Let us ask the reader to imagine what would take place if the earth were to come into collision with another planet. Some of our readers, at any rate, will know that the Sun and all its planets--the earth among them--is moving swiftly to some unknown destination among the stars.[6] Suppose that some great planet, not of the Solar System, barred our path. We should not be taken wholly unawares, for astronomers would know of the approach of the star and our earth to one another months, and perhaps years, beforehand. That would be because the light of the Sun falling on it would be reflected, just as the reflection of the Sun's rays light up the Moon for our eyes. If the strange planet were a very large body, like the sun for bigness, it would become visible far beyond the confines of the Solar System. It might first be taken for a new star such as sometimes blazes up in the sky and then sinks into darkness again. But its steadiness would make it an object of suspicion. There would be another brief period in which it might be taken for a comet; but comets have a light quite different from that of reflected sunlight. So that anxious expectation would be dissipated, and the world would begin to recognise the monster for what it really was. If its size were the same as that of the Sun, then it would first become visible to us when 15,000,000,000 miles distant,[7] or let us say 1600 times farther away from us than we are from the Sun. We and it would approach slowly at first. It would be nearly ten years before the distance had been reduced to 6,000,000,000 of miles, and the intruder had begun to be visible to the naked eye. In fourteen years it would have reached the outer edge of the Solar System, and would be the brightest star in the heavens. In another year it would be twice as bright as Venus at her brightest, and would be coming nearer with appalling swiftness. In less than two months it would be as near the Sun as we are. In a week more it would have plunged into the Sun at the rate of 400 miles a second, and in the awful heat born in that collision, Sun and earth and planets would be molten, and the Solar System overwhelmed In unremorseful folds of rolling fire. [Footnote 6: It is usually supposed that this movement, amounting to perhaps ten miles a second, is in the direction of the constellation of Vega.] [Footnote 7: See an article by Mr. Ellard Gore in _Knowledge_, November, 1905. The object would not be visible at this distance except through large telescopes.]