rome. A unique example of Medieval Equatorium. 
  12. "Third Solution: The Equant Point - SliderBase" (in en-US). 
  13. 13.0 13.1 "the definition of astrolabe". 
  14. Fosmire, Michael (2014). "Richard of Wallingford" (in en). Biographical Encyclopedia of Astronomers. Springer, New York, NY. pp. 1831–1832. doi:10.1007/978-1-4419-9917-7_1167. ISBN 978-1-4419-9916-0. 

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