ASTRONOMY
Our picture of the universe

• Long,long ago,it was belived that the world is really flat plate supported on the
back of gaint tortoise

As long ago 340 BC the greek
philoshoper Aristotle,in his book
“on the heavens”was able to put
forward two good arguments for
beliving that the earth was a round
sphere rather than a flate plate.

• First Aristotle,realized that eclipses of the moon were caused by the
earth coming between the sun and the moon.

Second,the Greeks knew form their travels that the north star appeared
lower in the sky when the viewed in the sourth than it did in more
northernly.

• Aristotle thought that the earth was stationery and
that the sun,the moon,the planet,and the stars moved
in the circular orbits about the earth
• This idea was elaborated by Ptolemy in the second

A simpler model, however, was
proposed in 1514 by a Polish
priest,Nicolas Copernicus.
His idea was that that the sun was
stationery at the centre and that
the earth and the planets moved
in circular orbits around the sun.
Galileo Galilei started publicly to
support the copernican theory.

• When he looked at the planet jupiter, Galileo found
that it was accompaniedby several small satellites or
moon that orbited around it. This implied that
everything did not have to be in orbit directly around
the earth, as Aristotle and Ptolemy had tought.
• In 1609,Galileo Galilei started observing the night
sky with a telescope, which had just been invented

• An explanation was provided
only much later, in 1687, when
sir Issac Newton published his
“philosophiae Naturalis Principia
Mathematica”in which he not
only put forward a theory of
how bodies move in space and
time but also developed the
complicated mathematics
needed to analyse those
motions.

• In addition Newton postulated a law of universal gravitation according to which
each body in the universe was attracted toward every other body by a force that
was stronger,the more massive the bodies and the close they were to each other.

• Later in 1929, Edwin hubble made the
landmark observation that whatever
you look distant galaxies are moving
rapidly away from us.
• In other word universe is expanding.
This means that a earlier times objects
would have been closer together.
• Infact it seemed that there was a time,
about ten or twenty thousand million
year ago, when they were all at exactly
the same place when, therefore the
density of the universe was infinite.
• This discovery finally brought the
question of the beginning of the

Hubbles observations suggested that there was a time, called the Bigbang, when
the universe was infinitesimally small and infinitely dense under sucjh condition all
the laws of science, and therefore all ability to predict the future, would break
down.

In short, the Big Bang hypothesis states that all of the current and past
matter in the Universe came into existence at the same time, roughly
13.8 billion years ago. At this time, all matter was compacted into a very
small ball with infinite density and intense heat called a Singularity.

Our solar system began to
form around 5 billion years
ago, roughly 8.7 billion years
after the Big Bang. A solar
system consists of a
collection of objects orbiting
one or more central stars. All
solar systems start out the
same way. They begin in a
cloud of gas and dust called a
nebula .

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