Size or diameter of a planet can be measured with the help of parallax method. Distance D of the planet from earth is measured with the help of

relation D = b/\theta where b is the distance between observatories on earth and \theta is the angle between two directions along which planet was viewed from two observations.

Let d be the diameter of planet and a the angle subtended by the diameter d at a point E on earth.

then a being very small,d/D < < 1. Let AB be an arc

(of length d) of a circle with centre E. Then distance

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