in the
spectrum of Rigel.]

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
t