planet, is produced by the extraordinary eccentricity of its orbit. 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. The large arrows show the direction in which
the planet travels in its revolution about the sun, and the small curved
arrows the direction in which it rotates on its axis. Now, in moving
along its orbit from _A_ to _B_ the planet, partly because of its
swifter motion when near the sun, and partly because of the elliptical
nature of the orbit, traverses a greater angular interval with reference
to the sun than the cross, moving with the uniform rotation of the
planet on its axis, is able to traverse in the same time. As drawn in
the diagram, the cross has moved through exactly ninety degrees, or one
right angle, while the planet in its orbit has moved through
considerably more than a right angle. In consequence of this gain of the
angle of revolution upon the angle of rotation, the cross at _B_ is no
longer exactly under the sun, nor in the center of the illuminated