also crossed by the Milky Way. According to one myth Lupus is held
in the right hand of the Centaur as an offering upon the altar which
is represented by the constellation of Ara next to Centaurus on the
east. Ara also is crossed by the Milky Way. Neither Lupus nor Ara
contain any objects that are worthy of special attention.
Triangulum Australe, The Southern Triangle, a little to the southeast
of Alpha Centauri, is far more conspicuous than the Triangulum of the
northern hemisphere.
The accompanying charts give two views of these principal southern
constellations that lie within 40° of the south pole of the heavens
and that are below the horizon in 40° north latitude. The first of
these charts shows the constellations that are nearest the meridian
in the early evening hours in February. Canopus in Argo Navis and the
Greater Magellanic Cloud then lie close to the meridian. Argo Navis
with its subdivisions Puppis, Vela and Carina are found east of the
meridian lying directly in the path of the Milky Way, which stretches
diagonally across the sky from the northwest to the southeast. Far
over in the southeast appears Crux, the Southern Cross, also directly
in the path of the Milky Way. In the western heavens may be seen the
Lesser Magellanic Cloud in Hydrus, brilliant Achernar in Eridanus
and the inconspicuous star-group of Tucana.
In the early evening hours of July we find as shown on the second
chart, Alpha and Beta Centauri in Centaurus close to the meridian,
Lupus due north of Centaurus, Ara and Triangulum Australe in the
southeast and Crux in fine position for observation just west of the
meridian. Carina of Argo Navis lies to the southwest of Crux. The
Milky Way now arches magnificently across the heavens from Carina
through Crux, Centaurus and Lupus and Ara to the zodiacal groups of
Scorpio and Sagittarius in the northeast.
In the northern part of the heavens, as seen from the southern
hemisphere, appear the familiar zodiacal constellations that we of
the northern hemisphere find south of the zenith, as well as the
constellations of Orion, Lepus and Canis Major, Hydra, Corvus and
Crater, Ophiuchus and Serpens and Aquila, all finely in view in their
appropriate seasons.
It is only our familiar circumpolar constellations of the north--The
Two Bears, Draco, Cassiopeia, and Cepheus, Andromeda and Perseus and
Auriga that are invisible in mid-latitudes of the southern hemisphere
just as the constellations shown in the diagrams, and a few
additional groups such as Pavo, Grus, Phoenix, Apus, Mensa and Volans
which we have not shown, lie hidden from view beneath the southern
horizon in mid-latitudes of the northern hemisphere.
The northern visitor to the southern hemisphere familiar with the
constellations of his own land is filled with a queer sensation of
being in topsy-turvydom as he sees familiar Orion standing on his
head and all of the zodiacal constellations passing in their daily
motions to the north instead of to the south of his zenith while
by day the sun passes across the northern part of the heavens and
culminates north instead of south of his zenith. He misses the
familiar Dippers of his own land and searches in vain for a pole-star
in the unfamiliar circumpolar regions of the south.
XV
THE MILKY WAY OR GALAXY
"Broad and ample road whose dust is gold,
And pavement stars, as stars to thee appear
Seen in the galaxy, that milky way
Which nightly as a circling zone thou seest
Powder'd with stars."
--MILTON, _Paradise Lost_.
On clear, winter evenings one may see a portion of the zone of
the Milky Way, which encircles the heavens, arching magnificently
across the heavens as it passes from Cassiopeia and Cepheus in the
northwest, through Perseus and Auriga and the eastern part of Taurus,
across the feet of Gemini, between Canis Minor and Orion and through
the eastern part of Canis Major to the southern horizon.
At this point it passes beyond our range of vision into the
star-groups of Puppis, Vela and Carina, subdivisions of the huge
southern constellation of Argo Navis. It reaches its greatest
distance south of the celestial equator and also attains its greatest
brilliancy in Crux, the far-famed Southern Cross. From here it turns
northward once more, passing into Centaurus, Musca, Circinus Ara and
Lupus constellations of the southern hemisphere and comes within our
range of vision again in Sagittarius and Scorpio. Here the Milky Way
divides into two branches, though some astronomers now believe that
this apparent division into two branches is due to the presence of an
enormous cloud of non-luminous matter lying along the course of the
Milky Way at this point, similar in its nature to the dark "holes"
and "caves" and streaks that appear in all portions of the Milky Way
and most noticeably athwart its course in Argo and Centaurus.
One of these branches of the Milky Way passes from Sagittarius
through Aquila to Cygnus and the other through Scorpio, Ophiuchus and
Serpens to Cygnus, the two extending diagonally across the heavens
in the late summer and early fall evenings from the northeast to
the southwest. From Cygnus, the Milky Way passes into Cepheus and
Cassiopeia and thus completes its circuit of the heavens.
It is not seen to advantage in spring or early summer evenings
because it then rests nearly on the horizon. Its plane is inclined
about 63° to the celestial equator and its poles lie in the
constellations of Coma Berenicis and Cetus. These are the two points
that lie farthest from the Milky Way.
The Milky Way has been called the groundwork of the universe. By far
the greater number of all the stars are crowded towards its plane in
the form of an enormous flattened disk or lens.
Our solar system, it has been estimated, lies close to the plane of
the Milky Way and at a distance of some 50,000 or 60,000 light-years
from its center. The diameter of the Milky Way as deduced from Dr.
Harlow Shapley's work on globular star clusters is about 300,000
light-years in extent, or ten times greater than the limit set some
years ago.
The apparent crowding together of the stars into dense clouds in the
Milky Way is partly an effect due to our position in the Milky Way.
When we look at the heavens in a direction at right angles to this
plane we find comparatively few stars lying along our line of vision
because the stars are actually fewer in number in this direction.
If we look _along_ the plane of the Milky Way, however, we see
to a greater distance through an enormous depth of stars. Though
individual stars may not be much closer together in the Milky Way
than they are outside of it, there are on the whole more of them and
the effect of greater density is produced.
Father Hagen of the Vatican Observatory, who has for years made a
study of the dark clouds of obscuring matter and dark nebulæ that
abound in space, has found evidence of the existence of many vast
clouds of dark obscuring matter over the entire heavens above and
below the plane of the Milky Way as well as surrounding the Milky
Way in its own plane. The existence of such clouds of non-luminous
matter would account partly for the comparative fewness of stars in
space outside of the plane of the Milky Way since many stars would be
concealed from our eyes by these obscuring clouds. There is, however,
in addition, an actual crowding of all the visible stars toward this
plane.
The peoples of all ages have honored the Milky Way in story and
legend. It has been universally referred to as The Sky River and The
Pathway of Souls. To the little Hiawatha, we remember, the "wrinkled
old Nakomis"
"Showed the broad white road in heaven
Pathway of the ghosts, the shadows,
Running straight across the heavens,
Crowded with the ghosts, the shadows.
To the Kingdom of Ponemah
To the land of the hereafter."
In _The Galaxy_, Longfellow thus describes the Milky Way:
"Torrent of light and river of the air
Along whose bed the glimmering stars are seen
Like gold and silver sands in some ravine
Where mountain streams have left their channels bare!"
In Sweden, where the Milky Way arches high through the zenith in
winter, it is called the Winter Street, and Miss Edith Thomas writes
thus beautifully of it in her poem entitled, "The Winter Street":
"Silent with star dust, yonder it lies--
The Winter Street, so fair and so white;
Winding along through the boundless skies,
Down heavenly vale, up heavenly height.
Faintly it gleams, like a summer road
When the light in the west is sinking low,
Silent with star dust! By whose abode
Does the Winter Street in its windings go?
And who are they, all unheard and unseen--
O who are they, whose blessèd feet
Pass over that highway smooth and sheen?
What pilgrims travel the Winter Street?
Are they not those whom here we miss
In the ways and the days that are vacant below?
As the dust of that Street their footfalls kiss
Does it not brighter and brighter grow?"
Beautiful indeed are these poetic fancies but none of them picture
even remotely the awe-inspiring grandeur of the Milky Way as it
actually exists.
[Illustration: A DARK NEBULA: THE DARK BAY OR DARK HORSE NEBULA IN
ORION
Taken with 100-inch Hooker Telescope of the Mt. Wilson Observatory]
Millions upon millions of far distant suns equal to or surpassing
our own sun in brilliancy are gathered within this vast encircling
zone of the heavens, their combined light giving to the naked eye the
impression of a milky band of light. Nine-tenths of all the stars, it
has been estimated, lie close to the plane of the Galaxy, as well as
all the vast expanses of luminous gaseous nebulæ and clouds of dark
obscuring matter all seemingly intermingled in chaotic confusion;
yet law and order govern the motions of all. Here also are the great
moving star clusters such as the Pleiades and the Hyades and all of
the brilliant "Orion" stars.
The structure of the Milky Way is not clearly understood but many
astronomers believe there is evidence that it takes the form of a
vast spiral nebula along whose arms the stars pass to and fro.
Beyond the Milky Way at enormous distances of many thousands of
light-years, but apparently influenced by it, lie the globular
star-clusters and the spiral nebulæ. The spirals appear to avoid the
plane of the Milky Way for they are receding in the direction of its
poles at high velocities; the globular clusters on the other hand
are drawing in toward the Milky Way on either side, and in time will
cross it.
Whether these objects external to the Milky Way form with it one
enormous universe or whether the spiral nebulæ are in turn galaxies
or "island universes," as the astronomer calls them, similar in form
and structure to our own galaxy and at inconceivably great distances
of millions of light-years from it, is still one of the riddles of
the universe which the astronomers are attempting to solve.
XVI
THE SURFACE OF THE SUN
The visible surface of the sun is called the _photosphere_. Even the
smallest telescopes will show its peculiar "rice-grain" structure,
consisting of intensely brilliant flecks or nodules about 500 miles
in diameter, which can be resolved by the more powerful telescopes
into smaller particles about 100 miles in diameter, against a darker
background. It has been estimated that these bright nodules or
rice-grains occupy only one-fifth of the total surface of the sun,
yet radiate three-fourths of the total light.
It is generally believed that the "rice grains" are the summits of
highly heated columns of gas, arising from the sun's interior, and
that the darker portions between are cooler descending currents.
It is well known that the photosphere or visible surface of the sun
appears to be much brighter in the center of the disk than near its
circumference. This is due entirely to its gaseous nature and to
the fact that it is surrounded by an atmosphere of dense enveloping
cooler gases. Rays from the center of the disk travel a shorter
distance through this atmosphere than the rays from the rim and
therefore are absorbed less by surrounding gases. We look further
down into the sun's interior near the center of the disk than in the
direction of its circumference and so the light appears more intense
there.
The photosphere is the region where sun-spots appear and they are
found in zones extending from 8° to 35° on either side of the solar
equator, never appearing exactly at the equator or near the poles.
The disturbances that produce sun-spots and many allied phenomena
occur cyclically in periods of eleven years on the average. The
first outburst of the disturbance is manifested by the appearance
of sun-spots in high solar latitudes. These break out and disappear
and break out again with increased vigor, working gradually downward
toward the solar equator, the maximum spottedness for a given period
occurring in solar latitude about 16°. The disturbance finally dies
out within 8° or 10° of the equator, but even before one cycle of
disturbance has entirely passed away a new cycle has broken forth in
high latitudes. So during the period of minimum spottedness there
are four distinct belts, two in low latitudes, due to the dying
disturbance, and two in high latitudes, due to the new disturbance.
At sun-spot maximums there are two well-marked zones of great
intensity, approximately 16° north and south of the sun's equator.
Sun-spots are solar cyclones, occurring usually in groups, though
large single spots appear less frequently. Each spot is quite sharply
divided into an umbra and a penumbra. The umbra is the darker central
portion, the funnel of the whirling cyclone, and the penumbra is
composed of the outspreading gases, and is less dark than the umbra.
The peculiar "thatch-straw" structure of the penumbra is due, it
is believed, to the fact that the columns of gases that usually
rise vertically from the sun's interior and from the "rice grains"
of the photosphere are drawn into a horizontal position by the
whirling motion that exists in the penumbra regions of a sun-spot and
therefore we get a longitudinal rather than a cross sectional view of
them.
The umbra of a sun-spot is anywhere from a few hundred miles to fifty
thousand miles in diameter, frequently exceeding the earth in size,
while the penumbra occasionally reaches a diameter of two hundred
thousand miles. Sun-spots of exceptional size can be seen even
without the aid of a telescope.
The darkness of sun-spots is only by comparison with their more
brilliant background. Owing to the rapid expansion and cooling of
gases the temperature in sun-spot regions is far below the normal
solar temperature of 6,000° Centigrade, lying between 3,000° and
4,000° Centigrade. At this temperature it is possible for the more
refractory chemical compounds to form, the oxides and the hydrides,
and the spectra of sun-spots reveal the presence of titanium oxide
and magnesium and calcium hydride. At the higher solar temperatures
that exist elsewhere in the photosphere and in its overlying gaseous
envelopes all chemical elements occur in a free state, intermingling
as incandescent vapors without the formation of any chemical
compounds.
Strong magnetic fields exist in sun-spot regions and magnetic
storms in our own atmosphere frequently accompany the appearance of
exceptionally large sun-spots.
Directly above the photosphere of the sun lies the "reversing layer,"
which is about five hundred miles in depth and is composed of the
incandescent vapors of all the chemical elements that exist on the
sun, which are also the same familiar elements that exist on the
earth, with the exception of coronium, the unknown element in the
solar corona, there is no element in the sun that has not been found
on our own planet.
The "reversing layer" receives its name from the fact that it
reverses the solar spectrum. It produces by its absorption of the
rays of light from gases below the dark absorption lines found in the
spectrum that serve to identify all the elements existing in the sun.
During the time immediately preceding and following a total eclipse
of the sun this reversing layer produces what is known as the flash
spectrum. When the photosphere, which gives the bright continuous
background of the solar spectrum, is concealed by the moon, the
normally dark lines of the reversing layer--dark only by contrast
with the bright background--become momentarily intensely bright lines
against a dark background. The flash spectrum only lasts a second or
so, as the reversing layer itself is soon covered by the moon.
Just above the reversing layer lies the _chromosphere_, which is
between five thousand and ten thousand miles in depth. Many of the
gaseous vapors of the reversing layer are found in the chromosphere,
thrown there continually by the vast upheavals of gases that are
constantly disturbing the surface of the sun. The greater the solar
activity the more is the chromosphere charged with the vapors of
the lower strata of the sun's atmosphere. The gases that are most
characteristic of the chromosphere, however, are the incandescent
gases of hydrogen and calcium, which give it the pink or reddish
tinge so noticeable during total solar eclipse. Helium is also found
in great abundance in the solar chromosphere.
Shooting upward from the photosphere with the tremendous velocity of
one hundred or more miles per second, can be seen at all times, by
properly screening off the light from the photosphere, the vast solar
eruptions known as the _prominences_. These are composed chiefly
of hydrogen and calcium gas, though other elements also appear,
especially near the bases of the prominences. Prominences are of two
varieties, the quiescent, or cloud-like prominences, that float high
above the solar surface for days at a time in some instances and
resemble terrestrial clouds in form, and the eruptive, or metallic
prominences, that dart up from the surface of the sun in an infinite
variety of forms that may be entirely changed in the short interval
of fifteen or twenty minutes.
These eruptive prominences usually attain heights of thirty or forty
thousand miles on the average, but _exceptional prominences reach
heights of more than one hundred thousand miles and in a few rare
cases have reached elevations of over five hundred thousand miles,
or more than one-half of the solar diameter_.
Prominences are the most spectacular and beautiful of all solar
phenomena, with the possible exception of the solar corona, which is
the outermost of all the solar envelopes and also the most tenuous.
The extent of the corona is enormous. Its outer streamers extend
usually to distances of several million miles from the center of the
sun. Measurements of the coronal light during total eclipses of the
sun have shown that its intensity is only about one-half that of
full moonlight, and it seems almost impossible to devise methods for
detecting it, except during total eclipses, on account of the extreme
faintness of its light.
The sun, it is now known, is surrounded by a strong magnetic field in
addition to the magnetic fields that exist in sun-spots. The cycle
of sun-spot change is attended by marked changes in many forms of
solar activity. The frequency of outbursts of eruptive prominences,
the brightness and form of the corona, magnetic storms and weather
changes on the earth are all closely associated with the sun-spot
cycle.
The cause of this sun-spot cycle, with all the attendant changes
in the general solar activity, and the source of the apparently
limitless supply of solar energy still remain the two chief unsolved
secrets of the sun.
XVII
THE SOLAR SYSTEM
Our sun is but a star traveling through the universe. It is
accompanied in its journey to unknown parts of space, that lie in
the general direction of the constellation Hercules, by an extensive
family of minor bodies consisting of the eight planets and their
encircling moons, one thousand or more asteroids, numerous comets,
and meteors without number, all moving in prescribed paths around
their ruler.
The most important members of the sun's family are the planets,
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune,
named in the order of their position outward from the sun. We hear
occasionally of the possibility of the existence of intra-Mercurial
and trans-Neptunian planets and it is possible that some day an
additional planet may be discovered within the orbit of Mercury or
beyond the orbit of Neptune. The gravitational control of the sun
extends far beyond the orbit of Neptune and there are reasons for
believing in the existence of at least one or two additional planets
on the outskirts of the solar system. The existence of a planet
within the orbit of Mercury is now, after long continued and diligent
search, believed to be very doubtful.
Were it possible to view the sun from the distance of the nearest
star with the aid of the greatest telescope on earth all the members
of his family would be hopelessly invisible. So, also, we cannot
tell as we point our powerful telescopes at the stars whether these
other suns are attended by planet families. We may only argue that it
is very unlikely that there should be only one star among hundreds
of millions that is attended by a group of comparatively small dark
bodies that shine by the reflected light from the star they encircle.
With the exception of the two planets, Mercury and Venus, which are
known as the inferior planets, since their paths lie between the
earth and the sun, all the planets have moons or satellites of their
own that encircle the planet just as the planet encircles the sun.
Our planet earth has one satellite, the moon, that has the
distinction of being the largest of all the moons in proportion to
the size of the planet it encircles. Jupiter and Saturn have moons
that surpass our moon in actual size; in fact, two of the moons
of the outer planets are actually larger than the smallest planet
Mercury, but they are very small in proportion to the size of the
planets around which they revolve. Mars, the next planet beyond the
earth, the nearest of the superior or outer planets, has two tiny
moons that bear the names of Deimos and Phobos, respectively. They
are both less than twenty miles in diameter and revolve very near
to the surface of Mars. They can only be seen with the aid of very
powerful telescopes. The inner moon, Phobos, is unique in the solar
system for it makes three trips around Mars while the planet is
turning once on its axis.
Jupiter, the next planet outward from the sun, is almost a sun itself
to its extensive family of nine moons. Four of these moons were first
seen about three hundred years ago when Galileo pointed his first
crude telescope at the heavens and any one can now see them with the
aid of an opera glass. One of the four is equal in size to our own
moon; the others surpass it in size. These moons are most interesting
little bodies to observe. Their eclipses in the shadow of Jupiter,
occultations or disappearances behind his disk, and the transits of
the shadows as well as the moons themselves across the face of the
planet can be easily seen even with the smallest telescope. The five
remaining moons have all been discovered in modern times. They are
extremely small bodies visible only in large telescopes. Satellite
V is the nearest of all the moons to Jupiter. The other four are at
great distances from the planet.
The planet Saturn has nine moons. Titan, the largest, is nearly
equal in size to Jupiter's largest moon, and is larger than Mercury;
four of the other moons have diameters between one thousand and two
thousand miles in length. Since Saturn is nearly twice as far from
the sun as Jupiter his moons are more difficult to observe, though
the two largest are visible in small telescopes.
Saturn is unique in the solar system in possessing in addition to
his nine satellites a most wonderful ring system, composed of swarms
of minute moonlets, each pursuing its individual path around the
planet. It is this unusual ring system that makes Saturn the most
interesting to observe telescopically of all the planets.