Lecture 2
PLANET
EARTH
•
third planet in distance from the Sun in
the solar system
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the only planet known to harbor life, and the
“home” of human beings.
•
About 71 % of Earth’s surface is covered
by water, which is essential to life.
•
rest is land, mostly in the form of
continents that rise above the oceans.
Origin of the Earth
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EARLY THEORIES
Nebular Hypothesis
•
German philosopher, Kant and French
mathematician, Laplace
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Earth, planets and sun originated from
Nebula.
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Nebula was large cloud of gas and dust.
It rotates slowly.
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Gradually it cooled and contracted and
its speed increased.
•
A gaseous ring was separated from nebula
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Later the ring cooled and took form of a
planet
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On repetition of the process all other
planets came into being
•
The central region, nebula became sun.
v Planetesimal
Hypothesis
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Chamberlin and Moulton proposed the
theory in 1904
•
the solar system originated from matter
which was in form of tiny dust grains called "Planetesimals".
•
The planets evolved from a cloud of planetesimals
which revolved around the sun.
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The planetesimals, the tiny cosmic
fragments collided with each other and united together to form larger bodies
which became planets and satellites revolving around the sun.
v Gaseous
Tidal Theory
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Jeans and Jeffrey proposed the theory in
1925
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Large star came near the sun.
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Due to gravitational pull a gaseous tide
was raised on the surface of the sun.
•
As the star came nearer, the tide
increased in size.
•
Gaseous tide detached when star move
away.
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The shape of the tide was like spindle.
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It broke into pieces-forming nine
planets of the solar system
Modern
Theory
•
scientists in later period took up the
problem of origin of universe rather than
that of just the earth or the planets.
v Big
Bang theory (expanding universe hypothesis)
•
According to cosmologists, the dawn of
the universe started with a Big Bang
•
entire matter of the universe was
condensed into a cloud of gas and dust
•
after
consecutive expansion and contraction it exploded into the Big Bang
•
The Big Bang theory is the
prevailing cosmological model that explains the early development of the
Universe.
•
According to the Big Bang theory, the
Universe was once in an extremely hot and dense state which expanded rapidly.
•
This rapid expansion caused the Universe
to cool and resulted in its present continuously expanding state.
•
According to the most recent
measurements and observations, the Big Bang occurred approximately 13.75
billion years ago, which is thus considered the age of the Universe.
•
After its initial expansion from a
singularity, the Universe cooled sufficiently to allow energy to be converted
into various subatomic particles, including protons, neutrons, and electrons.
•
protons and neutrons combined to form
the first atomic nuclei only a few minutes after the Big Bang,
•
it took thousands of years for electrons to
combine with them and create electrically neutral atoms.
•
The first element produced was hydrogen,
along with traces of helium and lithium.
•
Giant clouds of these primordial
elements would coalesce through gravity to form stars and galaxies, and the
heavier elements would be synthesized either within stars or during supernovae.
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