Angular diameter

Angular diameter, or apparent size, is a magnitude describing how massive a sphere or circle seems from a point of view. In astronomy, the magnitude (size) of objects in the sky are given in terms of their angular diameter as seen from earth, instead of their actual sizes, with the units in arcseconds. 

An arcsecond is 1/3600th of 1°, and a radian is 180/pi°, therefore by rearranging 1 radian is 3600*180/pi arcseconds, which is 206265 arcseconds. Thus, the angular diameter is given by the formula

Where d represents the diameter of the body and D is the distance to the body from earth.

Angular diameter : The angle subtended by an object

The angular diameter of the sun can be calculated by plugging in :

Which gives a value of approximately 1919" (arcseconds).