the movements of the separate stars in a given time: you will see that many of these arrows are about in the same direction and about of the same length. The stars are shown as round black dots (as they would be on a photographic negative) in their present positions, and the arrow-heads show where they will get to after thousands of years.
Since the arrows are not quite equal in length, and quite the same in direction, the arrow-heads form a figure rather different from the arrow-tails: that is to say, after this lapse of time the partners will have taken up rather different positions. They have felt the pulls of gravitation and responded to them—at any rate this is a reasonable explanation. They are waltzing to a certain extent, but not enough to prevent them all keeping together and moving down the room as a group of partners.