|WHY IS EARTH A BALL SHAPE?
METEOROLOGIST JEFF HABY
For most of human history it was not known how big earth was or the shape of the planet. Through
observations of the skies and earth-sun geometry it became more logical that the earth
is round. Once satellites and space craft could see earth from space there was no doubt that
the earth is round.
In examining the sun and moon in the sky they take on a disk or ball shape. Stars and planets appear
more as points of light but when examined by telescope they also have the disk or ball shape. The
moon is the easiest object to analyze since it is close to earth, can easily be looked at without
hurting eyes and it has topography. From the shadow casting on the moon by the sun it can be concluded it
must be a ball shape and not a disk.
Gravity is the attraction that holds mass together. Gravity increases as the mass of an object
increases. To get a significant amount of gravity it takes a very large object such as a moon
or planet but even the smallest objects have some gravity. Since gravity acts from all directions the
shape of least resistance is a ball. From space earth appears as a rather smooth ball. On the
surface though we notice significant topographic and elevation changes. Gravity over time
tries to even out these changes but other forces are able to counter gravity such as the
uplift produced when tectonic plates collide.
If a theoretical planet could be a square or triangle shape the edges would be eroded by
gravity over time since they are weak points in the shape. Over time the object will
become more and more circular.
Earth is not a perfect ball. The main reason for this is because of the earth's spin. When laundry
is in the spin cycle you notice the clothes go out to the edges of the washing machine. The spin
of the earth puts extra force on the equatorial regions thus the shape of earth slightly bulges
outward at the equator. The shape of earth is described as an oblate spheroid.