The Earth's True
The Earth is not a perfect sphere. Due to it's
rotation, the Earth (like all rotating planets) has a slightly
distorted shape. The rotational momentum tends to force the
matter to bunch up in the middle. In the case of the Earth,
this "middle" is the equator.
The true shape of the Earth called an
Oblate Spheroid. The term "Oblate" refers to it's slightly
oblong appearance. The term "Spheroid" means that it is
almost a sphere, but not quite. One of the most important
things to remember about the Earth's shape is that it is only
very slightly oblate. The diameter from the
North Pole to the South Pole (the shortest diameter) is
approximately 12,714 km. The equatorial diameter (the longest
diameter) is approximately 12,756 km. This is not a big
difference, but it does make the Earth not quite a sphere.
Evidence of the
Earth's True Shape
There is a
substantial amount of evidence which
the conclusion that the earth is a slightly oblate
spheroid. Three examples of such evidence are:
- Most obviously,
pictures taken from space provide evidence of the
Earth's shape. The Earth is so close to being a
perfect sphere that when viewed from any point in
space the Earth appears spherical. If accurate
measurements are performed, however, it can be shown
that the Earth is not quite a perfect circle.
- Very accurate
measurements of the positions of the stars also
provide evidence of the Earth's shape. The position of
the stars (and Sun) appear to change as we move over
great distances on the earth's surface. The most
notable example of this involves the changing
positions of the North Star (Polaris).
Precise gravity measurements also
support the conclusion that the Earth is not perfectly spherical.
The pull of gravity on an object changes as the distance of the
object from the center of the earth changes. The further an object
gets from the center of the Earth, the less it weighs. If the Earth
were a perfect sphere, then objects would weigh exactly the same
any place on Earth (as long as they were at the same elevation).
In reality, the weight of an object varies as it changes it's position
on the Earth's surface. At the equator, where the Earth's diameter
is greatest, objects weigh a little less. At the Poles, where the
Earth's diameter is the least, objects weight a little more.
The Earth's surface has many different
elevations. The changing elevations of the land
(mountains, canyons, etc.) are called relief.
Even though these features may seem quite impressive to
us, they are insignificant when compared with the size
of the Earth. Even the tallest mountain, Mount Everest,
has a height that is only about .07% the diameter of the
Earth! The bottom line is that the Earth is really very
Models of the
Models are representations of objects that
aid in our understanding. If we were to choose a scale
model of the Earth, what objects would make a good
miniature Earth? We might be tempted to choose an oblate
object to show the true shape, but that would be
inaccurate! Since the Earth is so slightly
oblate, and the Earth's relief is so insignificant in
comparison with its size, the best scale models of the
Earth would actually be very round and very smooth.
Billiard balls, marbles, ping pong balls, and other
smooth spheres are the best representation of the
Earth's true appearance.