Hipparchus (190-120 BC) was the greatest astronomer of ancient times. He took astronomy almost as far as it would go before the invention of the telescope.
Ptolemy is more famous but most of his astronomy is warmed-over stuff he got from Hipparchus, whom he called a great “lover of truth”.
A Greek from Nicaea, Hipparchus lived on the island of Rhodes where he studied the sun, the moon and the stars.
Hipparchus was the first to find out how far away the moon is. In order to work out the answer, he came up with a new field of mathematics: trigonometry.
Hipparchus made the best star map of ancient times, with some 2000 stars.
It was so good that his practice of using latitude and longitude has been used to map the heavens and the earth ever since.
It was so good that it had Uranus on it, a planet that was not discovered till 1900 years later.
It was so good that he discovered the precession of the equinoxes: that the sun does not appear in quite the same position on the first day of spring every year. Instead it moves backwards against the background of the stars, going all the way round in 26,700 years.
He started his map in 134 BC after seeing a new star in the constellation of Scorpio. He thought it was new, but could not be sure. He checked the star maps of Eudoxus and Erastosthenes. They did not have the star, but then he saw how bad their maps were. So he made his own.
The Greeks thought the heavens were perfect and unchanging, so finding a new star was a serious matter.
Hipparchus was the first to measure stars by their brightness. The 20 brightest stars he called first magnitude stars, the next brightest stars he called second magnitude and so on.
Most of what you see in Ptolemy comes from Hipparchus. Those circles within circles (the epicycles) and even most of the numbers. Hipparchus put the earth in the centre because that is what the best science of the day said: Aristotle’s.
The earth-centred model of Hipparchus was so good at working out where planets would be on any given day that few doubted it.
His model was just that: a model. But it worked so well that most mistook it for the truth.
Hipparchus and many others knew that Aristarchus had put the sun in the centre, but it went against common sense (the earth does not seem to be moving), the best science (Aristotle) and, besides, no one had worked it up into a model as good as that of Hipparchus. Not Aristarchus, not even Copernicus himself over 1600 years later.
Hipparchus was not overthrown till the 1600s when Aristotle was overthrown by Newton. And not until Kepler made some changes to the model of Copernicus.
All but one of his books is lost. Most of what we know of his work comes through Ptolemy.