rfaces equal to only
twice the diameter of the larger one. Again, in the system of Spica we
have two great suns swinging round each other in only four days, at a
distance equal to that between Saturn and his sixth satellite. But
although we have at present nothing analogous to this in our solar
system, it can be proved mathematically that it is perfectly possible
for a system of this kind to preserve its stability, if not for ever, at
any rate for ages, and we shall see in our last chapter that there was
in all probability a time when the earth and the moon formed a peculiar
system of two bodies revolving rapidly at a very small distance compared
to the diameters of the bodies.

It is possible that we have a more complicated system in the star known
as b Lyrae. This is a variable star of great interest, having a
period of twelve days and twenty-two hours, in which time it rises from
magnitude 4-1/2 to a little above 3-1/2, sinks nearly to the fourth
magnitude, rises again to fully 3-1/2, and finally falls to magnitude
4-1/2. In 1891 Professor Pickering discovered that the bright lines in
the spectrum of this star changed their position from time to time,
appearing now on one side, now on the other side of corresponding dark
lines. Obviously these bright lines change their wave length, the
light-giving source alternately receding from and approaching to the
earth, and the former appeared to be the case during one-half of the
period of variation of the star's light, the latter during the other
half. The spectrum of this star has been further examined by Belopolsky
and others, who have found that the lines are apparently double, but
that one of the components either disappears or becomes very narrow from
time to time. On the assumption that these lines were really single (the
apparent duplicity resulting from the superposition of a dark line),
Belopolsky determined the amount of their displacement by measuring the
distances from the two edges of a line of hydrogen (F) to the artificial
hydrogen line produced by gas glowing in a tube and photographed along
with the star-spectrum. Assuming the alternate approach and recession to
be caused by orbital revolution, Belopolsky found that the body emitting
the light of the bright lines moved with an orbital velocity of
forty-one miles. He succeeded in 1897 in observing the displacement of a
dark line due to magnesium, and found that the body emitting it was also
moving in an orbit