The rotation of the sun must produce an ellipticity or bulging out of the equator, as in the case of the earth. But this effect is so small as to elude all astronomical measurement. To all appearance the sun, notwithstanding its rotation, is a perfect sphere.
Two very remarkable laws govern the solar spots, one relating to their frequency, the other to the region of the sun's disc on which they are seen. We recall the fact 9f the sun having north and south latitude as we have on the earth. The first peculiarity of the spots is that they are rarely seen at more than to 40° of solar latitude, north or south. They are most numerous at about 15° latitude, and from that parallel grow less numerous both toward and from the equator. At the equator they are com paratively scarce. Thus we see that there are two zones of spots; one north and the other south of the sun's equator. The following table of the number of spots observed by Carnngton in different zones of latitude illustrates the law: Limiting Number lat. of zones of spots to 471 5 to 10 1,940 10 to 15 2,522 15 to 20 2,158 20 to 25 1.303 25 to 30 740 30 to 35 203 35 to 40 84 The other remarkable feature of the spots is their periodicity. At intervals of 11 years they are very numerous, while at the inter mediate times they are comparatively scarce, none being visible for perhaps half the time. It is found that the period is almost exactly 11 years 47 days. It was at one time supposed to be the same as the period of revolution of Jupiter, which is somewhat less than 12 years. Had such been the case we should have con cluded that the spots were in some way pro duced by the action of that planet. But it is now proved that the period is more than six months less than that of Jupiter, and does not coincide with any other period known in the solar system. Its average duration is found to be 11.13 years; but it is subject to changes of a year or more. We, therefore, conclude that the cycle of change in the spots is due to a round of processes going on inside the sun itself. What these may be we have no means of determining.
The very careful series of photographs of the sun made at the Greenwich Observatory the past 30 years show a singular law of variation in the spots with the 11-year period.
After a space of two or three years, during which, as we have said, the spots are few and small, the first evidence of a renewal of activity is seen in the occasional appearance of a spot at an unusually high latitude, say to 35° north or south. This may continue for several months, or even a year. Then the spots begin to be more frequent nearer and nearer the equa tor, while there are fewer beyond of lati tude. Finally, after three or four years, they become thickest of all, as we have said, in about 15° of latitude. About five years from the time when fewest are seen, they will be most numerous; then they will also be seen nearer the equator, or even on the equator itself. After five years they begin to diminish rather more slowly than they increased, until they gradually seem once more almost to disappear. The years of minimum sun spots are 1889, 1900, 1911, etc. The years when they are most numerous were 1882, 1893, 1904, etc. But these dates are only approximate, as the intervals be tween the minima are not always exactly the same.
The Facula.— Another curious feature of the sun is the occasional appearance of spots brighter than the rest of the photosphere. These commonly appear in bunches in the neighbor hood of the spots, and derive their name from the Latin word facula, a torch. They follow a law similar to that of the spots not only in having the same period of frequency, but in being most numerous where the spots are most numerous. But they are seen over a very much wider region of the sun's disc than the spots, sometimes, although rarely, near the poles of the sun.
A third feature of the sun associated with the spots and the faculm is known as the These can be actually seen through an ordinary telescope only during total eclipses of the sun. But they may now be seen at any time, when the air is clear, by means of a powerful spectroscope. Like the faculse, they are commonly, but not always, found in the neighborhood of the spots, being seen from time to time all round the sun's limb. (See ECLIPSE). They will be described more fully hereafter.