Philips Atlas of the Universe

EXPLORING THE UNIVERSE

Ironically, the next character in the story, the Danish

astronomer Tycho Brahe, was no Copernican. He believed

in a central Earth, but he was a superbly accurate observer

who produced a star catalogue which was much better

than anything compiled before. He also measured the

positions of the planets, particularly Mars. When he died,

in 1601, his work came into the possession of his last

assistant, the German mathematician Johannes Kepler.

Kepler had implicit faith in Tycho’s observations, and

used them to show that the Earth and the planets do indeed

move round the Sun – not in circles, but in ellipses.

Kepler’s Laws of Planetary Motion may be said to

mark the beginning of modern-type astronomy. The first

two Laws were published in 1609, though the change in

outlook was not really complete until the publication of

Isaac Newton’s Principiaalmost 80 years later. Mean-

while, the first telescopes had been turned towards the sky.

 Stonehengeis probably

the most famous of all

‘stone circles’. It stands

on Salisbury Plain, and

is a well-known tourist

attraction! Contrary to

popular belief, it has

nothing to do with the

Druids; its precise function

is still a matter for debate,

but it is certainly aligned

astronomically. It has, of

course, been partially

ruined, but enough remains

to show what it must

originally have looked like.

 The Ptolemaic theory –

the Earth lies in the centre

of the universe, with the

Sun, Moon, planets and stars

moving round it in circular

orbits. Ptolemy assumed that

each planet moved in a small

circle or epicycle, the centre

of which – the deferent –

itself moved round the Earth

in a perfect circle.

 The Copernican theory

• placing the Sun in the
  centre removed many of the
  difficulties of the Ptolemaic
  theory, but Copernicus kept
  the idea of circular orbits,
  and was even reduced to
  bringing back epicycles.

▲ The Tychonic theory–

Tycho Brahe retained the

Earth in the central position,

but assumed that the other

planets moved round the

Sun. In effect this was a

rather uneasy compromise,

which convinced

comparatively few people.

Tycho adopted it because

although he realized that the

Ptolemaic theory was

unsatisfactory, he could not

bring himself to believe that

the Earth was anything but

of supreme importance.

▲ Kepler’s Laws:

Law 1 A planet moves in an

ellipse; the Sun is one focus,

while the other is empty.

Law 2 The radius vector –

the line joining the centre of

the planet to that of the Sun

• sweeps out equal areas in

equal times (a planet moves

fastest when closest in).

Law 3 For any planet, the

square of the revolution

period (p) is proportional

to the cube of the planet’s

mean distance from the Sun

(a). Once the distance of any

planet is known, its period

can be calculated, or vice

versa. Kepler‘s Laws make it

possible to draw up a scale

model of the Solar System;

only one absolute distance

has to be known, and the

rest can then be calculated.

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