body would undergo after the lapse of a stated period of time. Twenty miles a second means 1,200 miles a minute. That is of course 72,000 miles per hour, 1,728,000 miles per day, or 630,720,000 miles per annum. It therefore follows that in a million years the distance through which a star will move, on the assumption we have made, cannot be less than 600,000,000,000,000 miles.
I do not think that the effect of these considerations on the continuance of the visibility of stars throughout vast periods of time has always been quite fully appreciated. The figures given will provide a demonstration that there must have been a vast change in the appearance of the heavens within the lapse of the last million years. To carry the inquiry much farther, we ought, however, to be acquainted with the distance of each star under consideration, and this is an element of which we are ignorant in the great majority of cases. A single instance will, however, suffice for an illustration. The nearest star as far as we yet know in the northern hemisphere is 61 Cygni. There have been, it is true, some discrepancies between the various determinations of its distance which different astronomers have obtained. I think, however, that we cannot be far wrong in adopting a value of 50,000,000,000,000 miles. I shall therefore take this magnitude as a typical distance to which we may apply the arguments of the present chapter. It appears that in the course of a million years a star with the average speed of 20 miles a second would move over a distance which was about a dozen times as great as the distance between 61 Cygni and the solar system. It will be noted that in expressing the speed of
