Let us approach even nearer.  We see a clearly recognized even disk, of equal dimensions in every direction.  This is the photosphere.  We here reach some definitely measurable data for estimating its visible size.  We already know its distance.  Its disk subtends an angle of 32’ 12".6, or a little more than half a degree.  Three hundred and sixty such suns, laid side by side, would span the celestial arch from east to west with a half circle of light.  Two lines drawn from our earth at the angle mentioned would be 860,000 miles apart at the distance of 92,500,000 miles.  This, then, is the diameter of the visible and measurable part of the sun.  It would require one hundred and eight globes like the earth in a line to measure the sun’s diameter, and three hundred and thirty-nine, to be strung like the beads of a necklace, to encircle his waist.  The sun has a volume equal to 1,245,000 earths, but being only one-quarter as dense, it has a mass of only 326,800 earths.  It has seven hundred times the mass of all the planets, asteroids, and satellites put together.  Thus it is able to control them all by its greater power of attraction.

Concerning the condition of the surface of the sun many opinions are held.  That it is hot beyond all estimate is indubitable.  Whether solid or gaseous we are not sure.  Opinions differ:  some incline to the first theory, others to the second; some deem the sun composed of solid particles, floating in gas so condensed [Page 90] by pressure and attraction as to shine like a solid.  It has no sensible changes of general level, but has prodigious activity in spots.  These spots have been the objects of earnest and almost hourly study on the part of such men as Secchi, Lockyer, Faye, Young, and others, for years.  But it is a long way off to study an object.  No telescope brings it nearer than 200,000 miles.  Theory after theory has been advanced, each one satisfactory in some points, none in all.  The facts about the spots are these:  They are most abundant on the two sides of the equator.  They are gregarious, depressed below the surface, of vast extent, black in the centre, usually surrounded by a region of partial darkness, beyond which is excessive light.  They have motion of their own over the surface—­motion rotating about an axis, upward and downward about the edges.  They change their apparent shape as the sun carries them across its disk by axial revolution, being narrow as they present their edges to us, and rounder as we look perpendicularly into them (Fig. 35).

[Illustration:  Fig. 35.—­Change in Spots as rotated across the Disk, showing Cavities.]

These spots are also very variable in number, sometimes there being none for nearly two hundred days, and again whole years during which the sun is never without them.  The period from minimum to maximum [Page 91] of spots is about eleven years.  We might look for them again and again in vain this year (1878).  They will be most numerous in 1882 and 1893.  The cause of this periodicity was inferred to be the near approach of the enormous planet Jupiter, causing disturbance by its attraction.  But the periods do not correspond, and the cause is the result of some law of solar action to us as yet unknown.