- The shape of Earth:
Earth is very close to spherical in shape (like a ping-pong ball).
It is actually a bit flatter at the poles and a bit wider around the Equator:
The Earth's equatorial diameter is 12,756 km but the polar diameter is only 12,714 km.
This shape is called an oblate spheroid. Scientists think it results from the planet's rotation.
The rotation creates a centrifugal force perpendicular to the axis of rotation, and this centrifugal force pulls the Equatorial region outward and sideways.
The centifugal force slightly lessens (or counteracts) the gravitational pull that keeps the Earth spherical.
Other planets have this same shape, too.
- Eratosthenes:
The earliest known estimate of the size of the Earth was made in ancient times by the Greek scientist Eratosthenes (died 194 B.C.E.).
Eratosthenes lived in Alexandria, Egypt. He heard about a water-well in the town of Syene (modern name, Aswan), far to the south.
(Water wells need to be perfectly vertical to allow you to lower a bucket straight down.)
Most of the time, the sun would shine at an angle on this well and you could only see one side lit up.
One day each year, however, when the sun was at its highest in the sky (summer solstice), it would shine straight down the well and you could water at the bottom.
So, on the day of the summer solstice, he measured the angle of the sun in Alexandria. It was 7.2 degrees away from the vertical, or 1/50 of a full 360 degree circle.
He concluded that the circumference of the Earth was 50 times the distance from Alexandria to Syene (Aswan).
He was pretty close in his estimate! In those days, however, he didn't have a good way to measure the distance from Alexandria to Syene. Too bad.
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