few facts will enable us to estimate the stretch of space that divides us from the other world in which so much interest is now being taken.
The longest distance that could possibly intervene between the two globes is found when the sun lies between them and when they are each at their greatest possible distance from it. On the other hand the most favourable condition for the observation of Mars will be when the planet is making its nearest approach to the sun, and when the earth happens to be in the same direction as Mars from the sun.
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Fig. 13.—Orbits of the Satellites of Mars.
It can be shown that the very lowest value which the planet's distance from the earth can possibly assume would be about 35,000,000 miles. Nor is the condition of things which we have supposed one which will be often realised. No doubt every two years and two months, or more accurately every 780 days, the sun and Mars and the earth come nearly into a straight line, the earth being between the other two bodies; whenever this happens we have what is called the opposition of Mars. If the orbits of both Mars and the earth were circular, then any one opposition would be as good as any other, so far as proximity is concerned: for the distance between the earth and
