The physical length of the ruler as a fraction of the circumference of the big circle is the same as the angular length of the ruler as a fraction of 360: length circumference = 0 360 Since circumference = 2 X radius and the radius of the big circle is the distance to the ruler/planet: length = 2 distance 0 360 Solving for the physical length of the ruler yields: length = 2 distance x 360 This general equation can be used along with the distance to your planet to work out a conversion factor that will convert your moon's orbital semi-major axis from arcseconds to AU; i.e. in the above equation, we will set length = a [AU], 0 = a ["], and calculate conversion factor ["/AU] : = 2T 360 1 3600" distance. conversion factor = type your answer... "/AU. Note: in order for units to work out, we had to multiply by the number of degrees per arcsecond (") in the above formula.