It is well known that the photosphere or visible surface of the sun appears to be much brighter in the center of the disk than near its circumference. This is due entirely to its gaseous nature and to the fact that it is surrounded by an atmosphere of dense enveloping cooler gases. Rays from the center of the disk travel a shorter distance through this atmosphere than the rays from the rim and therefore are absorbed less by surrounding gases. We look further down into the sun's interior near the center of the disk than in the direction of its circumference and so the light appears more intense there. The photosphere is the region where sun-spots appear and they are found in zones extending from 8° to 35° on either side of the solar equator, never appearing exactly at the equator or near the poles. The disturbances that produce sun-spots and many allied phenomena occur cyclically in periods of eleven years on the average. The first outburst of the disturbance is manifested by the appearance of sun-spots in high solar latitudes. These break out and disappear and break out again with increased vigor, working gradually downward toward the solar equator, the maximum spottedness for a given period occurring in solar latitude about 16°. The disturbance finally dies out within 8° or 10° of the equator, but even before one cycle of disturbance has entirely passed away a new cycle has broken forth in high latitudes. So during the period of minimum spottedness there are four distinct belts, two in low latitudes, due to the dying disturbance, and two in high latitudes, due to the new disturbance. At sun-spot maximums there are two well-marked zones of great intensity, approximately 16° north and south of the sun's equator. Sun-spots are solar cyclones, occurring usually in groups, though large single spots appear less frequently. Each spot is quite sharply divided into an umbra and a penumbra. The umbra is the darker central portion, the funnel of the whirling cyclone, and the penumbra is composed of the outspreading gases, and is less dark than the umbra. The peculiar "thatch-straw" structure of the penumbra is due, it is believed, to the fact that the columns of gases that usually rise vertically from the sun's interior and from the "rice grains" of the photosphere are drawn into a horizontal position by the whirling motion that exists in the penumbra regions of a sun-spot and therefore we get a longitudinal rather than a cross sectional view of them. The umbra of a sun-spot is anywhere from a few hundred miles to fifty thousand miles in diameter, frequently exceeding the earth in size, while the penumbra occasionally reaches a diameter of two hundred thousand miles. Sun-spots of exceptional size can be seen even without the aid of a telescope.