NASA's horizon system shows the angle (S-O-T) of Sun and planet from the view of Earth. Could you please tell me how one measure the angle of Sun and Mars when opposition nearly takes place. In other words, if S-O-T approaches 175/176/178 degrees, how does one from the view of Earth measure this angle? In general, if S-O-T is small (30 degrees, for instance), an instrument may be easily put to do this measurement, however, if S-O-T is too large (nearly approach 180 degrees), because of the spherical structure of the Earth body, an instrument is very difficult to be put. For instance, when opposition nearly takes place, for an observer on Earth, if Sun is at noon, Mars must be approach midnight. At the moment it is unlikely to for the observer to put an instrument to make measure. However, if Sun is at morning (dawn), Mars must be approach evening (dusk). At the moment it is also very difficult for the observer to put an instrument to measure. Because of the unevenness of ground, I even guess nobody can use an instrument to measure an angle of 177/178/179 degrees for Sun and Mars.