Why is the Earth round (planets and the sun, too)?
It's the gravity, of course.
Imagine
Earth were liquid--consisting, say, only of water. Gravity
would of course pull that water towards the center, and if any could
flow closer to the center, it would do so.
Therefore,
if such an Earth were not a sphere--if some points were
higher than the average--their water would quickly flow down. Water
would also flow into any valley deeper than the average and fill it up.
The final shape MUST be a sphere. Only then does every point on the
surface have the same distance from the center.
Gas
planets like Jupiter, Saturn, Uranus and Neptune behave the same
way, since gas flows like a fluid. The Sun is gas, too, so it must be
spherical. But Earth? The Moon? Why should a planet composed of solid
rock have the same shape as one composed of water?
Because,
dear friend, once you descend a few hundred miles (or
kilometers), rocks flow like a fluid, too. A rock 300 miles underground
bears the weight of a 300-mile layer of rock, and under such pressure
even solid material will gradually yield, more so because of the intense
heat there. Hence the rocky Earth is close to a sphere.
Mountains
may stick up 30000 feet, but not much more, otherwise the
ground will slowly sink under their weight. Olympus Mons on Mars can
reach 80000, but the weaker gravity of Mars makes it possible. Asteroids
100 miles across may still be potato-shaped (not enough gravity to give
the outer layers enough weight), but by the time you reach 300 miles,
round is the rule.
Round
... but not always completely round. Earth bulges a little at
the equator, because rotation flings matter outwards, weakening gravity
there by a fraction of 1%. Jupiter spins faster than Earth--just under
10 hours--and bulges much more: telescope pictures of Jupiter show it
visibly fatter at the equator. And our Moon--why do you think it always
presents the same face to the Earth? Yes, it is lightly longer in that
direction, and the pull on the part sticking out towards Earth keeps it
aligned that way.
This week's question is provided by Dr. David Stern. Dr. David Stern is a physicist retired from Goddard. You will find a lot about gravity, physics, space and more on his web collection "From Stargazers to Starships", home page http://www.phy6.org/stargaze/Sintro.htm. Section #4a, for instance, tells about the Moon and explains how its bulge keeps one side of it facing Earth.