When we see anything, whether it be the book which we hold in our hands or a star millions of miles away, we do so by means of a vibration in the ether, commonly called a ray of light, which passes from the object seen to our eyes. Now the speed with which this vibration passes is so great--about 186,000 miles in a second--that when we are considering any object in our own world we may regard it as practically instantaneous. When, however, we come to deal with interplanetary distances we have to take the speed of light into consideration, for an appreciable period is occupied in traversing these vast spaces. For example it takes eight minutes and a quarter for light to travel to us from the sun, so that when we look at the solar orb we see it by means of a ray of light which left it more than eight minutes ago. From this follows a very curious result. The ray of light by which we see the sun can obviously report to us only the state of affairs which existed in that luminary when it started on its journey, and would not be in the least affected by anything that happened there after it left; so that we really see the sun not as he _is_, but as he was eight minutes ago. That is to say that if anything important took place in the sun--the formation of a new sun-spot, for instance--an astronomer who was watching the orb through his telescope at the time would be quite unaware of the incident while it was happening, since the ray of light bearing the news would not reach him until more than eight minutes later. The difference is more striking when we consider the fixed stars, because in their case the distances are so enormously greater. The pole star, for example, is so far off that light, travelling at the inconceivable speed above mentioned, takes a little more than fifty years to reach our eyes; and from that follows the strange but inevitable inference that we see the pole star not as and where it is at this moment, but as and where it was fifty years ago. Nay, if to-morrow some cosmic catastrophe were to shatter the pole star into fragments, we should still see it peacefully shining in the sky all the rest of our lives; our children would grow up to middle age and gather their children about them in turn before the news of that tremendous accident reached any terrestrial eye. In the same way there are other stars so far distant that light takes thousands of years to travel from them to us, and with reference to their condition our information is therefore thousands of years behind time.