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Revolution of Moon around the Earth
The moon revolves around the Earth at an
average distance of 384000 km and completes on revolution in 27 days 7 hours and
43 minutes. The moon keeps the same face towards the Earth throughout its
motion because the rotation period is exactly same as the period revolution of moon.
The moon revolves around the Earth because
of the force of gravitation between the Earth and the moon. The moon also
applies force on the Earth and both revolve around their common Center of mass.
As the mass of moon is 81 times smaller than the mass of Earth so the Earth
does not move much but it is only the moon which appears to revolve around the
Earth.
Effect
of gravity of moon
The effect of gravity of moon can be seen
in the phenomena of tides. The moon exerts gravitation pull on the surface of
Earth but due to rigidity of the solid surface its effect is very less. The
oceans are not rigid so they easily deform under the effect of gravitational
pull. The level of water of the ocean increases on the surface of Earth which
is facing moon. The water level on the back side of Earth also increases. This
results in a bulge on the front and back surface of Earth.
Drag between the Earth and moon
The bulge is not localised to a particular
place because the speed of rotation of Earth is larger than the speed of
revolution of moon around the Earth. The surface below the water bulge
moves with the rotation of Earth but bulge is pulled by the moon. The Earth tries to drag the bulge along the
direction of rotation. Thus the bulge is not exactly along the direction of
line joining the Earth and moon but bulge lies slight towards the direction of
rotation of Earth.
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This situation results in a torque on the
bulge due to moon. Since a friction force exists between the surface of Earth
and the bulge in the ocean water, the torque also acts on the surface of Earth
which is in the opposite direction of the motion of Earth. So this torque tries
to slow down the rotation of Earth. The torque also acts on the bulge on the
back surface of Earth and this torque is in the same direction of rotation of
Earth. This torque on the back surface tries to increase the speed of rotation
of Earth but the magnitude of torque on the back surface is less than the
torque on the front surface. The net result is the torque in the opposite direction
of rotation of Earth which tries to decrease the speed of rotation of Earth.
Result of decrease in speed of rotation of Earth
The slowing of the speed of rotation of Earth also results in the increase in speed of
revolution of moon around the Earth. Due to decrease in speed of Earth the
angular momentum of Earth decreases and the angular momentum of moon increases.
So we can say that the angular momentum is transferring from the Earth to moon.
The net angular momentum of Earth moon system remains constant and follows the
law of conservation of angular momentum.
The increase in speed of revolution of moon
around the Earth also results in the increase in radius of orbit of moon. The
moon is moving away from us at the rate of 3.78 cm per year.
The Earth will keep on slowing down and
moon will keep on speeding up until they reach at the common speed. Then the
period of rotation of Earth and period of revolution of moon will be same. But
it will take billions of years.
The sun will become red giant much before
that and the oceans on the Earth will be evaporated by the extreme heat of sun.
So we don’t need to worry much as this process is going take very long time. If
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