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16 to be very difficult to determine with accuracy. Fortunately this is matter chiefly of theoretic regret, as we now know the actual sizes to within a degree of exactness practically sufficient for most purposes but perturbations; to within about 1/300 part of the whole, so far as our ultimate measure is concerned, the distance we are off from the Sun.

A good idea of the method and some appreciation of the difficulty involved in it can be got by considering a precisely similar case, that of determining the distance of a spire a mile and three fifths away by shutting first one eye and then the other and noting the shift of the spire against its background. It is needless to add that without telescopic aid the determination is impossible, and that it is exceeding difficult with it.

Nevertheless, from the distance of the Sun determined in this manner, we find from measurements of the apparent disk of the planet made at Flagstaff that Mars is about 4,215 miles in diameter. This makes his surface a little more than a quarter that of the Earth and his volume about one seventh of hers.

The next point to find out is his mass, that is, the amount of matter he contains. This is very easy to determine when a planet has a satellite, and very difficult to determine when