pearance. In 1839, Captain Davis saw one which he computed was not less than 186,000 miles long, and had an area of twenty-five billion square miles. If these are deep openings in the luminous atmosphere of the sun, what an abyss must that be at "the bottom of which our earth could lie like a boulder in the crater of a volcano !"

The spots consist of distinct parts.-From the accompanying representation it will be seen that the spots generally consist of one or more dark portions called the umbra, and around that a grayish portion styled the penumbra (pene, almost, and um

Fig. 6.

SUN SPOTS.

this, the umbra is sometimes divided by luminous bridges.

The spots are in motion.-They change from day to day; but they all have a common movement. About fourteen days are required for a spot to pass

across the disk of the sun from the eastern side or limb to the western; in fourteen days it reappears, changed in form perhaps, but generally recognizable.

The spots change their rapidity and apparent form as they pass across the disk.-A spot is seen on the eastern limb; day by day it progresses, with a gradually increasing rapidity, until it reaches the cen

tre

; it now gradually loses its rapidity, and finally disappears on the western limb. The diagram illustrates the apparent change which takes place in the form. Suppose at first it is of an oval shape; as it approaches the centre it apparently widens and becomes circular. Having passed that point, it becomes more and more oval until it disappears.

This change in the spots proves the sun's rotation on its axis.-These changes can be accounted for only on the supposition that the sun revolves on its axis indeed, they are the precise effects which the

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.)

b

SYNODIC AND SIDEREAL REVOLUTION.

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

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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.'

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.