A VOYAGE IN SPACE
stars moving so rapidly that we can see their change of position in a week—almost in a day; but such cases are quite exceptional, and we may go much further and say that it is quite exceptional to find a star which has moved appreciably even after ten years. We have spent a great deal of time at Oxford lately in looking for such changes, by comparing photographs of the same region in the sky taken at intervals of at least ten years, and out of the hundreds of stars on the plates we only find one or two which have moved appreciably in ten years. Moreover, we find about the same number on a plate whether the region represented be in the Milky Way or far from it. In the former case there may be one thousand stars shown on the plate: in the latter case only one hundred: and yet we are just as likely to find two stars which have moved appreciably in the latter case as in the former. The inference seems to be that the extra 900 stars are all too far away to show the slightest motion in ten years: we must wait fifty or one .hundred years to see them move. If any one had photographed them fifty or one hundred years ago, we might to-day have known something of their movements; but unfortunately astronomers had not learnt how to photograph faint stars at that time; they began only twenty or thirty years ago.
It is time that we said a word as to the position of our own Earth in all this organized movement. Our Earth and all the other planets and satellites must be regarded as being carried along with the Sun. Compared with the distances of the stars our distance from the Sun is really very small,