Aphelion, the point where the planet is the most remote from the Sun. In like manner the points marked Perihelion and Aphelion on the inner circle show the corresponding points of the Earth's orbit, which is much more nearly circular. Now as the two planets revolve in different periods of time, Mars taking 686.98 of our days to complete his circuit, and the Earth 365.26 days to complete hers, the one planet will overtake the other only once every two years and two months or so. Meanwhile they are at great distances apart. But even when they do meet, they do not always meet equally near. For the one orbital period is not an exact multiple of the other, and as the orbits are both ellipses, it is evident that these meetings of the two planets will occur at different points of their orbits, and, therefore, at different distances. If the meeting occur when Mars is in perihelion the planets approach one another within 35,050,000 miles; if in aphelion, only within 61,000,000 miles.

But even this difference in distance does not measure the full extent of the variation in brilliancy. As the brightness of an illuminated body varies inversely as the square of its distance from the source of light, and as the total amount of light it reflects to an observer varies inversely as the square of his distance from it, it makes every difference in the apparent bril-