when nearest the Sun his distance is 129,500,000 miles, when at his mean distance 141,500,000 miles, and when most remote 154,500,000 miles. The proportion of light and heat he receives respectively is therefore roughly as 16 to 20 to 24; or half as much again at certain times as at others.
So much in our knowledge of Mars is pretelescopic. Men might have and practically did learn this much without ever seeing the planet other than as a point of light. Its orbit was tolerably accurately known and could have been known still more accurately without telescopic aid; not so, until we become much more nearly omniscient than we at present are, the planet's self.
III. SIZE AND SHAPE
With the telescope we enter upon a new phase in our knowledge of the planet: the determination of its shape and size.
The relative plan of the solar system can be learned with great accuracy from observations of the motions of its members; not so easily learned is the scale upon which it is constructed. Although the former is intrinsically a very complicated, the latter a very simple problem, two characteristics of the actual system makes it possible to solve the former much more nearly than the latter. One of these