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The Astronomer Tycho Brahe

On 11th November 1572 Tycho Brahe observed a new, brilliant star in the constellation of Cassiopeia. His measurements showed that it really was adistant star and not any local phenomena. This was very intriguing at that time, since the sphere of the stars was considered to be divine and perfect, hence no changes ought to take place there. Tycho followed its brightness evolve until it faded away the next year. He reported the event in his book "De stella nova", which made him famous all over Europe. Today we know that Tycho's "stella nova" was a dying star, spreading its matter in space in a violent explosion, caused by the collapse of the burnt-out core of the star. The glowing gas clouds are still observable today, through powerful telescopes.

Cassiopeia with the New Star De nova Stella" 1573

Basically, to measure positions and motions of celestial objects, angles have to be measured with the greatest possible accuracy. Tycho built his first instrument in 1564 following old drawings. The resolution of such an instrument is not better than one degree, far too inaccurate for Tycho's purposes. He started to design his own instruments, and proved to be a brilliant innovator. Apart from designing large instruments such as sextants, quadrants and armillary spheres, he also designed a very accurate aiming device and found a way to divide the angular scales to an accuracy of half an arcminute, i.e. 1/120 degree. Hence Tycho's observations were a hundredfold more accurate than the old ones.

The comet of 1577 - Tycho Brahe

Tycho used his accurate instruments to measure positions and motions of stars, planets and comets. Based on the immeasurably small angular position shifts (parallax) of the stars, he could say for sure that they were very distant, far beyond the planets. He compiled a catalogue with positions for 1004 stars that was unsurpassed in accuracy for more than 150 years. For the planets, in particular for Mars, he measured the positions and motions so accurately, that Kepler later could use these to deduce his famous three laws of planetary motion, the first of which is that all planets move in elliptical orbits with the Sun in one of the focii. Finally, Tycho could deduce that the comet of 1577 moved along a non-circular orbit beyond the Moon, crossing the orbits of the other planets. This proved definitively that the celestial bodies were not attached to any crystal spheres, but moved in empty space.

The first instrument that Tycho Brahe constructed

The generally accepted world system, the geocentric system, was formulated by Aristotle about 350 BC, and further elaborated by Ptolemy about 150 AD. It held that the Earth was the centre of the universe, and that the Moon, Sun and planets were attached to crystal spheres revolving around the Earth. All motions were perfectly circular. Gravitation was a phenomenon applying to the Earth only, as an expression for things seeking their natural place. However, the ptolemean system could not explain certain aspects of planetary motion that were frequently observed. Instead, Copernicus suggested in his book "De Revolutionibus Orbium Coelestium", 1543, that a system with the Sun in the centre, with the Earth as a planet among the others, would explain the observed discrepancies. Although Tycho's data would fit the Copernican model nicely, he was in doubt because of the problem with gravitation. If the natural equilibrium for things was on the Earth, how could it be outside the centre in the world system? Tycho therefore devised his own system, with the Earth still in the centre, but no crystal spheres and some of the planets orbiting the Sun instead of the Earth. Tycho Brahe published his worlds system in 1588 in the book "De Mundi Aetherei Recentioribus Phaenomenis" (On Some New Phenomena).


The universe of Ptolemaios The universe of Copernicus The universe of Tycho Brahes


With time Copernicus' theory about the Sun at the centre of the cosmos gained increasing support. Tycho Brahe had no telescope. Galileo was one of the first astronomers to use a telescope. He is believed to have constructed his in 1609. But several astronomers, working independently of each other, began to use telescopes at around the same time. The time was "ripe". The telescope made it possible to study the surface of the planets and to discover heavenly bodies that are not visible to the naked eye. In the beginning there were some who did not dare to look through a telescope: it was considered to be a delusion of the devil and black magic. In 1610, Galileo discovered with the help of his telescope that the Earth is not the only planet with a moon. Jupiter was seen to have four moons. Aided by Tycho Brahe's observations, Johann Kepler deduced that the orbits of the planets, including the Earth's, ought to be elliptical. This supports Copernicus' theory.

In 1687 Sir Isaac Newton presented his laws of gravity, which explained how the planets could stay in their orbits without being fixed to solid spheres. Gravity also meant that the smaller bodies, the planets, ought to orbit around the massive body, the Sun. We now know that our Sun is just an ordinary star in a stellar system, the Milky Way, which contains about a thousand billion other stars. There are more than a hundred billion similar stellar systems in the universe.

Johann Kepler - 1620



The Weighing of the World Systems. - J. B. Ricciolis nya Almagest, 1651