Page:Astronomy for Everybody.djvu/118

round the earth, the speed of rotation must be more than four times that of the earth's rotation to make it complete the circuit in the time that it does. At the sun's equator the speed is more than a mile a second.

The most curious feature of this rotation is that it is completed in less time at the equator than at a distance on each side of the equator. Were the sun a solid body, like the earth, all its parts would have to rotate at the same time. Hence the sun is not a solid body, but must be either liquid or gaseous, at least at its surface.

The equator of the sun is inclined six degrees to the plane of the earth's orbit. Its direction is such that in our spring months the north pole is turned six degrees away from us and the central point of the apparent disk is about that amount south of the sun's equator. In our summer and autumn months this is reversed.

By the mean density of the sun we refer to the average specific gravity of the matter composing it, or the ratio of its weight to that of an equal volume of water. It is known that the density is only about one fourth that of the earth, and about four tenths greater than that of water. Stated with more exactness, the figures are:

Density of sun : Density of earth = 0.2554.

Density of sun: Density of water =: 1.4115.

The mass or weight of the sun is about 334,000 times that of the earth.

The force of gravity at the sun's surface is 27 times that of the earth. If it were possible for a human being