Thursday, October 11, 2012

PLANET EARTH



Lecture 2

 
PLANET EARTH
         third planet in distance from the Sun in the solar system
          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
         EARLY THEORIES 

 Nebular Hypothesis                                       
         German philosopher, Kant and French mathematician, Laplace
         Earth, planets and sun originated from Nebula.
         Nebula was large cloud of gas and dust. It rotates slowly.
         Gradually it cooled and contracted and its speed increased.
         A gaseous ring was separated from nebula
         Later the ring cooled and took form of a planet
         On repetition of the process all other planets came into being
                               The central region, nebula became sun.
v         Planetesimal Hypothesis
               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.
              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
         Jeans and Jeffrey proposed the theory in 1925
         Large star came near the sun.
         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.
         The shape of the tide was like spindle.
         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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