How will you measure the diameter of the Moon using parallax method?

Explanation

O - observation point on earth.

(i) In diagram, O is the observation point on the earth and d is the diameter of moon. An astronomical telescope held at O is focussed on moon, the image is observed into moon of a circular disc.
(ii) `angleAOB=theta`
S - average distance between moon and the surface of earth.
(iii) As 'S' is very large compared to the diameter, d of the moon, the diameter of the moon is considered as a circular arc of radius, S.
`d=S times theta`.
Hence d can be calculated, when 'S' is known and `theta` is measured.