The wobble of the world: discovering precession 

The discovery came from a boring comparison. Ptolemy reports that Hipparchus measured the distance of the bright star Spica from the autumn equinox point and compared his figure with observations made about a century and a half earlier by Timocharis at Alexandria. The star had shifted along the zodiac by roughly two degrees. Not the kind of error a careful observer makes, and not confined to one star.

What Hipparchus concluded, as later reported, is that the equinox points themselves are sliding slowly westward along the ecliptic, so that all stellar longitudes creep upward over time while the pattern of the constellations stays intact. The rate he arrived at was in the region of one degree per century, an underestimate of the true value of about one degree every seventy-two years, but the right order of magnitude and the right phenomenon. This is the precession of the equinoxes.

The physical cause was not available to him, and would not be for eighteen centuries. Earth spins on an axis tilted about twenty-three and a half degrees from the plane of its orbit, and it bulges at the equator. The gravitational pull of the Moon and the Sun on that bulge torques the planet, so the spin axis traces a slow cone, completing a circuit in roughly twenty-six thousand years. Newton gave the mechanism in the Principia. Hipparchus gave the measurement.

The consequences are everywhere once you look. The pole star is a temporary appointment: Polaris is near the north celestial pole now, Thuban in Draco held the position when the Egyptian pyramids were built, and Vega will hold it in some twelve thousand years. The zodiac signs used in Western astrology were fixed to the equinox in antiquity and have drifted about a full sign away from the constellations bearing the same names. The tropical year, measured equinox to equinox, differs slightly from the sidereal year, measured against the stars, which is why calendar reform is a recurring headache.

Detecting precession also required trust in other people's numbers. Hipparchus had to assume that Timocharis had measured competently, in a stated frame, with stated units, and that the record had been copied without corruption. His result is as much an argument for keeping archives as it is an astronomical finding. Modern science runs on the same assumption every time someone reanalyses old data.

Ancient sources also credit him with improved values for the length of the tropical year and the distance of the Moon, and with the observation that the Sun's apparent motion is not uniform through the year. Precession, though, is the one that made his reputation permanent.

It is also the reason his catalog was worth copying, and the person who copied it most consequentially was an astronomer in Alexandria nearly three centuries later.