The vast Size of the Sun--Hotter than Melting Platinum--Is the Sun the Source of Heat for the Earth?--The Sun is 92,900,000 miles distant--How to realise the magnitude of this distance--Day and Night--Luminous and Non-Luminous Bodies--Contrast between the Sun and the Stars--The Sun a Star--Granulated Appearance of the Sun--The Spots on the Sun--Changes in the Form of a Spot--The Faculae--The Rotation of the Sun on its Axis--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 elaborate furnaces, but it does not attain the temperature of the sun.