is--View of a Typical
Sun-Spot--Periodicity of the Sun-Spots--Connection between the
Sun-Spots and Terrestrial Magnetism--Principles of Spectrum
Analysis--Substances present in the Sun--Spectrum of a Spot--The
Prominences surrounding the Sun--Total Eclipse of the Sun--Size and
Movement of the Prominences--Their connection with the
Spots--Spectroscopic Measurement of Motion on the Sun--The Corona
surrounding the Sun--Constitution of the Sun.

In commencing our examination of the orbs which surround us, we
naturally begin with our peerless sun. His splendid brilliance gives him
the pre-eminence over all other celestial bodies.

The dimensions of our luminary are commensurate with his importance.
Astronomers have succeeded in the difficult task of ascertaining the
exact figures, but they are so gigantic that the results are hard to
realise. The diameter of the orb of day, or the length of the axis,
passing through the centre from one side to the other, is 866,000 miles.
Yet this bare statement of the dimensions of the great globe fails to
convey an adequate idea of its vastness. If a railway were laid round
the sun, and if we were to start in an express train moving sixty miles
an hour, we should have to travel for five years without intermission
night or day before we had accomplished the journey.

When the sun is compared with the earth the bulk of our luminary becomes
still more striking. Suppose his globe were cut up into one million
parts, each of these parts would appreciably exceed the bulk of our
earth. Fig. 10 exhibits a large circle and a very small one, marked S
and E respectively. These circles show the comparative sizes of the two
bodies. The mass of the sun does not, however, exceed that of the earth
in the same proportion. Were the sun placed in one pan of a mighty
weighing balance, and were 300,000 bodies as heavy as our earth placed
in the other, the luminary would turn the scale.

[Illustration: Fig. 10.--Comparative Size of the Earth and the Sun.]

The sun has a temperature far surpassing any that we artificially
produce, either in our chemical laboratories or our metallurgical
establishments. We can send a galvanic current through a piece of
platinum wire. The wire first becomes red hot, then white hot; then it
glows with a brilliance almost dazzling until it fuses and breaks. The
temperature of the melting platinum wire could hardly be surpassed in
the most elabo