The study of the Earth and the universe has fascinated many scientists since ages. The
knowledge of the universe, like a water drop in an ocean, is the result of the work of scientists
and philosophers since ages. The galaxies, the solar system and the motion of planets and
other celestial bodies in the solar system had puzzled many in the past. The brilliant thoughts
of certain people clubbed with their incessant efforts to understand the nature produced
some theories and concepts that enable us to comprehend nature better. The question of
how the motion of celestial bodies in the universe are governed, was debated on till around
the 2nd century. This was when Ptolemy, a Greek scientist, put forward his theory regarding
the motion of planets and the sun.
The force that keeps the celestial bodies intact in the universe was called ‘gravitational force’
or the ‘force of gravitation’. According to the theory put forward by Ptolemy, all the planets
in the solar system and the sun revolve around the Earth in concentric circular orbits with
the Earth as the centre. This theory is called ‘Ptolemic theory’ or ‘geocentric theory’. The
suffix ‘geo’ refers to the Earth and so the theory is named ‘geocentric theory’. This geocentric
theory was accepted as established for centuries till a Polish monk, Copernicus, proposed a
theory in around 16th century AD. According to the theory proposed by Copernicus, it is not
the Earth but the sun which is the centre of the universe and all other planets including the
Earth revolve around the sun in circular orbits. Since ‘helios’ refers to the sun in Greek, this
theory is called ‘heliocentric theory’. The quest to understand the universe continued and many
observations were made regarding planetary motions by Tyco Brahe. His assistant, Johannes
Kepler analysed the observations of his master and proposed three laws for planetary motion.
These laws proposed by Kepler are called Kepler’s laws of planetary motion
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