As the planet alternately approaches and recedes from the sun its orbital velocity, as we have already remarked, varies between the limits of twenty-three and thirty-five miles per second, being most rapid at the point nearest the sun.

But this variation in the speed of its revolution about the sun does not, in any manner, affect the rate of rotation on its axis.

The latter is perfectly uniform and just fast enough to complete one axial turn in the course of a single revolution about the sun.

The accompanying figure may assist the explanation.
[Illustration: DIAGRAM SHOWING THAT, OWING TO THE ECCENTRICITY OF ITS ORBIT, AND ITS VARYING VELOCITY, MERCURY, ALTHOUGH MAKING BUT ONE TURN ON ITS AXIS IN THE COURSE OF A REVOLUTION ABOUT THE SUN, NEVERTHELESS EXPERIENCES ON PARTS OF ITS SURFACE THE ALTERNATION OF DAY AND NIGHT.] Let us start with Mercury in perihelion at the point _A_.

The little cross on the planet stands exactly under the sun and in the center of the illuminated hemisphere.