The Solar Spots.— When the Galilean tele group of these objects close together. The spots of the group may run into each other to any extent, forming an irregular and jagged mass. Sometimes a spot has almost the appear ance of a crack in the photosphere. In the spot two portions can generally be distinguished, a dark interior called the uumbrap and a shaded border much brighter than the umbra, though not quite so bright as the photosphere, called the "penumbra.* When the atmosphere is steady, the latter is seen, with a good telescope, to be not of uniform shade, but to be striated, presenting an appearance somewhat like that of a thatched roof. This can be seen better by a figure than by a long description. A spot seems black only by contrast with the brilliancy of the sun. If it were possible to cut off the sun's light, the light from the spot itself would be of dazzling brightness.
It was formerly supposed that the spots were openings in the photosphere through which a darker interior was seen. This conclusion was reached because it was supposed that when the spot approached the edge of the solar disc the penumbra loolced broader on the side of tht spot next the edge. But careful observations made in recent times show that this is not the case. Sometimes the penumbra is broader at one edge and sometimes at another. It was also supposed that the spots might be something in the nature of cooler dark metals floating on the photosphere. But this view also has been abandoned. It has recently been shown by Hale that a sun spot is an immense vortex, in which whirling electrically charged particles produce an intense magnetic field. Thus a sun spot re sembles a terrestrial tornado. The observations of Evershed and St. John indicate that the gases are rising from the interior of the sun toward the surface, flowing nearly radially out ward above the surface from the centre of the vot. Measurements of the heat radiated from the spot, as compared with that of the neighbor ing disc, show that the spot is really cooler than the rest of the photosphere. Spectroscopic ob servations agree with this by showing a great absorption of the light coming from the interior of a spot.
Another salient feature of the sun is its rotation on an axis deviating only six degrees from a line perpendicular to the ecliptic. The time of rotation is shown by observations of the spots on the sun, which we see to move from east toward west. The relation of the sun to its axis of rotation is much the same as in the case of the earth. The sun's axis inter sects the photosphere at two opposite points called the poles of the sun. A circle passing round the sun midway between the poles is called the solar equator. Distances north and
south of the equator are called solar latitude. When we look at the sun at noon its north and south poles are near the upper and lower points of the disc; and the equator passes horizontally, or nearly so, across the centre of the disc. The position of the sun's equator is more exactly defined by the following numbers: Inclination to the ecliptic r 15' Longitude of the node 74° 29' The earth in its annual course around the sun passes through the line of the nodes about 5 June and 5 December of each year. At these times the apparent paths of the spots across the sun's disc are straight lines. At the interme diate times they are more or less curved. In March the south pole is slightly turned toward us, and the paths of the spots are curved up ward. In Septetnber the reverse is the case. We see only the north pole, and the paths are curved downward.
Observations of the spots lead to the unex pected conclusion that the equatorial regions of the sun rotate in less time than those nearer the poles, although the distance they have to go is greater. The sun is so much larger than the earth that, although the time of rotation is more than .25 times as long, yet the absolute linear velocity of the rotation near the equator is four nnies as great as that of the earth's rotation, being very nearly one mile per second.
:The observations of Carrington give the period of rotation as follows : At the sun's equator 24. 9 days.
At me of latitude 26.4 • The apparent time of rotation, as we see it, is nearly two days longer, because the earth has carried us forward in its annual motion while the sun is rotating, and the spot has to catch up to our direction before a rotation seems complete.
The rotation of the sun can also be de termined by means of the spectroscope, through this instrument enabling us to determine whether a luminous body is approaching the earth, or receding from it. In consequence of the rotation, the photosphere on the east side of the sun is continually moving toward us, and that on the opposite side moving away from us. The observations made by this method agree with those made on the solar spots in giving a period of about 25 or 26 days; but they are dis cordant as to the variation with latitude. The angular motions in the different latitudes, found by two observers with the spectroscope, Duner and Adams, are as follows: This difference agrees with much other evi dence on the same point in indicating that the different elements effective in producing the lines lie at different levels around the ball of the sun.