142

Logic 142

gave a triangel of which two angles and the intermediate side (the distance between the two positions of the earth) were known (the mean distance of the sun from the earth being taken as unit of distance).
From that he could calculate the distance of Mars from the sun with no assumption except that Mars was really of the same point of his orbit about which there could (for a reason too long to set forth here) hardly be the least doubt whether the elliptic orbit were correct or not.
By trying this at times when Mars was at the two extremes of his orbit and when he was at intermediate places Keppler could get a test of the secerest character as to whether the elliptic Theory really flattened the orbit by the right amount or not.
In the cases of the few but well situated pairs of observations which could be found that were suitable to this test the accord of observation and theory was all that could