text: trigonometry: apparent size????? ok, can someone help me learn how to calculate this?  the diameter of the moon is about 3500 km. it's apparent sizeis about 0.0087 radians. about how far is it from earth? picture a right triangle.  moon's distance is the base, moon's diameter is the height.  long base, small height.  angle between base and hypotenuse is 00087 radians.  height / base is tangent of that angle.  so tan(0.0087) = 3500 / distance.  rewrite as distance = 3500 / tan(0.0087) and solve with calculator.  answer should be in ballpark of 240,000 miles which is 384,000 km.