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To measure the distance between two points on an astronomical object we use angular measurements.

For example if  ? radians is the angle subtended by two diametrically opposite ends of the moon at a point on the earth,then the diameter of the moon x = r?,where r is the distance of the moon from the earth.                                                                                                                                              

2)Parallax method 

 To measure the distance r of a far away object S by parallax method we look at it from two positions A and B separated by a distance x.If  ? radians is the angle between the two directions,then

                                  ? = x/r            
       Therefore             r = x / ?