Explain method for measurement of dimension of a planet or star.

Explanation

Angular diameter: From a point on surface of the earth angle subtended at diameter of the earth is called angular diameter `(alpha)`.
Let diameter of the earth is d.
As shown in figure from point of observation on surface of the earth a can be obtained at diametrically opposite point.
Distance between the earth and planet (Star) is D.

`alpha=(d)/(D)` (radian)
isused to measure diameter (d) of the planet.
`I^(@)`(degree) =60. (min)
I. (min)=60.. (sec)
`I^(@)=3600..` (sec)