This calculates the angle subtended by an object of a certain size at a certain distance. Complete any 2 of the numerical fields, having first selected appropriate units, and have the empty field calculated when the button is clicked. Changing units automatically converts any value in the associated field so this can also be used as a units converter on single fields.
Example: Jupiter on 2011 Apr 13: equatorial radius 71492 km; geocentric distance 5.9467474 au (Δ from ephemeris).
So enter: size units km, value (2 x radius) 142984; distance units au, value 5.9467474; angle units arc seconds.
Calculate gives apparent equatorial diameter of 33.1518 arc sec.