TYCHO BRAHE (1546-1601)

1577 – Denmark

‘The heavens are changeable, and the comets move through space. The Earth is the centre of the Universe, and round it rotates the Moon and the Sun. The planets orbit the Sun’


Up to now it had been believed that planets were carried on ‘heavenly spheres’ that fit tightly around each other.

Brahe dissented from the Copernican doctrine and accepted the dogma that the Earth stood still. His real contribution to astronomy was as an observer, rather than as a theorist. He accurately measured the position of 777 stars, a remarkable achievement considering it was done without a telescope. He also measured the movement of planets, but was unable to determine their orbits.

His observations paved the way for the discoveries of his assistant, Kepler. After Brahe’s death Kepler inherited Brahe’s vast accumulation of data on planetary observations.

portrait of tycho brahe


Brahe’s observation of the supernova of 1572 and the comet of 1577 convinced him that the Universe was not unchangeable as was believed by philosophers of his time. The notion of celestial spheres was not possible because comets moved through these spheres. But he still placed the Earth at the centre of the Universe. His contemporary, the Italian philosopher GIORDANO BRUNO (1548-1600), believed in the Sun centered Copernican system and for these heretical beliefs was burned at the stake.

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1609-19 – Germany

‘1600 – Kepler works in Prague with TYCHO BRAHE the imperial mathematician, under the patronage of Rudolph II
1601 – On Brahe’s death, Kepler inherits his position (and crucially, his astronomical notes)’

portrait of KEPLER ©


  • First Law: The planets move in elliptical orbits with the Sun at one focus

  • Second Law: The straight line joining the Sun and any planet sweeps out equal areas in equal periods of time

  • Third Law: The squares of orbital periods of the planets are proportional to the cube of their mean distances from the Sun

Modern measurements of the planets show that they do not precisely follow these laws; however, their development is considered a major landmark in science.

Kepler’s ardent faith in the Copernican system – ‘The Sun not only stands at the centre of the universe, but is its moving spirit’, he asserted – brought him the disfavour of religious leaders. With his realisation that the planets do not rotate in perfect circles but in fact orbit in an ellipse, he provided the mathematical explanation for planetary motion, which had eluded Copernicus and Ptolemy.

The first two laws were published in 1609 ( Astronomia Nova – New Astronomy ) and the third in 1619 ( Harmonicses Mundi – Harmonics of the World ). Their publication put an end to PTOLEMY’s cycles & epicycles. His work provided the observational and arithmetical proof to support COPERNICUS‘ theories.

His second law states that an imaginary line between the Sun and the planets sweeps out an equal area in equal periods of time.

Stating that the planets ‘sweep’ or cover equal areas in equal amounts of time regardless of which location of their orbit they are in means that, as the Sun is only one of two centres of rotation in a planet’s orbit, a planet is nearer to the Sun at some times than at others. Thus the planet must speed up when it is nearer the Sun and slow down when it is further away.

His third law finds that the period (the time for one complete orbit – a year for the Earth, for instance) of a planet squared is the same as the distance from the planet to the Sun cubed (in astronomical units). This allows distances of planets to be worked out from observing their cycles alone.

Kepler was a versatile genius who, besides discovering these three laws, compiled tables of star positions ( Tabulae Rudolphinae – 1627 ) and developed the astronomical telescope.

Kepler also studied the anatomy of the human eye and founded the science of geometrical optics ( ‘Dioptrics’ – 1611 ), proposing the ray theory of light after ALHAZEN’s discussion in Opticae Thesaurus ; he described the eye in the same terms – as a pinhole camera, with light entering through the pupil and forming an image of the outside world on the retina at the back of the eye.

His credible solution to predicting planetary motion would act as the stimulus for questions that would lead to ISAAC NEWTON‘s theory of gravity.

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