An equatorium (plural, equatoria) is an

astronomical Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest include ...

calculating instrument. It can be used for finding the positions of the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

, and

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

s without arithmetic operations, using a geometrical model to represent the position of a given

celestial body An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists within the observable universe. In astronomy, the terms ''object'' and ''body'' are of ...

History

In his comment on Ptolemy's ''Handy Tables'', 4th century mathematician Theon of Alexandria introduced some diagrams to geometrically compute the position of the planets based on Ptolemy's epicyclical theory. The first description of the construction of a solar equatorium (as opposed to planetary) is contained in

Proclus Proclus Lycius (; 8 February 412 – 17 April 485), called Proclus the Successor (, ''Próklos ho Diádokhos''), was a Greek Neoplatonist philosopher, one of the last major classical philosophers of late antiquity. He set forth one of th ...

's fifth-century work ''Hypotyposis'', where he gives instructions on how to construct one in wood or bronze. The earliest known descriptions of planetary equatoria are in the Latin translation of an early eleventh century text by Ibn al‐Samḥ and a 1080/1081 treatise by al-Zarqālī, contained in 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 Alfonso X (also known as the Wise, ; 23 November 1221 – 4 April 1284) was King of Castile, Kingdom of León, León and Kingdom of Galicia, Galicia from 1 June 1252 until his death in 1284. During the April 1257 Imperial election, election of 1 ...

in the thirteenth century. The ''Theorica Planetarum'' (c. 1261–1264) by Campanus of Novara is the earliest extant description of the construction of an equatorium in

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

Europe. 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.

Variations

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 Richard of Wallingford (1292–1336) was an English mathematician, astronomer, horologist, and cleric who made major contributions to astronomy and horology while serving as abbot of St Albans Abbey in Hertfordshire. Biography Richard was b ...

at the beginning of the 14th century. 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.

Astrolabe compared with equatorium

The roots of the equatorium lie in the

astrolabe An astrolabe (; ; ) is an astronomy, astronomical list of astronomical instruments, instrument dating to ancient times. It serves as a star chart and Model#Physical model, physical model of the visible celestial sphere, half-dome of the sky. It ...

. The history of the astrolabe dates back to roughly 220 BC in the works of

Hipparchus Hipparchus (; , ; BC) was a Ancient Greek astronomy, Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry, but is most famous for his incidental discovery of the precession of the equinoxes. Hippar ...

. 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. 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.

Uses

The equatorium can further be specialized depending on the

epicycle In the Hipparchian, Ptolemaic, and Copernican systems of astronomy, the epicycle (, meaning "circle moving on another circle") was a geometric model used to explain the variations in speed and direction of the apparent motion of the Moon, ...

. 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. 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 The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

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. A

horoscope A horoscope (or other commonly used names for the horoscope in English include natal chart, astrological chart, astro-chart, celestial map, sky-map, star-chart, cosmogram, vitasphere, radical chart, radix, chart wheel or simply chart) is an ast ...

of that era would have required the positions of seven astronomical objects, requiring nearly two hours of manual calculation time.

History

Variations

Astrolabe compared with equatorium

Uses

See also

References

Further reading