It is possible to determine the size of any planet
once we know the distance S of the planet. The image of
every heavenly body is a disc when viewed through a
optical telescope. The angle θ between two extreme points
A and B on the disc with respect to a certain point on the
Earth is determined with the help of a telescope. The
angle θ is called the angular diameter of the planet. The
linear diameter d of the planet is then given by
d = distance × angular diameter