THE SUN.
MOTION OF THE SUN—THE SEASONS—CHARACTER OF THE SUN—SUNSPOTS—ZODIACAL
LIGHT.
Suppose that we rise early in the morning we shall, as the reader will
say see the sun rise—that is, he appears to us to rise as the earth
rotates. By the accompanying diagram (fig. 544) we can understand how
Sol makes his appearance, and how he comes up again; not, it will be
observed, after the manner stated by the Irishman, who declared that
the sun “went down, and ran round during the night when nobody was
looking.” The earth rotates from west to east, and so the sun appears
to move from east to west. If we look at the diagram we shall see that
after rising at O, the sun advances towards the meridian in an oblique
arc to A, the highest or culminating point—midday. He then returns,
descending to W; this path is the diurnal arc. At Q similarly, during
his passage in the nocturnal arc, he reaches the lowest or inferior
culmination. HH´ is the meridian.
[Illustration: Fig. 544.—Sun’s motion.]
[Illustration: Fig. 545.—The ecliptic.]
On the 21st of March, this path brings the sun on the “ equinoctial”
line mentioned at the close of the last chapter. Day and night are
then of equal duration as the arcs are equal. So this is the _Vernal_
(or spring) _Equinox_. Some weeks after the sun is at midday higher up
at S´, so the diurnal arc being longer, the day is longer, (Z is the
zenith, Z´ is the nadir, P P´ is the celestial axis). From that time he
descends _again_ towards the equinoctial to the autumnal equinox, and
so on, the diurnal arc becoming smaller and smaller until the _winter
solstice_ is reached (S).
From what has been previously said, it is evident that the sun has a
twofold apparent motion—viz., a circular motion obliquely ascending
from the horizon, which is explained by the rotation of the earth,
and by our position, _o_, to the earth’s axis, P P´, and also by a
rising and setting motion between the solstitial points, S and S´,
which causes the inequality of the days and nights. Independently of
the daily motion of the sun, we observe that at the summer solstice,
on the 21st of June, at midday, the sun is at S´, and one half year
later—viz., on the 21st of December, at midnight, the sun is at _s_,
from which he arrives again in the space of half a year at S´; so
we are able to represent this annual motion of the sun by a circle,
the diameter of which is the line, S´ _s_. This circle is called the
_Ecliptic_.
The plane of the ecliptic, S´ _s_, cuts the plane of the equinoctial,
A Q, at an angle of 23½°, and the axis of the ecliptic, S´´´ _s´´_,
makes the same angle with the axis of the heavens, P P. The two
parallel circles, S´ _s´_ and S _s_, include a zone extending to both
sides of the equinoctial, and beyond which the sun never passes. These
circles are called the _Tropics_, from τρέπω, _I turn_, because the sun
turns back at these points, and again approaches the equinoctial. The
parallel circles, S´´ _s´´_, and S´´´ _s´´´_, described by the poles of
the ecliptic, S´´´ _s´´_, about the celestial poles, P P, are called
the _arctic_ and _antarctic circles_.
Whenever the sun crosses the equinoctial, there is the equinox; but the
points of intersection are not invariably the same every year. There
is a gradual westerly movement, so it is a little behind its former
crossing place every year. (_See_ diagram, fig. 547.)
[Illustration: Fig. 546.—The Seasons.]
[This is the “Precession of the Equinoxes,” because the time of
the equinoxes is hastened, but it is really a retrograde movement.
Hipparchus discovered this motion, which amounts to about fifty seconds
in a year. So the whole revolution will be completed in about 28,000
years.]
[Illustration: Fig. 547.—Precession of Equinoxes.]
It is obvious, then, that the sun is the most important star in the
universe; and when we come to speak about the earth we shall consider
the seasons, etc., more fully. Now we must endeavour to explain what
the sun is like, and this can only be done with specially darkened
glasses, for a look at the sun through an ordinary telescope may result
in great, if not permanent, injury to the eye.
The sun is not solid so far as we can tell. It is a mass of “white-hot”
vapour, and is enabled to shine by reason of its own light, which the
planets and stars cannot do; they shine only by the sun’s reflected
light. So we may conclude the sun to be entirely gaseous, but, thanks
to the recent researches in _spectrum analysis_ (already explained),
by which the light of the sun has been examined by means of the
spectroscope, and split up into its component colours, Mr. Lockyer and
other scientists have discovered that a number of elements (metals)
exist in the sun in a fused, or rather vapourous state, in consequence
of the intense heat. Hydrogen exists in the sun, with other gases
unknown to us here, and many metals, discovered by their _spectra_,
which are the same under similar circumstances.
The sun is supposed to be spherical in shape,—not like the earth,
flattened at the poles,—and to be composed of materials similar to what
the earth is composed of, and what it would be if it were as hot as
the sun is. Thus we can argue by analogy from the _spectra_ of earthly
elements, that, as the sun and star light gives us similar _spectra_,
the heavenly bodies are composed of the same elements as our globe. We
can thus form our opinion of the sun’s constitution. Mr. Neisen says:—
“With the aid, therefore, of the additional information given us by
the spectroscope, it is not very difficult to form a true idea of the
probable condition of the surface of the sun, which is all that we can
see. It is the upper-lying strata of a very dense atmosphere of very
high temperature—an atmosphere agitated by storms, whirlwinds, and
cyclones of all kinds, traversed by innumerable currents, and now and
then broken by violent explosions. Above the brilliant surface which we
see is a less dense and somewhat cooler upper stratum, which, though
hot enough to shine quite brightly, is quite invisible in the presence
of the brighter strata beneath it.”
[Illustration: Fig. 548.—Sun spots.]
SUN SPOTS, as they are generally called, are hollows in the sun’s
vapoury substance, and are of enormous extent; and there are brilliant
places near those spots, which are termed _faculæ_. These spots have
been observed to be changing continuously, and passing from east to
west across the sun, and then to come again at the east to go over the
same space again. Now this fact has proved that the sun turns round
upon his axis, and although he does not move as we imagine, from east
to west, round the earth, the orb _does_ move—in fact, the sun has
three motions: one on his axis; secondly, a motion about the centre of
gravity of the solar system, and a progressive movement towards the
planet Hercules.
If we examine the surface of the sun through a proper telescope, we
shall find that the even surface we can plainly distinguish at sunset
is marked, and the brightness is greater towards the centre of the orb.
We can perceive various irregularities; we shall find spots, faculæ
(little torches), etc. These spots were discovered by Galileo and other
astronomers, and were, as we have stated, found to be surface markings,
and not a series of bodies passing between the earth and the sun.
The rotation of the sun was measured, and it was found that the orb
revolved in about 25⅓ days, and in such a manner as to be slightly
inclined to the plane of the ecliptic.
Herschel observed a spot at least 50,000 miles in diameter, which
is more than six times the diameter of the earth. The sun spots
are observed to be constantly changing, and are naturally observed
differently as the revolution proceeds. The dark pole, or “nucleus”
(_umbra_), as it is called, is surrounded by a less dark surface called
the _penumbra_, but the umbra is not really dark; it is extremely
bright when viewed alone, as has been proved by Professor Langley,
while the heat is even greater in proportion. But the umbra of a sun
spot must be below the level of the penumbra, for the shape changed
as the sun revolved on its axis. The penumbra was wider on the side
nearest the edge of the solar disc, and the umbra may be due to the
uprushing or downpouring of gas or vapour like “whirlpools in the solar
atmosphere.”
Near the sun spots the long streaks, or _faculæ_, are often observed
by the borders of the disc, and a transition of the luminous part of
the photosphere[26] into darkness has been observed, and bright bridges
crossing the spots, and then gradually getting dark, were seen by M.
Chacomac. The sun spots vary in direction, but the same general course
is continued. Sometimes they describe curves, sometimes lines.
[Illustration: Fig. 549.—Direction of sun spots.]
During solar eclipses the sun exhibits what are termed “red
prominences,” which are the luminous vapours existing around the sun.
When the orb is eclipsed, we can see the bright-coloured vapours
shooting out from underneath the dark shadow, and this light is
termed the “coronal atmosphere”; the vapours are called the sun’s
_chromosphere_. In the coronal atmosphere are certain curious shapes
of vapour thrown up, and frequently changing,—projecting, in fact,
from the gaseous envelope. These red prominences were first observed
in 1842, and in 1851 it was proved that they appertained to the sun,
for the moon hid them as the eclipse began. Before the prominences
were discovered, the red light surrounding the solar disc was known,
and called the “sierra” (now chromosphere), or chromatosphere. “The
luminosity of these prominences is intense,” says Secchi, “and
they rise often to a height of 80,000 miles, and occasionally to
more than twice that; then bending back, they fall again upon the
sun like the jets of our fountains. Then they spread into figures
resembling gigantic trees, more or less rich in branches.” We give some
illustrations of the appearances of these prominences.
[Illustration: Fig. 550.—Solar prominences.]
The _zodiacal light_ is often observed. It is a glow, and frequently
of a rosy tint. It may be seen in England in March or April before
sunset, or in the autumn before sunrise; and it is doubtful whether it
be a terrestrial or an extra-terrestrial light—a lens-shaped object
surrounding the sun. Some philosophers maintain that the light is
caused by multitudes of minute bodies travelling round the sun; but
Herr Gronemann has lately fully discussed the observations, and the
drift of his contention is under stated:—
[Illustration: Fig. 551.—Solar prominences.]
There are valid observations against two items in the support of
the old theory—viz., the affirmed connection of the evening and
morning cones seen on the same night (if the corresponding sides be
prolonged), and the participation of the cones in the daily motion of
the heavens. The zodiacal light is sometimes seen when daylight has
not yet disappeared; and, on the other hand, it sometimes fails to
appear, though there is complete darkness. There would seem to be a
real lengthening and shortening. It has been observed by Schiaparelli,
that the light is much more difficult to make out when it passes
through the meridian, than when it is only 30° above the horizon,
and is less easily seen when the air is clearest than when a sort of
mist is present. Indeed, the bright parts of the Milky Way may be
seen to be weakened by mist, while the zodiacal light at the same
height is unaffected. The zodiacal light has temporary variations of
light intensity, and it shows from time to time remarkable changes
of form and position, so sudden and short as to be hard to explain
on the planetary hypothesis. The elongations of the cones show a
half-yearly period, which is independent of the transparency of the
air. The cone follows the observer northwards or southwards, so that
there is no parallactic action; and this peculiarity (so adverse to
the extra-terrestrial hypothesis) cannot be explained by reflection
or absorption of light. As to spectroscopic observations, the author
finds (1) that the zodiacal light consists partly of proper light; (2)
that its connection with polar light is but secondary, temporary, and
accidental; (3) that the cause of the second phenomenon is such that it
may strengthen the zodiacal light and modify its spectrum; and (4) that
the results of spectrum analysis rank with other arguments tending to
find the source of the zodiacal light in the neighbourhood of the earth
(like the polar light). Herr Gronemann, then, thinks the zodiacal light
a terrestrial phenomenon, though he will not say that it cannot be
influenced by cosmic action. He throws out the suggestion that the cone
may be a kind of optical illusion, arising from some fine matter—gas
or dust—being more accumulated near the observer in one direction
than another. The apparent length of the cone might be conditioned by
the conical shadow of the earth, and the changes of length be due to
cosmic and electric influences. In any case, there is need of a more
scientific theory than the old one.
[Illustration: Fig. 552.—On the sun’s disc.]
We may conclude our brief notice of the great luminary to which we are
indebted for everything, by a _resumé_ of his distance from us, his
diameter, and a few other plain facts and figures. In the first place
the actual distance of the sun from the earth has never accurately
been determined, but perhaps the next transit of Venus will assist
the observers to a nearer estimate. It is quite sufficient for our
purpose, however, to state that the sun’s distance from the earth is
92,000,000 of miles. The distance varies in winter and summer. In the
former period the sun is _nearer_ than in summer, and yet as the rays
strike over us, and pass us, as it were, we feel less heat. When, as in
summer, we are more in the focus of the rays, we feel the greater heat.
We have already spoken of _parallax_, and it is by finding the solar
parallax that the distance of the sun from us is found. This parallax
has not been exactly ascertained, or rather authorities differ, and as
difference of 0·01´´ in the solar parallax means something over 100,000
miles of distance, it is evident that exactness is almost impossible.
If 8´´·80 be settled as the solar parallax, 92,880,000 miles is the
distance of the sun from the earth. If 8´´·88 be taken we have nearly
92,000,000 exactly.
The _volume_ of the sun is 1,253,000 times that of the earth, and yet
the _density_ of the former is only about one-fourth of the latter, so
the attraction of gravitation at the sun must be more than that of the
earth’s surface twenty-seven times. A body dropped near the surface
of the sun would fall 436 feet in the first second, and have attained
a velocity of ten miles a minute at the end of the first second. The
diameter of the sun depends in our calculations upon its distance from
the earth. If we suppose that to be 92,880,000 miles, the diameter is
866,000 miles. If we take 91,000,000 of miles as the distance from
the earth the diameter is 850,467 miles. The sun makes (apparently)
the circuit of the heavens in 365 days, 6 hours, 9 minutes, and 9·6
seconds; the transit from one vernal equinox to the next being only 365
days, 5 hours, 48 minutes, 48·6 seconds, owing to the precession of the
equinoxes already mentioned.
When we consider the power and grandeur of the sun we may well feel
lost in the contemplation. The sun balances the planets and keeps
them in their orbits. He gives us the light and heat we enjoy, and
coal-gas is merely “bottled-up sunlight.” In darkness nothing will come
to maturity. We obtain rain and dew owing to the sun’s evaporative
power; and no action could go on upon earth without the sun; and yet we
receive only about 1/2070650000 part of its heat and light.
As to the colour of the sun, Professor Langley states that it is really
blue, and not the white disc we see. The whiteness is due to the effect
of absorption exerted by the vapourous metallic atmosphere surrounding
our luminary; and if that atmosphere were removed, his colour would
change.
FOOTNOTES:
[26] The bright part of the sun.