For a great deal of our knowledge about the sun we are indebted to
the moon. It will sometimes happen that the moon comes in between
us and the sun, and produces an eclipse. At first you might think
that an eclipse would only have the effect of preventing us from
seeing anything of the sun, but it really reveals most beautiful
and interesting objects, of whose existence we should otherwise be
ignorant. The great luminary has curious appendages which are quite
hidden under ordinary circumstances. In the full glare of day the
dazzling splendor of the sun obliterates and renders invisible these
appendages, which only shine with comparatively feeble light. It
fortunately happens that the moon is just large enough to intercept
the whole of the direct light from the sun, or rather, I should
say, from the central parts of the sun. Surrounding that central
and more familiar part from which the brilliancy is chiefly derived
is a remarkable fringe of delicate and beautiful objects which are
self-luminous no doubt, but with a light so feeble that when presented
to us amid the full blaze of sunlight they are invisible. When,
however, the moon so kindly stops all the stronger beams, then these
faint objects spring into visibility, and we have the exquisite
spectacle of a total eclipse. The objects that I desire to mention
particularly are the corona and the prominences.
[Illustration: FIG. 18.--Total Eclipse of the Sun, May 6, 1883 (_drawn
by Trouvelot_).]
A pretty picture of the total eclipse of the sun which occurred on May
6, 1883, is here shown (Fig. 18). It is taken from a drawing made by
M. Trouvelot, who was sent out with a French observing party. They
went a very long way to see an eclipse, but what they saw recompensed
them for all their trouble. The track along which the phenomenon could
be best seen lay in the Pacific Ocean, and a place had to be selected
which was so situated that the sun should be high in the heavens at
the important moment, and also that the duration while the total
eclipse lasted should be as long as possible. They accordingly went to
Caroline Island, and all this journey to the other side of the earth
was taken to witness a phenomenon that only lasted five minutes and
twenty-three seconds. Short though these precious minutes were, they
were long enough to enable good work to be done. Careful preparations
had been made so that not a moment should be thrown away. Each member
of the party had his special duty allotted to him, and this had been
rehearsed so carefully beforehand that when the long-expected moment
of “totality” arrived there was neither haste nor confusion; every one
carefully went through his part of the programme. M. Trouvelot, for
instance, occupied himself for two minutes and a few seconds in making
the sketch that we now show. No doubt an accomplished astronomical
artist like M. Trouvelot would gladly have taken longer time for his
sketch of so unique a sight, but brevity was imperative. He had already
had experience of similar eclipses, so that he was prepared at once
to note what ought to be noted, and the picture we have shown is the
result. This was completed within less than half of the duration of
totality, and the artist had still three minutes left to devote to
another and quite different part of the work, which does not concern us
at present.
[Illustration: FIG. 19.--The Corona of the Sun, 1882 (_by Schuster_).]
I want you particularly to look at these long branches or projections
which we see surrounding the sun when totally eclipsed. They shine
with a pearly light, and, in fact, it is stated that even during the
gloomiest portion of the time there was still as much illumination as
on a bright moonlight night. All that light came from this glorious
halo round the sun which astronomers call the “corona.” We do not
under ordinary circumstances obtain even the slightest glimpse of this
object. Even during a partial eclipse of the sun it is not visible, but
directly the moon quite covers the sun, so as to cut off all the direct
light, then the corona springs into visibility. It is always there, no
doubt, though we cannot see it.
One of the most interesting photographs of the eclipsed sun which has
ever been taken was that by Professor Schuster in 1882 (Fig. 19). The
corona is well shown, and also a comet.
The other appendages to the sun which can be seen during an eclipse are
the objects which we call “prominences.” They are of a ruddy color, and
seem to be great flames, which leap upwards from the glowing surface
of the sun below. Though the existence of the prominences was first
discovered by their presence during eclipses, it fortunately happens
that we are no longer wholly dependent on eclipses for the purpose
of making our observations of these remarkable objects. It is true
that we may look at the sun with even the biggest and most powerful
telescope in the world, and still not be able to perceive anything of
the prominences. We require the aid of a special appliance called the
spectroscope to render them visible. But I am not now going to describe
this ingenious contrivance. I am only going to speak of the results
which have been obtained by its means. We shall here again avail
ourselves of the experience of M. Trouvelot for a picture of two of
these wonderful appendages.
[Illustration: FIG. 20.--Solar Prominences (_drawn by Trouvelot_).]
The view (Fig. 20) shows the ordinary aspect of the sun diversified
with groups of dark spots. The fringe around the margin of the globe
is of some ruddy material, forming the base of the flames which rise
from the glowing surface. No doubt these flames are also often present
on the face of the sun, but we cannot see them against the brilliant
background. They are only perceptible when shown against the sky
behind. At two points of this ruddy fringe, which happen curiously
enough to be nearly opposite to each other, two colossal flames have
burst forth. They extend to a vast distance, which is quite one-third
of the width of the sun. The vigor of these outbreaks may be estimated
by the remarkable changes which are incessantly going on. These great
flames may indeed be said to flicker; only, considering their size, we
must allow them a little more time than is demanded for the movements
of flames of ordinary dimensions. The great flame on the left was
obviously declining in brilliancy when first seen. In a quarter of an
hour it had broken up into fragments, some of which were still to be
seen floating in the sun’s atmosphere. In ten minutes more the light of
this flame had almost entirely vanished. Surely these are changes of
extraordinary rapidity when we remember the size of this prominence. It
was nearly 300,000 miles in height--that is to say, about thirty-seven
times the width of our earth.
Great as are these prominences, others have been recorded which are
even larger. One of them has been seen to rush up with a speed of
200,000 miles an hour--that is, with more than two hundred times the
pace of the swiftest of rifle-bullets.