of changes. Some go through
the whole of these changes in three days, and others take much longer.
The periods, as the intervals between the complete round of changes are
called, vary, in fact, between three days and six hundred! It may seem
impossible that changes covering so long as six hundred days could be
known and followed, but there is nothing that the patience of
astronomers will not compass.

One very well-known variable star you can see for yourselves, and as an
ounce of observation is worth a pound of hearsay, you might take a
little trouble to find it. Go out on any clear starlight night and look.
Not very far from Cassiopeia (W.), to the left as you face it, are three
bright stars running down in a great curve. These are in the
constellation called Perseus, and a little to the right of the middle
and lowest one is the only variable star we can see in the sky without a
telescope.

This is Algol. For the greater part of three days he is a bright star of
about the second magnitude, then he begins to fade, and for four and a
half hours grows steadily dimmer. At the dimmest he remains for about
twenty minutes, and then rises again to his ordinary brightness in three
and a half hours. How can we explain this? You may possibly be able to
suggest a reason. What do you say to a dark body revolving round Algol,
or, rather, revolving with him round a common centre of gravity? If
such a thing were indeed true, and if such a body happened to pass
between us and Algol at each revolution, the light of Algol would be cut
off or eclipsed in proportion to the size of such a body. If the dark
body were the full size of Algol and passed right between him and us, it
would cut off all the light, but if it were not quite the same size, a
little would still be seen. And this is really the explanation of the
strange changes in the brightness of Algol, for such a dark body as we
are imagining does in reality exist. It is a large dark body, very
nearly as large as Algol himself, and if, as we may conjecture, it is a
mighty planet, we have the extraordinary example of a planet and its sun
being nearly the same size. We have seen that the eclipse happens every
three days, and this means, of course, that the planetary body must go
round its sun in that time, so as to return again to its position
between us and him, but the thing is difficult to believe. Why, the
nearest of all our planets to the sun, the wee Mercury, takes
eighty-seve