We now use the small angle formula to convert the linear distance Neptune moves in one day, which you found in Step 2, into an angular size on the sky from Earth. So we need the distance from Earth to the planet. 2.06 105 = d D where D is the distance from the planet to the Sun minus the distance of 1.50 108 km between the Earth and the Sun. (Enter the linear distance in km and then calculate the angular size in arc seconds.) = 2.06 105 dkm km = arc seconds