laws of perspective demand in that case.About twenty-seven days (27 d., 7 h.) elapse from the appearance of a spot on the eastern limb before it reappears a second time. During this time the earth has gone forward in its orbit, so that the location of the observer is changed; allowing for this, the sun's time of rotation is about twentyfive days (25 d., 8 h., 10 m.: Langier.) X

X Synodic and sidereal revolution of the spots.-We can easily understand why we make an allowance for the motion of the earth in its orbit. Suppose a

solar spot at a, on a line passing from the centre of the earth to the centre of the sun. For the spot to pass around the sun and come into that same position again, requires about twenty-seven days. But during this time, the earth has passed on from T to T'. The spot has not only travelled around to a again, but also beyond that to a', or the distance from a to a' more than an entire revolution. To do this requires, as we have said, about two days. A revolution from a around to a' is called a synodic, and one from a around to a again is called a sidereal revolution.

The spots apparently do not always move in straight lines. Sometimes their path curves toward

MARCH.

JUNE.

SEPTEMBER.

Fig. 9.

the north, and sometimes toward the south, as in the figure. This can be explained only on the supposition that the sun's axis is inclined to the ecliptic (7° 15').

The spots have a motion of their own.-Besides the motion already named as assigned to the sun's rotation, the spots seem to have a motion of their own,

and this fact is undoubtedly the cause of the variation in the estimates made of the time of the sun's revolution on its axis. A spot near the equator

performs a synodic revolution in about twenty-five days, while one half way to either pole requires twenty-eight days. One spot was noticed which had a motion three times greater than that of clouds driven along by the most violent hurricane. Again, immense cyclones occasionally pass over the surface with fearful rapidity, producing rotation and sudden changes in the spots. At other times, however, the spots seem to set sail and move across the disk of the sun like gondolas over a silver sea."

66

The spots change their real form.-Spots break out and then disappear under the very eye of the astronWollaston saw one that seemed to be shat

omer.

tered like a fragment of ice when it is thrown on a frozen surface, breaking into pieces, and sliding off in every direction. Sometimes one divides itself into several nuclei, while again several nuclei combine into one. Occasionally a spot will remain for six or eight rotations, while often one will last only half an hour. In one case, Sir. W. Herschel relates that when examining a spot through his telescope, he turned away for a moment, and on looking back it was gone.

X The appearance of the spots is periodical.—It is a remarkable fact that the number of spots increases and diminishes through a regular interval of about 11.11 years. These variations seem also to be connected with periodical variations in the aurora, and magnetic earth-currents, which interfere with the telegraph. The regular increase and diminution in the spots was discovered by Schwabe of Prussia, who watched the sun so carefully that it is said, "for thirty years the sun never appeared above the horizon without being confronted by his imperturbable telescope." Besides this, it has now been found that the activity of the sun's spots goes through another regular period of about 56 years. Independently of this conclusion, it has also been discovered that the aurora has a similar period of 56 years.

The spots are influenced by the planets.-They appear to be especially sensitive to the approach of Venus, on account of its nearness, and of Jupiter, because of its size. The area of the spots exposed

to view from the earth is uniformly greatest when Venus is on the opposite side of the sun from us, and least when on the same side. When both Venus and Jupiter are on the side of the sun opposite to us, the spots are much larger than when Venus alone is in that position. In part explanation of this influence of the planets, we may suppose that they, in some manner, modify reflection on the disk of the sun exposed to their action, and thus cause a condensation of gases.

The spots do not influence the fruitfulness of the season.-Sir W. Herschel first advanced the idea that years of abundant spots would be years also of plentiful harvest. This is not now generally received. What two years could be more dissimilar than 1859 and 1860? Both abounded in solar spots, yet one was a fruitful year and the other almost one of famine in Europe.

× The spots are cooler than the surrounding surface.It seems that the breaking out of a spot sensibly diminishes the temperature of that portion of the sun's disk. The faculæ, on the other hand, do not increase the temperature. (Secchi.)

The spots are depressions below the luminous surface. -This was thought probable before, but is conclusively proved by the photographs of the sun, which have been taken in large numbers of late at Kew Observatory.

Comparative brightness of spots and sun.—If we represent the ordinary brightness of the sun by