To get some conception of what a "light-year" means, let us remember that light could travel right round the earth at its equator seven times in the space of a single second, and that there are 31,556,925 seconds in a year. Light then could girdle the earth a thousand million times whilst it comes from Alpha Centauri. Or we may put it another way. The distance from Alpha Centauri exceeds the equator of the earth by as much as this exceeds an inch and a half; or by as much as the distance from London to Manchester exceeds the hundredth of an inch. Of all the rest of the innumerable stars, as far as actual measurement is concerned, for us, as for the Hebrews, they might all actually lie on the texture of a curtain, at practically the same distance from us. We have measured the distances of but a very few stars; the rest--as every one of them was for the Hebrew--are at a greater distance than any effort of ours can reach, be our telescopes ever so great and powerful, our measuring instruments ever so precise and delicate. For them, as for us, the heaven of stars is "for height," for a height which is beyond measure and therefore the only fitting image for the immensity of God. So Zophar the Naamathite said-- "Canst thou find out the Almighty unto perfection? It is as high as heaven; what canst thou do?" and Eliphaz the Temanite reiterated still more strongly-- "Is not God in the height of heaven? And behold the height of the stars, how high they are." God Himself is represented as using the expanse of heaven as a measure of the greatness of his fidelity and mercy. The prophet Jeremiah writes-- "Thus saith the Lord; if Heaven above can be measured, and the foundations of the earth searched out beneath, I will also cast off all the seed of Israel for all that they have done, saith the Lord." As if he were using the figure of a great cross, whose height was that of the heavens, whose arms stretched from east to west, David testifies of the same mercy and forgiveness:-- "For as the heaven is high above the earth, So great is His mercy toward them that fear Him. As far as the east is from the west, So far hath He removed our transgressions from us." [Illustration: THE GREAT COMET OF 1843. "Running like a road through the constellations" (_see_ p. 105).] CHAPTER IX COMETS Great comets are almost always unexpected visitors. There is only one great comet that we know has been seen more than once, and expect with reasonable certainty to see again. This is Halley's comet, which has been returning to a near approach to the sun at somewhat irregular intervals of seventy-five to seventy-eight years during the last centuries: indeed, it is possible that it was this comet that was coincident with the invasion of England by William the Conqueror.