If a star, say for instance Sirius, were motionless, or rather if it retained a constant distance from the earth, Fraunhofer's lines would occupy exactly the same position in the spectrum as they do in that of the Sun. On the contrary, if Sirius were approaching, the lines would be slightly shifted towards the blue, or if it were receding towards the red. Fig. 55 shows the displacement of the hydrogen line in the spectrum of Rigel, due to the fact that it is receding from us at the rate of 39 miles a second. The Sun affords us an excellent test of this theory. As it revolves on its axis one edge is always approaching and the other receding from us at a known rate, and observation shows that the lines given by the light of the two edges differ accordingly. So again as regards the Stars, we obtain a similar test derived from the Earth's movement. As we revolve in our orbit we approach or recede any given star, and our rate of motion being known we thus obtain a second test. The results thus examined have stood their ground satisfactorily, and in Huggins' opinion may be relied on within about an English mile a second. The effect of this movement is, moreover, independent of the distance. A lateral motion, say of 20 miles a second, which in a nearer object would appear to be a stupendous velocity, becomes in the Stars quite imperceptible. A motion of the same rapidity, on the other hand, towards or away from us, displaces the dark lines equally, whatever the distance of the object may be. We may then affirm that Sirius, for instance, is receding from us at the rate of about 20 miles a second. Betelgeux, Rigel, Castor, Regulus, and others are also moving away; while some--Vega, Arcturus, and Pollux, for example--are approaching us. By the same process it is shown that some groups of stars are only apparently in relation to one another. Thus in Charles' Wain some of the stars are approaching, others receding. I have already mentioned that Sirius, though it seems, like other stars, so stationary that we speak of them as "fixed," is really sweeping along at the rate of 1000 miles a minute. Even this enormous velocity is exceeded in other cases. One, which is numbered as 1830 in Groombridge's _Catalogue of the Stars_, and is therefore known as "Groombridge's 1830," moves no less than 12,000 miles a minute, and Arcturus 22,000 miles a minute, or 32,000,000 of miles a day; and yet the distances of the Stars are so great that 1000 years would make hardly any difference in the appearance of the heavens. Changes, however, there certainly would be. Even in the short time during which we have any observations, some are already on record. One of the most interesting is the fading of the 7th Pleiad, due, according to Ovid, to grief at the taking of Troy. Again, the "fiery Dogstar," as it used to be, is now, and has been for centuries, a clear white.