Thales, a Greek, believed that the Earth was flat and that it was covered by a great dome that carried the Sun stars and planets.
Aristarchus simplified the whole idea by proposing that the Earth in fact moved round the Sun, but his theory was not accepted as people thought that if the Earth moved they would fall off it!
Ptolemy returned to the Earth centered idea of the universe, but he modified it by introducing the idea of the epicycle. This was to help explain the apparent backward or retrograde motion of the planets when seen from the Earth. He said that the planets moved around small circles which themselves moved around big circles round the Earth.
Copernicus
revived the model of the Solar System that had been held by the Greek, Aristarchus.
However this idea was still not well received, this time by the church. Just as the Greeks insisted that the Earth must stay still and that all orbits must be circular as the circle was the
'perfect shape' so the church argued that the Earth must be the centre of the universe as it
was on Earth that Christ was born. They used such quotations as: God has founded the
Earth and it shall not be moved. (David in Psalm 89).
In his book, De Revolutionibus, Copernicus proposed the heliocentric or Sun-centred theory of the Solar system. However, maybe because he was concerned about criticisms from some members of the scientific community and the church, he delayed its publication, only organising the final printing due to encouragement from a friend and pupil, Rheticus. As a result he only saw the completed work on the day of his death .
Gradually however the heliocentric, idea of the Solar
System was accepted, and it has been proved by the space flights of this
century.
Tycho was a Danish nobleman
who worked in an observatory built for him by the King of Denmark.
Over a period of 20
years he made a series of extremely accurate observations of the positions of the planets:
These were all done with unaided eyes, as the telescope had not yet been invented. He was
attempting to produce a theory of the solar system as a compromise between those of
Ptolemy and Copernicus, but this never received much support.
Kepler was a pupil of Tycho and when Tycho died Kepler inherited all his
observations. Using these observations Kepler proposed his three laws of planetary
motion.
1. Each planet moves in an ellipse with the Sun at one focus.
2. The line
joining the planet to the Sun sweeps out equal areas in equal times.
3. The ratio
of the cube of the radius of a planet's orbit to the square of its period of revolution about the
Sun is the same for all planets
Galileo
was an enthusiastic supporter of the Sun centred theory but he also made many individual
contributions to Physics and Astronomy.
He investigated the way bodies fell and came
to what we now call Newton's First Law. In 1609 he was the first person to use a telescope
for astronomy.
With this telescope he made four major discoveries in Astronomy:
(a)
The craters on the Noon
(b) Sunspots
(c) The moons of Jupiter
(d) The
phases of Venus.
Galileo was forced to abandon his support for the Copernican theory being threatened with torture and sentenced to life imprisonment by the Inquisition, immediately commuted to house arrest.
Newton's major contribution to the theory of astronomy was his Universal law of Gravitation which explained the orbital motion in terms of gravitational attraction. He also developed the reflecting telescope and studied the spectrum of sunlight.