Why does the Earth rotate? Why is it a sphere?
Hi,
I have been unable to get answers to the following questions concerning
the earth, can you help?
Why does the earth rotate - what are the forces causing the rotation?
Why did it start rotating in the first place?
Why are planets round?
Any help to get these answers is most appreciated
Reply
Hi, Mike
You have asked some very fundamental questions. Why does the Earth rotate? Because whatever it arose from--probably a cloud of gas and dust--was rotating to begin with.
The thing to keep in mind that even a very slow rotation of a cloud of
objects gets greatly speeded up as it condenses at the center. The reason
is a basic law (a consequence of Newton's laws of motion) by which a
quantity known as ANGULAR MOMENTUM (or rotational momentum) is conserved. The angular momentum can be defined as the average radial distance, TIMES the average velocity of motion, TIMES the mass.
The mass does not change when matter collects near the axis of rotation, so we neglect the last part. Then, if the material collectes in the middle, where its average radius of rotation is 10, 100 or 1000 times smaller, the average velocity of its particles increases by the same factor.
You see this happen every day. When water drains from a filled bathroom sink, even if the water in the sink is rotating so slowly that you do not notice, by the time it reaches the drain it is spinning rapidly enough
to form a funnel. (It does not spin in the opposite direction in Australia:
the effect on which this claim is based is far too weak to have much effect.
See "Stargazers," section 24). You also see this in hurricanes and tornadoes.
And you see it happen when a large object in space collapses. In 1054
a star "went supernova" in the constellation of the Crab, a process
in which the top layers blow off and the core collapses to a tiny
"neutron star," perhaps 15 km across and as massive as the Sun. The
collapsed core of the Crab Nebula apparently rotates 30 times a second,
because that is the frequency at which it blinks in x-rays and radio
(and I believe in its light, too).
In the solar system all planets orbit in the same direction and
nearly in the same plane, and they and the Sun rotate in the same
direction too (= counterclockwise, viewed from north). This suggests that they all condensed from the same cloud.
Now that other question: why are planets round? Because of their
gravity. On the surface of the Earth, solid material--say, rock cliffs
--can easily stand the pull of gravity without deforming. But go
just a few hundred kilometers inside the Earth, and you find everything
under enormous pressure, from the weight of the layers heaped up
on top. Under such pressure (and helped by the heat down there!), even
solid rock deforms like putty.
If the Earth were all fluid, gravity would pull it into a symmetric
sphere--the same way as it shapes the oceans. The Earth is not fluid, but as
mentioned above, it makes no great difference. Actually, a ROTATING
fluid Earth would be deformed by the centrifugal force, with the
equator bulging out slightly. Gravity is weakened there, by the centrifugal
force and by the greater distance from the center. That was observed
in Newton's time, and Newton explained it by essentially using
a fluid analogy.
Jupiter is much bigger than Earth and rotates much faster: its
equator bulges out so much that pictures taken through a telescope
suggest a definite ellipticity.
Voyager 2 and other space probes have by now cruised through most
of the solar system and have imaged its planets and their moons.
The rule seems to be that objects with a radius above 150 km are
spherical; smaller ones do not have a strong-enough gravity and may
be potato-shaped, e.g. the moons of Mars.
(A different slant, from later correspondence) Planets are pulled into their round shape by gravity, which evens them out all around. Anything sticking out is pulled down! For example, the highest mountain on Mars, Olympus Mons, is about 2 1/2 times the height of Everest. How come? Because smaller Mars has at its surface only 1/3 the pull of gravity we feel on Earth. On Earth, the greater weight of a mountain that high would make it sink into the surface.
Sincerely
David