Hipparchus: The Man Who Measured the Sky

Nearly everything the most important astronomer of the ancient world wrote has been lost.

One minor work survives, a commentary on an astronomical poem, and it is not the one anybody would have chosen to save. What we know about Hipparchus of Nicaea comes mostly from Ptolemy, writing roughly three centuries later and quoting him constantly, and from scattered mentions in Strabo and Pliny. That is a thin evidential base for a man who worked out that the whole sky is slowly slipping, catalogued the stars, invented the brightness scale astronomers still use, and built the mathematics that made all of it calculable.

A Life We Can Barely Sketch

He was born at Nicaea in Bithynia, in what is now northwestern Turkey, around 190 BCE, and he did most of his observing on the island of Rhodes. He was still working in 127 BCE. After that, silence.

Rhodes mattered. It sat on the trade routes between the Greek world and the east, which meant a working astronomer there could get his hands on Babylonian records, and Hipparchus plainly did. The Babylonian scribes had been logging eclipses, planetary positions and lunar cycles on clay for centuries, in a tradition that reaches back to the material covered in the Babylonian origins of astrology. What Hipparchus brought to that inheritance was Greek geometry. The combination is the whole story of his career: eastern data, western mathematics, and a temperament that refused to accept a number he had not checked.

The Star That Started a Catalogue

Pliny the Elder tells a story about why Hipparchus began mapping the stars, and it is a good story, which is exactly why it should be handled carefully.

According to Pliny, writing around 77 CE, a new star appeared in the sky in Hipparchus's lifetime, and the shock of seeing something new in a supposedly unchanging heaven drove him to record the position and brightness of every star he could see, so that future observers could tell whether anything had moved or arrived. Modern astronomers think the object was probably a nova. The trouble is that Pliny is our only source and he was writing two centuries after the event, so the tidy motive may be his rather than Hipparchus's. What is not in doubt is the catalogue itself. It listed positions for something in the region of 850 stars, the exact figure being uncertain because the original is gone and the surviving count of 1,025 belongs to Ptolemy's version.

Whether Ptolemy compiled his own catalogue or largely reproduced Hipparchus's with an adjustment for the intervening centuries has been argued about since the eighteenth century, and it is still not settled. In 2022 a team working on a palimpsest, a manuscript scraped clean and written over, reported that they had recovered a fragment of star coordinates that appears to come directly from Hipparchus. It is the first piece of his catalogue anyone has seen. One fragment does not resolve a two-hundred-year dispute, but it is a remarkable thing to have found at all.

Why Sirius Is Brighter Than a Star of the Sixth Magnitude

Cataloguing stars requires a way of recording how bright each one is, and there was no such system. So he made one.

Hipparchus sorted the visible stars into six classes. The brightest were of the first magnitude and the faintest he could see were of the sixth, with everything else distributed between. It is a crude scheme, built entirely on the naked eye and personal judgement, and it is still in use. Astronomers formalised it in the nineteenth century, fixing the scale so that a difference of five magnitudes equals a brightness ratio of exactly one hundred, then extended it in both directions to handle telescopes and very bright objects. Sirius came out at magnitude -1.46. The Sun sits at -26.7. Every one of those numbers is anchored to a rough sorting job done by a man on Rhodes with no instrument beyond his own eyes, and the fact that the scale runs backwards, with brighter objects getting smaller numbers, is his fingerprint on modern astronomy.

The First Trigonometry

To turn observations into predictions you need to convert angles into lengths, and in the second century BCE there was no tool for doing it.

Hipparchus built one. He constructed a table of chords, listing the length of the straight line joining two points on a circle for a series of angles, which is the direct ancestor of the sine table and the reason he is often called the founder of trigonometry. That table is what let him model the Sun's uneven motion through the year, predict eclipses, and estimate the distance to the Moon. His figure for the lunar distance, derived from parallax observations of a solar eclipse, worked out at somewhere around sixty Earth radii. The modern value averages about sixty as well. He was working with sight lines, geometry and arithmetic, and he landed on the right answer.

The Sky Had Slipped

The discovery he is best known for came out of comparing his own measurements with records made about a century and a half earlier by Timocharis and Aristyllus in Alexandria.

The star Spica, in the constellation of the maiden, was not where their figures said it should be relative to the autumn equinox point. It had shifted by roughly two degrees. Hipparchus checked other stars, found the same drift in the same direction, and concluded that the equinox points themselves were moving slowly backwards along the ecliptic. He put the rate at no less than one degree per century, which was cautious and slightly low; the true figure is about one degree every seventy-two years. Nobody would understand the cause for another eighteen hundred years, until Newton identified it as the gravitational pull of the Sun and Moon on Earth's equatorial bulge, setting the axis wobbling like a slow spinning top over a cycle of roughly 26,000 years. The full account of what that means for the zodiac, and for the headlines that periodically announce your star sign has changed, sits in the guide to precession and why your sign might be wrong.

What He Left Astrology

Hipparchus was an astronomer, and the line between astronomy and astrology in his world was not the line we draw now.

Pliny reports that he was regarded as having a close involvement with astrology, and some later sources credit him with work on it directly, though nothing survives to confirm what that amounted to. His practical bequest is not in doubt. The coordinate systems, the chord tables, the solar and lunar models and the star catalogue are the machinery that Ptolemy inherited and used to write both the Almagest and the Tetrabiblos, the two books that carried Greek astronomy and Greek astrology into the Islamic world and then into medieval Europe. Every horoscope cast for the next fifteen hundred years rested on calculations that trace back through Ptolemy to Rhodes.

There is something worth sitting with in that. The man who gave astrology its mathematical spine is also the man who proved that the sky does not stay put, which is the single fact most often used to argue that the whole system is broken. He would probably have enjoyed that.