Aim: What is the shape of the Earth?
I. Shape of the Earth
A. The Earth is NOT a perfect sphere, but it is
B. Oblate spheroid – the true shape of earth in
which its equatorial diameter is slightly larger than
its polar diameter.
1. Polar Diameter (measured from N.Pole to S.
Pole) – 12,714 Km
2. Equatorial Diameter – 12,757 Km
N Polar Diameter
C. Evidence of Earth’s Shape
1. Photographs of Earth from outer space show that
Earth is round
2. Years ago, sailors noticed that ships slowly
disappeared or appeared over the horizon.
3. Gravity measurements
a. Gravity is the force of attraction between any two
b. The Law of Gravitation – the gravitational force is
proportional to the square of the distance between the two
centers of attracted objects.
Distance Between two objects
c. If Earth was a perfect sphere, then the force of gravity
would be the same anywhere on the surface. But, the force of
gravity is actually greater at the poles than at the equator.
Thus, the distance from the poles to the center of the Earth is
less than the distance from the equator to the center of the
4. Altitude of Polaris (North Star)
a. The altitude (height or angle made between the observer’s eyea
and the observer’s horizon) of Polaris is equal to the observer’s
latitude (only in the northern hemisphere).