Hipparchus (c. 150 BC): Star Catalogue and Precession Found

Hipparchus of Nicaea, active roughly 162-127 BC and working mainly on the island of Rhodes, is often called the greatest astronomer of ancient Greece. Around 130 BC he discovered the precession of the equinoxes, the slow shift of the sky’s reference points against the stars. He also compiled one of the first star catalogues and produced the earliest known trigonometric table, turning astronomy into a precise, mathematical science.

Life and sources

Hipparchus was born in Nicaea in Bithynia, today Iznik in north-western Turkey, around 190 BC. The dated observations attributed to him run from 162 to 127 BC, and most were made on Rhodes, then a leading centre of learning. Little else is known of his life, including the date and place of his death, although later coins of Nicaea showed his portrait seated beside a celestial globe.

Only one of his works survives complete: a Commentary on the Phaenomena of Aratus and Eudoxus, a critique of an earlier poem about the constellations. Most of his achievements are known through Ptolemy’s Almagest, written in Alexandria around AD 150, which quotes and builds on him. In 2022 researchers reported text in a medieval palimpsest, the Codex Climaci Rescriptus, that appears to preserve star coordinates from his catalogue.

Discovery of precession

Hipparchus measured the positions of bright stars, such as Spica, relative to the equinoxes, the two points where the Sun crosses the celestial equator. Comparing his results with those of Timocharis and Aristyllus, who had worked in Alexandria around 150 years earlier, he found that the stars’ longitudes had all increased by a similar amount.

He concluded that the equinoxes themselves slowly move backwards along the zodiac. He estimated the rate at no less than one degree per century. The true value is about one degree in 72 years, so his figure was a cautious lower limit. The cause, a slow wobble of the Earth’s axis, was only explained by Isaac Newton in the 17th century.

The star catalogue

According to Pliny the Elder, Hipparchus was prompted to map the stars after seeing a new star, possibly the nova Chinese astronomers recorded in 134 BC. His catalogue, compiled around 129 BC, probably listed about 850 stars with their positions, so that later astronomers could detect changes in the sky.

He is also traditionally credited with grading stars by brightness, the basis of the magnitude scale still used today, although the evidence for this comes mainly from Ptolemy. Ptolemy’s own catalogue of 1,022 stars probably drew heavily on Hipparchus’ work, although how much Ptolemy copied rather than re-observed has long been debated by historians of astronomy.

Mathematics, Sun and Moon

To handle angles in his calculations, Hipparchus compiled a table of chords, which gave the length of the straight line across a circle for a given angle. It did the same job as a modern table of sines, and for this he is often called the founder of trigonometry.

He built geometric models of the motions of the Sun and Moon using eccentric circles and epicycles, drawing on Babylonian eclipse records. He measured the length of the solar year as 365¼ days minus about 1/300 of a day, only a few minutes too long, and used parallax to estimate the distance of the Moon, with results in the range of roughly 60-80 times the Earth’s radius, of the right order compared with the modern average of about 60. His lunar theory was sophisticated enough to be reproduced in the gearing of the Antikythera Mechanism.

Myths and facts

Myth: Hipparchus believed the Earth went round the Sun. He kept an Earth-centred model, unlike his predecessor Aristarchus of Samos. His aim was to predict positions accurately, and Earth-centred models did that well with the data available.

Uncertain: the Farnese Atlas. A 2005 study argued that the constellations on this Roman statue’s globe were based on Hipparchus’ lost catalogue. Many historians remain unconvinced.

Timeline

• c. 190 BC: Hipparchus is born in Nicaea, Bithynia.
• 162 BC: Earliest observation attributed to him.
• c. 134 BC: A new star is seen, which Pliny says prompted his catalogue.
• c. 130 BC: He announces the precession of the equinoxes.
• c. 129 BC: He compiles his star catalogue.
• 127 BC: Latest dated observation attributed to him.
• c. AD 150: Ptolemy’s Almagest preserves much of his work.

Legacy

Hipparchus built on Babylonian records and the measurements of scholars such as Eratosthenes, but he went further by testing theories against systematic, dated observations. Through Ptolemy, his methods shaped astronomy in the Islamic world and medieval Europe for more than 1,400 years. The European Space Agency named its star-mapping satellite Hipparcos, launched in 1989, partly in his honour.

Why it matters

Hipparchus showed that careful measurement over long periods could reveal motions too slow to see in a lifetime. His discovery of precession, his star catalogue and his chord tables made him a founder of scientific astronomy.