The Greek Who Counted the Stars and Found the New One

Hipparchus of Rhodes worked between roughly 162 and 127 BC. He is the most important ancient Greek astronomer of whom almost no original writing survives. Everything we know about him is filtered through Claudius Ptolemy, who lived three hundred years later and incorporated Hipparchus’s work into the Almagest (c. 150 AD), and through small surviving fragments of Hipparchus’s commentary on the constellations of Aratus and Eudoxus.

What survives is enough to be certain that he was the founder of quantitative observational astronomy. He produced the first comprehensive catalogue of stars in the Greek tradition — approximately 850 stars, with their celestial coordinates and visual magnitudes. He invented spherical trigonometry. He discovered the precession of the equinoxes, the slow westward drift of the celestial coordinate system, by comparing his own star positions with those measured a century and a half earlier by Timocharis and Aristyllus at Alexandria. He measured the lunar distance and the length of the solar year with accuracies that were not exceeded for fifteen centuries. He probably designed, or designed an early version of, the planetary gearing later realized in the Antikythera mechanism.

The biographical sketch

The name Hipparchus is attached to a town in Bithynia — Nicaea, on the Sea of Marmara — by the geographer Strabo writing two generations after his death. He probably moved to Rhodes early in life and spent the majority of his career there. Rhodes was, in the second century BC, the most prosperous Greek-speaking republic in the eastern Mediterranean and the site of one of the more important Greek observational programs of the Hellenistic period.

Rhodes is also approximately 36 degrees north latitude — a useful observing site for the stars Hipparchus needed to measure, because it allowed him to see most of the southern sky relevant to the ecliptic. The lighthouse at Alexandria stood approximately five hundred miles to the south. Hipparchus probably visited Alexandria once or twice but did most of his work at Rhodes.

He published, by Ptolemy’s count, at least fourteen books. Only one survives: the Commentary on the Phaenomena of Eudoxus and Aratus, which is a critique of two earlier descriptions of the constellations, the older by Eudoxus of Cnidus and the more recent by the poet Aratus of Soli. The Commentary is a workmanlike piece of corrective astronomy; it is not the work that established Hipparchus’s reputation.

His major astronomical works — the books that Ptolemy cited continuously — were three: On the Motion of the Moon, On Things Carried Round by the Sun, and a third book referred to in various ways including On the Length of the Year. These books were the foundation of Ptolemaic astronomy and, by transmission through Ptolemy, the foundation of all European astronomy until the seventeenth century. They were lost in late antiquity. Some fragments survive in Arabic translation and re-translation.

The supernova

One specific event in 134 BC seems to have triggered the star catalogue. According to Pliny the Elder, writing two centuries later, Hipparchus had observed a new star — a nova stella, in Pliny’s Latin — appear in the constellation Scorpius. The new star (which was probably a nova or, less likely, a supernova; modern radio surveys have located a candidate remnant in Scorpius that is consistent with the period) was bright enough to be unmistakable. It faded over several months.

Pliny continues: “Hipparchus, having discovered a new star which appeared in his own time, was led by its motion in the days when it shone to wonder whether such an appearance often occurred — whether the stars which we think to be fixed move in any way.” The result, according to Pliny, was that Hipparchus undertook to measure and catalogue every star visible to the naked eye, so that subsequent generations could detect any future change.

The star catalogue that resulted gave the position of each star in celestial coordinates (right ascension and declination, in modern terms) and assigned each star a magnitude on a scale from 1 (brightest) to 6 (faintest visible to the naked eye). The Hipparchian magnitude scale is the direct ancestor of the modern stellar magnitude system. The catalogue itself does not survive as a separate document but was incorporated, more or less wholesale, into Book VII–VIII of Ptolemy’s Almagest. Independent modern analysis of the Almagest star positions has confirmed that they were measured at approximately the latitude of Rhodes and at approximately the epoch of 128 BC.

There is one specific recent discovery worth noting. In 2022, an international team led by Victor Gysembergh published the recovered original text of Hipparchus’s star catalogue from a palimpsest in the library of St Catherine’s Monastery in Sinai. The underwriting had been overlaid by a Christian devotional text in the tenth century. Multispectral imaging recovered approximately a hundred lines of the original Hipparchus, with star positions that match the predicted Hipparchian observation date. It was the first direct recovery of any substantial Hipparchian text in the original Greek.

Precession

Hipparchus’s most consequential single discovery was the precession of the equinoxes. He had compared his own measurements of the position of the bright star Spica relative to the autumnal equinox with measurements made at Alexandria approximately 150 years earlier by the astronomers Timocharis and Aristyllus. The two measurements differed by approximately 2 degrees. The difference was systematic — every star Hipparchus could compare with the earlier records showed a similar westward shift.

The interpretation Hipparchus offered, conservatively stated, was that the celestial coordinate system — the equinox points and the celestial poles — was slowly drifting westward relative to the fixed stars at a rate of approximately one degree per century. (The modern figure is approximately one degree per 71 years; Hipparchus was about thirty percent low.) The cause was unknown to him. The phenomenon was real and is now understood as the slow gyroscopic wobble of the Earth’s rotational axis, produced by the gravitational effect of the Sun and Moon on the Earth’s equatorial bulge. The full cycle takes approximately 26,000 years.

The discovery was the first observation in the history of astronomy of a systematic motion of the entire celestial sphere, distinct from the rotation of the Earth and the apparent annual motion of the Sun and planets. It is roughly comparable in conceptual significance to the Tycho parallax measurement of the comet of 1577 — the moment at which the heavens were observed to be doing something the previous model had not accounted for.

The lunar distance

Hipparchus also produced the most accurate ancient measurement of the distance to the Moon. He used the lunar parallax visible during solar eclipses — specifically the eclipse of 14 March 190 BC (observed at the Hellespont) and the eclipse of 14 March 129 BC (observed at Rhodes) — to triangulate the Moon’s distance against the Earth’s radius previously measured by Eratosthenes. His result: the Moon is between 59 and 67 Earth radii distant. The modern figure is 60.3 Earth radii. He was correct to within ten percent.

The mechanism

The 2nd-century BC Greek tradition that produced Hipparchus also produced, somewhere in the eastern Mediterranean, the planetary computing device known to modernity as the Antikythera mechanism. The mechanism, recovered from a shipwreck off the island of Antikythera in 1901 and progressively decoded over the following century, models the motion of the Sun, Moon, and probably the five visible planets, with a complete eclipse-prediction system, using approximately thirty-five geared wheels.

The mechanism’s design assumes the same theoretical model of solar and lunar motion that Hipparchus is known to have developed: an eccentric circular solar orbit and an inclined lunar orbit with a regressing line of nodes. The eclipse-prediction system uses the saros cycle and the exeligmos cycle. The dating of the surviving Antikythera device, based on inscriptions and shipwreck context, is approximately 100–60 BC — within a generation of Hipparchus’s working life.

Direct attribution of the surviving device to Hipparchus is not possible. The strong scholarly position, articulated by Alexander Jones, Tony Freeth, and others, is that the surviving Antikythera mechanism is a copy or evolved descendant of a planetary computer designed in the late-2nd-century BC Rhodes tradition associated with Hipparchus and his school. The original may have been built by Hipparchus himself.

What survives

The single surviving original work of Hipparchus is the Commentary on Aratus and Eudoxus. The recovered Sinai palimpsest may, when fully processed, eventually contain a substantial fraction of the original star catalogue. Everything else of Hipparchus’s substantial corpus is lost, gestured at through Ptolemy, or implicit in the design of the Antikythera mechanism.

He died, according to one ancient source, at Rhodes around 127 BC. The death is not directly documented. The lunar crater Hipparchus is named after him. The first asteroid to be discovered with the new Mount Wilson 60-inch telescope, in 1908, was provisionally numbered 4000 and later named Hipparchus. The European Space Agency’s Hipparcos mission (1989–1993), the satellite that measured stellar positions and parallaxes for 118,000 stars with milli-arcsecond accuracy, was named for him. It produced the most complete catalogue of stellar positions since the Sinai palimpsest.