The great luminary which warms, lights, and rules the solar system is,
like the majority of its fellow stars, a gigantic bubble. In other
words, it is a globe of glowing gas, which is nowhere solid, though the
immense pressure which must exist in its interior probably causes this
gas to assume there a density greater than that of any solid which we
know.
[Illustration: =A PHOTOGRAPH OF THE SUN, SHOWING THE CLOUDS OF FIERY
VAPOR WHICH SURROUND IT=]
DIMENSIONS OF THE SUN. The sun appears to human vision as a brilliant
globe of a little more than half a degree in diameter. It is about the
same apparent size as the moon, since the size of the sun is to that of
the moon very nearly in the same proportion as their relative distances
from the earth. In reality, however, the sun is a gigantic orb, so huge
that if the earth were at its center the whole orbit of the moon would
lie well within its circumference. The diameter of the sun is about
866,500 miles.
The mass of the sun is about 332,000 times that of the earth, but its
specific gravity is only about a quarter that of the earth, 1.41, if
that of water be taken as unity. The mean distance of the sun from the
earth is about 92,800,000 miles; but, as the earth’s orbit is not
circular but elliptic, this distances varies by about 3,000,000 miles,
being smallest in January and greatest in July.
THE PHYSICAL CONDITION of the sun is very different from that of the
earth, though we know it is composed of very similar materials. The
white-hot surface that we see, called the _photosphere_, is believed to
be largely a shell of highly heated metallic vapors surrounding the
unseen mass beneath. Dark spaces seen in the photosphere are known as
_sun-spots_, and these are often surrounded by brighter patches, termed
_faculæ_. Above the photosphere a shallow envelope of gases, rising here
and there into huge prominences, and known as the _chromosphere_, is
seen in red tints when the sun is totally eclipsed. Beyond the
chromosphere, there is also seen, at the same time, a faint but far more
extensive envelope called the _corona_.
[Illustration: This diagram illustrates the theory that sun-spots are
formed by fragments struck from Saturn’s rings (which are in themselves
nothing more than a great meteoric swarm) by the swarm of meteors known
as the Leonids, which fragments fall into the solar furnace at a speed
of four hundred miles a second.]
The sun’s rays supply light and heat not only to the earth, but also to
the other planets which revolve round it. Its attraction confines these
planets in their orbits and controls their motions.