184. _The Rotation of the Sun._--The spots are evidently carried around by the rotation of the sun on its axis. It is evident, from Fig. 201, that the sun will need to make more than a complete rotation in order to bring a spot again upon the same part of the disk as seen from the earth. _S_ represents the sun, and _E_ the earth. The arrows indicate the direction of the sun's rotation. When the earth is at _E_, a spot at _a_ would be seen at the centre of the solar disk. While the sun is turning on its axis, the earth moves in its orbit from _E_ to _E'_: hence the sun must make a complete rotation, and turn from _a_ to _a'_ in addition, in order to bring the spot again to the centre of the disk. To carry the spot entirely around, and then on to _a'_, requires about twenty-seven days. From this _synodical period_ of the spot, as it might be called, it has been calculated that the sun must rotate on its axis in about twenty-five days. [Illustration: Fig. 202.] 185. _The Inclination of the Sun's Axis._--The paths described by sun-spots across the solar disk vary with the position of the earth in its orbit, as shown in Fig. 202. We therefore conclude that the sun's axis is not perpendicular to the plane of the earth's orbit. The sun rotates on its axis from west to east, and the axis leans about seven degrees from the perpendicular to the earth's orbit. 186. _The Proper Motion of the Spots._--When the period of the sun's rotation is deduced from the motion of spots in different solar latitudes, there is found to be considerable variation in the results obtained. Thus spots near the equator indicate that the sun rotates in about twenty-five days; while those in latitude 20° indicate a period about eighteen hours longer; and those in latitude 30° a period of twenty-seven days and a half. Strictly speaking, the sun, as a whole, has no single period of rotation; but different portions of its surface perform their revolutions in different times. The equatorial regions not only move more rapidly in miles per hour than the rest of the solar surface, but they _complete the entire rotation in shorter time_. [Illustration: Fig. 203.] There appears to be a peculiar surface-drift in the equatorial regions of the sun, the cause of which is unknown, but which gives the spots a _proper_ motion; that is, a motion of their own, independent of the rotation of the sun. [Illustration: Fig. 204.] 187. _Distribution of the Sun-Spots._--The sun-spots are not distributed uniformly over the sun's surface, but occur mainly in two zones on each side of the equator, and between the latitudes of 10° and 30°, as shown in Fig. 203. On and near the equator itself they are comparatively rare. There are still fewer beyond 35° of latitude, and only a single spot has ever been recorded more than 45° from the solar equator.