Light takes time to travel. This sounds singular, but it is true. Light can not pass from place to place in no time. Light, journeying through space, is invisible. Only when it strikes upon something, whether a solid body or water or air, does it become visible to our eyes. The shining all round us in the day-time is caused by the sunlight being reflected, not only from the ground, but from each separate particle of air. If we had no atmosphere, we should see still the bright rays falling on the ground, but the sky above would be black. Yet that black sky would be full of millions of light-rays, journeying hither and thither from sun and stars, invisible except where they alight upon something. The speed of light is far beyond that of an express train, far beyond that of the swiftest planet. In one tick of the clock, Mercury has rushed onward twenty-nine miles. In one tick of the clock, storm-flames upon the surface of the sun will sweep over two or three hundred miles. But in one tick of the clock a ray of light flashes through one hundred and eighty-six thousand three hundred and thirty-seven miles. One hundred and eighty-six thousand miles! That is the same as to say that, during one single instant, a ray of light can journey a distance equal to about eight times round and round our whole earth at the Equator. By using this wonderful light-speed as a measurement, we gain clearer ideas about the distances of the stars. A ray of light takes more than eight minutes to pass from the sun to the earth. Look at your watch, and note the exact time. See the hand moving slowly through the minutes, and imagine one single ray of light, which has left the sun when first you looked, flashing onward and onward through space, one hundred and eighty-six thousand miles each second. Eight minutes and a half are over. The ray falls upon your hand. In those few minutes it has journeyed ninety-three millions of miles. So much for the sun’s distance. How about the stars? Alpha Centauri, a bright star seen in the southern hemisphere, is one of our nearest neighbors. Yet each light-gleam which reaches the eye of man from that star, left Alpha Centauri four years and a third before. During four years and a third, from the moment when first it quitted the surface of the blazing sun, it has flashed ceaselessly onward, one hundred and eighty-six thousand miles each second, dwindling down with its bright companion-rays from a glare of brilliancy to a slender glimmer of light till it reaches the eye of man. Four years and a third sounds much, side by side with the eight minutes’ journey from the sun. Sound would take more than three millions of years to cross the same abyss. At the constant velocity of thirty-seven miles an hour, an express-train starting from the sun Alpha Centauri would not reach the earth until after an uninterrupted course of nearly seventy-five millions of years.