lowed in his circuit of the Sun to be situated with regard to the Earth's as in the following diagram.
Diagram of the Orbits of Mars and the Earth.
On consulting the diagram we shall at once perceive why it is that every fifteen years Mars becomes so unusually bright as to seem, to one who has not kept track of him, a new and startling star. His orbit, it will be seen, is an ellipse of some eccentricity, and deviates in consequence considerably from a circle. The point marked Perihelion denotes the point where the planet is nearest the Sun; the point marked