We sometimes speak of the sun as having a diameter of 860,000 miles.  We mean that that is the extent of the body as soon by the eye.  But that is a small part of its real diameter.  So we say the earth has an equatorial diameter of 7925-1/2 miles, and a polar one of 7899.  But the air is as much a part of the earth as the rocks are.  The electric currents are as much a part of the [Page 79] earth as the ores and mountains they traverse.  What the diameter of the earth is, including these, no man can tell.  We used to say the air extended forty-five miles, but we now know that it reaches vastly farther.  So of the sun, we might almost say that its diameter is infinite, for its light and heat reach beyond our measurement.  Its living, throbbing heart sends out pulsations, keeping all space full of its tides of living light.

[Page 80] [Illustration:  Fig. 31.—­Zodiacal Light.]

We might say with evident truth that the far-off planets are a part of the sun, since the space they traverse is filled with the power of that controlling king; not only with light, but also with gravitating power.

But come to more ponderable matters.  If we look [Page 81] into our western sky soon after sunset, on a clear, moonless night in March or April, we shall see a dim, soft light, somewhat like the milky-way, often reaching, well defined, to the Pleiades.  It is wedge-shaped, inclined to the south, and the smallest star can easily be seen through it.  Mairan and Cassini affirm that they have seen sudden sparkles and movements of light in it.  All our best tests show the spectrum of this light to be continuous, and therefore reflected; which indicates that it is a ring of small masses of meteoric matter surrounding the sun, revolving with it and reflecting its light.  One bit of stone as large as the end of one’s thumb, in a cubic mile, would be enough to reflect what light we see looking through millions of miles of it.  Perhaps an eye sufficiently keen and far away would see the sun surrounded by a luminous disk, as Saturn is with his rings.  As it extends beyond the earth’s orbit, if this be measured as a part of the sun, its diameter would be about 200,000,000 miles.

Come closer.  When the sun is covered by the disk of the moon at the instant of total eclipse, observers are startled by strange swaying luminous banners, ghostly and weird, shooting in changeful play about the central darkness (Fig. 32).  These form the corona.  Men have usually been too much moved to describe them, and have always been incapable of drawing them in the short minute or two of their continuance.  But in 1878 men travelled eight thousand miles, coming and returning, in order that they might note the three minutes of total eclipse in Colorado.  Each man had his work assigned to him, and he was drilled to attend to that and nothing else.  Improved instruments were put into his [Page 82] hands, so that the sun was made to do his own drawing and give his own picture at consecutive instants.  Fig. 33 is a copy of a photograph of the corona of 1878, by Mr. Henry Draper.  It showed much less changeability that year than common, it being very near the time of least sun-spot.  The previous picture was taken near the time of maximum sun-spot.