I have shown you that the sun is intensely hot, and a very long way
off, and enormously big. And now we have to describe the appearance of
the surface of the sun when we examine it closely.
[Illustration: FIG. 12.--Looking at the Sun.]
If you get a piece of very dark glass, or if you smoke a piece of glass
over a candle, then you can look directly at the sun with comfort. A
nicer plan is to prick a pinhole in a card, through which you can look
at the sun without any inconvenience. Generally speaking, a view of
the sun in this way will show you only a uniformly bright surface. To
study the face of our great luminary carefully, you must use the aid
which the telescope gives to the astronomer. A very good way of doing
this is shown in Fig. 12. A small telescope, fixed on a stand, is
pointed to the sun, and, the eyepiece being drawn out somewhat further
than when direct observations are being made, the sun draws its own
picture on a screen. This may be examined without any inconvenience, or
without the necessity for any protection to the eye, and a number of
young astronomers can all view the sun at the same moment. On such a
picture you will generally see the brilliant surface marked with dark
spots, which are sometimes as numerous as in the case represented in
Fig. 13. These spots present very different appearances according to
circumstances. One such spot when seen with a very powerful telescope
showed the wonderful structure which is represented in Fig. 14.
[Illustration: FIG. 13.--This is what the Sun sometimes looks like.]
The visible surface of the sun is entirely formed of intensely heated
vapors. We might almost say that the spots are holes, by which we can
look through the brilliant surface to the interior and darker parts.
Sometimes the spots close up, and fresh ones will open elsewhere. Now
and then the whole surface is mottled over in a remarkable way. I give
here a picture which was taken from Mr. Nasmyth’s beautiful drawing,
in which he shows how the sun sometimes assumes the appearance which
has been likened to willow leaves (Fig. 15). This appearance was very
noticeable in the great spot of September, 1898.
[Illustration: FIG. 14.--A Sun-spot (_after Janssen_).]
[Illustration: FIG. 15.--Nasmyth’s Drawing of the Willow-leaved
Structure of the Sun.]
[Illustration: FIG. 16.--Spot nearing the Sun’s Edge.]
The spots often last long enough to demonstrate a remarkable fact. We
must remember that the sun is a great globe, and that it is poised
freely in space. There is nothing to hold it up, and there is nothing
to prevent it from turning round. That it does turn round, we can prove
by careful observation of the spots. I can best illustrate what I want
by Fig. 17, which shows six imaginary pictures. The first represents
the sun on the 1st day of the month; the next shows it five days later,
on the 6th; another view is five days later still, on the 11th; and
so on until the last picture, which corresponds to the 26th. You see,
on the first day there is a spot near the left edge; by the 6th,
this spot is near the middle; by the 11th, it is near the right edge;
then you do not see it at all on the 16th, or on the 21st; but on the
26th it is back in the same place from which it started. We find
other spots to have a similar history. They appear to move across the
face, and then to return in a little less than four weeks to the same
place where they were originally noticed. These appearances can be
illustrated very simply by cutting a small hole through the rind of an
orange down to the white interior skin, which may be darkened with ink.
Put a knitting needle through the axis of the orange, and then turn it
slowly round. The spot will be found to go through the changes that we
have seen. We start with the spot near the left, it moves across the
face, and then passes to invisibility by moving behind the globe until
it reappears again, after having moved round the back. As the same may
be observed with every spot which lasts long enough, we learn that the
changes in the places must be produced by the turning round of the sun.
Here you see is the way in which an astronomical discovery is made. We
first observe the fact that the spots do always appear to move. Then
we try to account for this, and we find a very simple explanation, by
supposing that the whole sun, spots and all, turns steadily round and
round. It can also be proved in a very conclusive manner that no other
explanation is possible. This rotation of the sun is always going on
uniformly, and some curious consequences follow from it. The view of
the sun which is turned towards us to-day is quite different from that
which was towards us a fortnight ago, or from that which we shall see
in a fortnight hence. There is no actual or visible axis about which
the sun rotates. In this the sun is like the earth and other celestial
bodies.
[Illustration: FIG. 17.--How the Sun turns round.]