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Tycho

Tycho Brahe (1546-1601), a Danish nobleman, was one of the greatest astronomers of all time. Before Tycho’s time only Hipparchus was better. Tycho tried to prove Copernicus wrong but his work, continued by Kepler after his death, only proved Copernicus right once and for all.

Copernicus said that the planets went round the sun. Ptolemy said they went round the earth. Tycho said something in between: yes, the planets went round the sun, but the sun went round the earth!

Tycho turned to astronomy when he saw an eclipse while at university. He once got in a fight there in the middle of the night over a point of mathematics. He lost his nose and later got a metal nose made to put in its place.

Although he was a nobleman who was often full of himself, he did fall in love with a simple country girl and married her.

In the universities they taught Aristotle: the earth was the centre of the world, a place of endless change, but the heavens above the moon were perfect and unchanging. What about comets? Aristotle said they were below the moon, part of the earth’s weather.

Tycho proved the heavens were anything but unchanging. He became famous when he found a new star that was not there before. It was called Tycho’s star (we call it a nova). It soon became brightest star in the sky.

Tycho also proved that comets were not part of the weather but farther than the moon. By gathering observations from different parts of Europe he could tell that its position in the sky against the stars changed less than the moon’s, meaning it was farther away.

The king built an observatory for Tycho on the island of Ven in between Denmark and Sweden. There Tycho studied the stars with the best instruments in the world. He carefully recorded the motion of the sun and the planets. His measurements were five times better than anything ever made. He even took into account the effects of the air and the limits of his own instruments. He wanted to prove Copernicus wrong.

Tycho wrote a letter to Galileo and told him that if Copernicus were right, then we should be able to measure how far away the stars were. Galileo had no answer for that. What neither of them knew was how unimaginably far away the stars were.

When the king died Tycho had to leave the island. He travelled to Prague. There he met Kepler. Kepler knew what a gold mine Tycho’s tables of numbers were. He promised Tycho to continue his work after he died and prove Copernicus wrong once and for all.

Kepler did continue his work, but in the end he had to admit that Copernicus, with a few changes, was right after all.

Later in the 1600s Tycho’s old observatory was burned down by war. Riccioli, who named the craters of the moon, named the brightest one Tycho in his honour.

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Hipparchus

Hipparchus (-190 to -120) was the greatest ancient Greek astronomer. He took astronomy almost as far as it would go before the invention of the telescope some 1700 years later.

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”.

Hipparchus was born in Nicaea and 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 2,000 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 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 at 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 at 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 are lost. Most of what we know of his work comes through Ptolemy.

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