also at present. From the ancient time, man has observed stars and planets appearing
in the night sky and he has come up with various theories about them. Accordingly,
astronomy can be considered as the oldest science in the world.
It was the Greeks who introduced Astronomy as a science of studying the sun,
the moon and the stars. There is evidence to prove that they used the knowledge from
predecessors in Egypt and the Middle eastern countries for this purpose.
In the distant past, celestial bodies were venerated as Gods. The first philosopher
to attack the myths regarding these objects was a Greek scientist Thales (640 B:C).
According to his theory the earth is flat. It floats on the ocean like a ship. The sun, the
moon and the stars are balls of fire revolving around this flat earth.
The idea of a flat earth was rejected by the great philosopher and mathematician
Pythagorus (580- 500 B:C) who declared that the earth is a sphere. His name became
immortalised with the famous pythagorus theorem.
Another Greek philosopher Aristotle (384-322 B:C) also supported the idea of
a spherical earth and explained it more clearly. He stated that the edge of the shadow
of the earth falling on the moon during an eclipse being a part of a sphere is an
evidence of the spherical nature of the earth.
He further stated that if a person were to travel a certain distance from a point in
any direction the change of the ascending angle of a star is the same, as evidence for
the spherical shape of the earth.
The view of Aristotle was that the sun, the moon and the planets revolve round
the earth. This incorrect idea of a geo - centric (earth - centered) model prevailed for
about 2000 years.
The first astronomer who put forward the heliocentered idea (sun - centred) was
the Greek (national) Aristorcus of Samos (310-250 B:C) who stated that the earth
revolves round the sun.
He also stated that the stars appear to rise and set each day because of the rotation
of the earth. However, the heliocentred idea was not accepted due to lack of evidence.Although Aristotle presented this idea of a geo-centric model, it was further
established on a mathematical basis by Claudius Ptolemaeus (100-178 A:D) who also
named as Ptolemy. Ptolemy was also responsible for building up maps of the earth and
the celestial bodies.
Ptolemy was succesful in compiling a book called Almagest which was a
collection of all knowledge available at that time. He indicated the presence of 48
constellations in that book out of which 47 are still known by their original names.
His view was that the earth is at the centre of the universe and that the sun, the
moon and the planets known at that time revolved round the earth in crystal spheres.
The views of Aristotle and Ptolemy were accepted by the powerful religious
athorities at that time. Presenting any views against them was an act that could be
punished by law.The helio-centric model, that is, the universe and the other planets revolve around
the sun was strongly advocated by Polish Astronomer Nicolaus Copernicus
(1473- 1543 A:D). This was a radical and a courageous effort. This became a radical
effort because at that time it was believed that the earth we live in, moves through
space was an extremely novel idea. It became a courageous effort since at that time
speaking against the geo-centred idea that was accepted by the religious authorities
was an anti-religious crime that could be punished by law.
Copernicus argued that the sun is at the centre of the universe and that planets
revolve around it in mega circular orbits. He also stated that the further away a planet
is from the sun, the longer time it takes to revolve around the sun. As evidence for his
theory he showed that the closest planet to the sun being Mercury takes about 2
months, while the Saturn takes about 30 years to revolve around the sun, which is the
planet farthest from the sun known at that time.
This novel idea of Copernicus led to an intellectual revolution at that time. That is
why Copernicus is known as “the great man who rotates the earth”. However, his ideas
which contradicted the religious teaching, and therefore the book which contained his
ideas was published after his death.(A crater in the moon is named after Copernicus too)
as Tycho Brahe, Johannes Kelper and Gallelio
Galili continued to follow him and to improve
the field of astronomy as a science.
Tycho Brahe (1546 -1602 A:D) was also
a pioneer in constructing instruments
for studying about stars. He was able to record
the movement of nearly 700 stars and the solar
system correctly. He rendered a valuable
service at that time by correcting some of the
existing data on astronomy.
Brahe did not accept the helio-centric model
of Copernicus. He stated that the five planets
then discovered revolved around the sun and
that the sun and these planets revolved around
the earth once a year. (The ‘Tycho’ crater on
the moon is named after this astronomer)
Although Brahe’s ideas on the movement of planets were incorrect, the information
he gathered formed the ground work for later astronomers. Johannes Kelper made use
of his observations and measurements to establish the ideas of Copernicus in later years.
Johannes Kelper (1571-1630 A:D), a German astronomer was able to formulate
three laws of planetory motion.
The ideas of Kelper’s laws of planetary motion can be summerised as follows:
• Every planet moves in an elliptical orbit which keeps the sun at one focus.
• The planets which revolve around the sun in their orbits move faster when it is
closer to the sun, and move slowly when it is far away from the sun.
• The length of the year of the planets which are close to the sun is shorter than that
which are far of the planets away from the sun.
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