Standing on the Sun

I therefore once again think it to have happened by divine arrangement, that I arrived at the same time in which he was intent upon Mars, whose motions provide the only possible access to the hidden secrets of astronomy, without which we would remain forever ignorant of those secrets.

Mars moves around the sun. But, so does our Earth. This adds a complexity to observing Mars's motion, for we do not see Mars's location, but we rather see the direction of the line connecting the Earth and Mars as it points in the direction of the fixed stars behind Mars. To eliminate the effects of the Earth's motion, all we have to do is stand on the sun! But this is easier than it sounds. Every 780 days or so, the Sun, Earth, and Mars are lined up in a formation known as opposition. At that moment, the sun sees Mars in exactly the same direction as the Earth. By using these observations, astronomers remove what had come to be known as the "second inequality," leaving behind only the "first inequality," the ostensible subject of Part II of the New Astronomy: the motion of Mars proper to itself, and not our changing perspective of it.