From the western historical perspective, it is sometimes easy to focus on the astronomical works of early Greek astronomers too tightly, while overlooking the worthy contributions made by earlier philosophers from other cultures that have also greatly added to the development of astronomy as we recognize it today. For this reason, this list begins with some brief details on the work of four major Greek astronomers from antiquity, before moving on to exploring some other notable astronomers from other ancient cultures around the world.

**Western World** (310 BC to 170 AD)

**Aristarchus of Samos**

Lived: 310-230 BC

Country: Samos (Greek)

Aristarchus of Samos is almost universally accepted as having developed the first known theoretical model of the Universe, with the Sun as the centre, and Earth revolving around it. Although much of his work appears to have been influenced by Philolaus of Croton, Aristarchus nevertheless associated the “central fire” with the Sun, and placed the other planets in their correct order, and at their approximately correct distances from the Sun.

The following translation by Thomas Heath of a text, “The Sand Reckoner” written by Archimedes’, in which he describes the work of Aristarchus, perhaps describes Aristarchus’ contribution to modern astronomy best. After explaining that astronomers considered the “universe” to be a sphere centred on the Earth, and whose radius was equal to a straight line running from the Earth’s centre to the Sun, he then explains:

“His hypotheses are that the fixed stars and the sun remain unmoved, that the earth revolves about the sun on the circumference of a circle, the sun lying in the middle of the orbit, and that the sphere of the fixed stars, situated about the same centre as the sun, is so great that the circle in which he supposes the earth to revolve bears such a proportion to the distance of the fixed stars as the centre of the sphere bears to its surface.”

Some of Aristarchus’ other achievements includes correctly predicting the rotation of Earth around an axis, and like his predecessor Anaxagoras (496-428 BC), he stated that other stars were similar in nature to the Sun, although much further away from Earth.

Lived: 276-195 BC

Country: Cyrene (Greek)

Eratosthenes was a man of great learning, and possessed a deep understanding of the principal scientific disciplines of his time. Apart from becoming the Chief Librarian of the great Library of Alexandria, he invented the science of geography, as well as the terminology to describe it, which is still used today.

In the field of astronomy, he is credited with calculating the distance between the Sun and Earth, with one translation of his work placing the value to within a few percent of its actual value, as well as using “stadia”, a standard unit of length at the time, to calculate the approximate circumference of the Earth to within 1-2 percent of accuracy. Eratosthenes is also recognized as being the first to calculate the tilt of the Earths’ axis, the invention of the leap day, and for being the first person to construct a map of the world that used meridians and parallels, which later developed into a system to also indicate the positions of stars and other celestial bodies.

Lived: 190-120 BC

Country: Nicea (Greek)

Hipparchus is considered by many historians to have been the greatest astronomical observer and mathematician of ancient times. He is known to have been an active astronomer from at least 162 BC to 127 BC, and his accomplishments during this time are many and varied. For instance, he used observations and mathematical techniques developed in Babylonia to develop the first quantitative and accurate models that described the relative motions of the Sun and Moon.

He is also credited with having developed trigonometry, and particularly spherical trigonometry, which he used in conjunction with his theories on lunar motions to arrive at a method with which to predict solar eclipses. His other achievements include compiling the first star catalogue in the Western world, the discovery and measurement of precession of the equinoxes, and the reputed invention of the astrolabe and armillary sphere, both of which he is said to have used while compiling his catalogue of stars.

Lived: 100-170 AD

Country: Egypt (Greek)

Ptolemy’s magnum opus, Almagest, is the only comprehensive treatise on ancient astronomy that has survived the ages intact. Although much of what Ptolemy wrote about, such as the geocentric model in which the Earth lay at the centre of the universe, was later proven to be false, the great advantage his Almagest had over other treatises was the fact that much of it was laid out in convenient tables, which made quick work of computing the past and future positions of celestial bodies. Furthermore, his calculations were fairly accurate, an were used as the basis for a model of the Universe that reigned supreme for several centuries until a more accurate heliocentric model was developed using elliptical, rather than circular orbits of the planets.

The Almagest also contains a fairly comprehensive star catalogue, as well as detailed descriptions of 48 constellations that were visible to him at the time. These constellations are in many cases the predecessors of the 88 modern constellations in use today. Ptolemy claimed that his observations were in large part based on observations made by his predecessors going back several centuries.

**Rest of the World** (560 BC to 1131 AD)

Lived: 560-480 BC

Country: Chaldea (Mesopotamia)

Although there is some doubt about the authorship of clay tablets that record the motions of the Moon, Sun, and planets at any given time, around a century’s worth of scholarship seems to attribute the Babylonian A System to a Chaldean astronomer named Naburimannu. One particular tablet records an ephemeris (locations) of Mercury between the years 424–401 BC, while others that record lunar motions for the year 306 BC suggests that if Naburimannu was indeed the originator of the Babylonian A System, he is likely the author of the lunar tablets as well. Naburimannu is also credited with having calculated the synodic month (time from one new Moon to the next) to be 29.530614 days, which is a figure correct to three decimal places.

**Gan De**

Lived: 400-340 BC

Country: China

Also known as Lord Gan, Gan De is generally accepted to have been the first astronomer known by name to have compiled a star catalogue in association with a contemporary of his, the astronomer, Shi Shen. Among many other types of observations, Gan De is known to have made some of the very first recorded observations of the planet Jupiter, which he described as “large and very bright.” At this time, Gan De also recorded a naked–eye observation of one of Jupiter’s principal moons by using a tree to “occult” the planet itself. Along with Shen, Gan De recorded some very accurate movement observations of the planets Jupiter, Venus, and Mercury, and in 1973, a catalogue compiled by Gan De and Shi Shen was discovered as part of the second century BC Mawangdui Silk Texts.

Lived: 476-550 AD

Country: India

While Aryabhata was only 23 years old when he produced his most famous work called the Aryabhatiya, his accomplishments in the mathematics of astronomy are far too numerous to list here. Sadly, the original text is lost, and what is known about the work of this prodigy is known only through discussions of it by later Indian and other astronomers. Nevertheless, Aryabhata correctly stated that the Earth rotates once around its axis every day, and that the motion of the Moon and stars across the sky is the result of the rotating Earth. This translated excerpt from the Aryabhatiya explains his view on this matter:

“In the same way that someone in a boat going forward sees an unmoving [object] going backward, so [someone] on the equator sees the unmoving stars going uniformly westward. The cause of rising and setting [is that] the sphere of the stars together with the planets [apparently] turns due west at the equator, constantly pushed by the cosmic wind.”

Using his model of the solar system, Aryabhata also calculated the length of the sidereal day to arrive at a value of 23 hours, 56 minutes, and 4.1 seconds, which compares favorably with the modern value of 23 hours, 56 minutes, and 4.091 seconds. His calculations of the sidereal year is equally impressive; he arrived at a value of 365.25858 days, whereas the modern value is 365.25636 days, a difference of only 3 minutes and 20 seconds over the length of a modern year.

Lived: 598-665 AD

Country: India

If Aryabhata was the mathematical prodigy, Brahmagupta was the accomplished master, and contributions made to the mathematics of astronomy by this Indian astronomer would fill several volumes. Suffice to say that Brahmagupta used his magnum opus called Brahmasphutasiddhanta (“correctly established doctrine of Brahma”) to discredit rival astronomical theories, and especially the practical application of mathematics to astronomical parameters, as opposed to attacking the correctness (or otherwise) of the underlying mathematics itself.

One example will suffice. In chapter seven of the Brahmasphutasiddhanta, entitled Lunar Crescent, Brahmagupta disputed a statement in Vedic Scriptures that held that the Moon was further away from the Earth than the Sun. Below is a translation of his argument that Vedic Scripture was wrong:

7.1.” If the moon were above the sun, how would the power of waxing and waning, etc., be produced from calculation of the [longitude of the] moon? The near half [would be] always bright.”

7.2. “In the same way that the half seen by the sun of a pot standing in sunlight is bright, and the unseen half dark, so is [the illumination] of the moon [if it is] beneath the sun.”

7.3. “The brightness is increased in the direction of the sun. At the end of a bright [i.e. waxing] half-month, the near half is bright and the far half dark. Hence, the elevation of the horns [of the crescent can be derived] from calculation. […] “

Apart from successfully arguing against established doctrines, Brahmagupta also devised new and novel ways to calculate ephemerides for a wide variety of celestial bodies, as well as methods to calculate conjunctions, and both lunar and solar eclipses.

**Al-Khwarizmi **

Lived: 780-850 AD

Country: Persia (Iran)

Muhammad ibn Musa al-Khwarizmi’s work titled “Astronomical tables of Sindh and Hind” comprises 37 or so chapters of calendrical and astronomical calculations, as well as 116 tables of calendrical, astronomical and astrological data, plus a table of sine values. This work represents the first of many similar works by other contemporaneous astronomers, but this particular work contains definitive tables of the movements of the Sun, Moon, and the five known planets.

As such, this work represents a pivotal moment in Islamic astronomy, because prior to its publication, Islamic astronomy was almost exclusively focused on researching, translating, and re-learning existing knowledge. Moreover, al-Khwarizmi’ is widely recognized as the one of the fathers of algebra, and the word “algorithm” derives from “algorism”, a mathematical technique developed by Al-Khwarizmi’ to perform arithmetic using Hindu-Arabic numerals. In fact, the modern words “algorithm” and “algorism” both derive from the Latinized forms of al-Khwarizmi’s name.

**Omar Khayyam**

Lived: 1048-1131 AD

Country: Persia (Iran)

Khayyám is perhaps better known in the West today as a poet, but like most other Persian mathematicians of his time, he was also an accomplished astronomer, and achieved great fame in this role. For instance, in 1073 AD, Khayyám was invited to construct an observatory, along with several of his contemporaries, by Malik-Shah I, who was the sultan of the Seljuq Empire between 1072 to 1092 AD. The exact purpose of the invitation is uncertain, but what is known is that under Khayyám’s direction, the team of astronomers revised the Iranian calendar to arrive at a solar year of 365.24219858156 days, which value was based on direct observations made by the team.

It is also worth noting that not only is the Jalali calendar (named after the Sultan), the oldest calendar of its kind in the world, but it was also decidedly more accurate than the Gregorian calendar that was introduced five centuries later. Moreover, the Jalali calendar also remains the most accurate solar calendar still in use today.