The axis of the Earth not only rolls on itself, but it does so with a slight staggering, nodding motion, due to the attractions of the Sun and Moon, known as +Nutation+. And the axis does not remain fixed in the solid substance of the Earth, but moves about irregularly in an area of about 60 feet in diameter. The positions of the north and south poles are therefore not precisely fixed, but move, producing what is known as the +Variation of Latitude+. Then star-places have to be corrected for the effect of our own atmosphere, _i.e._ refraction, and for errors of the instruments by which their places are determined. And when all these have been allowed for, the result stands out that different stars have real movement of their own--their +Proper Motions+.
No stars are really "fixed"; the name "+fixed stars+" is a tradition of a time when observation was too rough to detect that any of the heavenly bodies other than the planets were in motion. But nothing is fixed. The Earth on which we stand has many different motions; the stars are all in headlong flight.
And from this motion of the stars it has been learned that the Sun too moves. When Copernicus overthrew the Ptolemaic theory and showed that the Earth moves round the Sun, it was natural that men should be satisfied to take this as the centre of all things, fixed and immutable. It is not so. Just as a traveller driving through a wood sees the trees in front apparently open out and drift rapidly past him on either hand, and then slowly close together behind him, so Sir WILLIAM HERSCHEL showed that the stars in one { 85} part of the heavens appear to be opening out, or slowly moving apart, while in the opposite part there seems to be a slight tendency for them to come together, and in a belt midway between the two the tendency is for a somewhat quicker motion toward the second point. And the explanation is the same in the one case as in the other--the real movement is with the observer. The Sun with all its planets and smaller attendants is rushing onward, onward, towards a point near the borders of the constellations Lyra and Hercules, at the rate of about twelve miles per second.
Part of the Proper Motions of the stars are thus only apparent, being due to the actual motion of the Sun--the "+Sun's Way+," as it is called--but part of the Proper Motions belong to the stars themselves; they are really in motion, and this not in a haphazard, random manner. For recently KAPTEYN and other workers in the same field have brought to light the fact of +Star-Drift+, _i.e._ that many of the stars are travelling in associated companies. This may be illustrated by the seven bright stars that make up the well-known group of the "Plough," or "Charles's Wain," as country people call it. For the two stars of the seven that are furthest apart in the sky are moving together in one direction, and the other five in another.