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Sphere
Every point on its surface is the same distance away from the centre. The radius is constant.
Spheroid
Sphere that is squashed or stretched along one axis
Oblate - squashed along an axis, wide, flat pebble
Prolate - stretched along an axis, rugby ball
What is the cross section of a sphere
Circle
What is the cross section of a spheroid
Ellipse (if cut through the poles
Polar radius
Distance from the centre of the spheroid to one of its poles
Equatorial radius
Distance from the centre of the spheroid to a point of its equator
Triaxial ellipsoid
Sphere that has been squashed or stretched along 2 of the principle axis
Triaxial because the diameter is different along each of the principle axes (up, across and forward)
What shape is the Earth?
Oblate spheroid - equatorial radius is greater than polar radius, bulging in the middle, little bulge beacuse little difference of radius
What shape is the Sun?
Oblate spheroid
What shape is the moon?
Triaxial ellipsoid
How to find oblateness?
f = a-b / a
f = oblateness
a = equatorial radius
b = polar radius