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
hemisphere. It appears to have shifted its position toward the west,
while the hemispherical cap of sunshine has slipped eastward over the
globe of the planet.

In the next following section of the orbit the planet rotates through
another right angle, but, owing to increased distance from the sun, the
motion in the orbit now becomes slower until, when the planet arrives at
aphelion, _C_, the angular difference disappears and the cross is once
more just under the sun. On returning from aphelion to perihelion the
same phenomena recur in reverse order and the line between day and night
on the planet first shifts westward, attaining its limit in that respect
at _D_, and then, at perihelion, returns to its original position.

Now, if we could stand on the sunward hemisphere of Mercury what, to our
eyes, would be the effect of this shifting of the sun's position with
regard to a fixed point on the planet's surface? Manifestly it would
cause the sun to describe a great arc in the sky, swinging to and fro,
in an east and west line, like a pendulum bob, the angular extent of the
swing being a little more than forty-seven degrees, and the time
required for the sun to pass from its extreme eastern to its extreme
western position and back again being eighty-eight days. But, owing to
the eccentricity of the orbit, the sun swings much faster toward the
east than toward the west, the eastward motion occupying about
thirty-seven days and the westward motion about fifty-one days.

[Illustration: THE REGIONS OF PERPETUAL DAY, PERPETUAL NIGHT, AND
ALTERNATE DAY AN