Kim wrote:

No, not too much. Very interesting. Does Mars' apparent size create problems for using the diurnal parallax method to determine distance?

Yep. That's part of the uncertainty factor. But here again, modern CCD astrophotography and astrophoto software like AIP4WIN makes short work of getting an estimate of the apparent position of bright Mars. Think of Mars as a bright Poisson distributed object that you can use a built-in reduction tool on to extract a near exact set of plate coordinates. Having reread this stuff after a few years, the short-cut reduction method is: 1) use JPL Horizons to extract the heliocentric x,y,z coordinates of your topocentric position at the start and end times of your photos. 2) Use the 3D distance formula between two 3D points to find the length of your Earth travel distance baseline. Take one-half the Earth-traveled distance. 3) Measure the small diurnal parallax angle from your photo. This will probably be on the order of less than 80 arcsecs for an asteroid belt object. Take one-half the measured diurnal parallax angle. 4) You now have a right-angle triangle to solve: y = sin ordinate = 1/2 Earth traveled distance in kilometers x = cos ordinate = distance to object in kilometers theta opposite angle = 1/2 the measured diurnal parallax in degrees tan(theta) = y/x x=y/tan(theta) Clear Skies - Kurt