Perseus, distinguished by its well-known festoon, or curve, of
stars. South of this lies the constellation Taurus or the Bull,
which contains the well-known groups or clusters, the Pleiades and
the Hyades. The Pleiades form perhaps the most remarkable group of
stars in the heavens, and are easily found, when above the horizon.
To ordinary eyesight the cluster consists of six stars. Some persons
gifted with exceptionally keen eyesight have, however, seen eleven
or twelve. A map of the Pleiades made in the sixteenth century
shows eleven stars very correctly. This was drawn, of course, from
observations made with a measuring instrument, but without the aid of
a telescope. The observer (I think it was Möstlin, Kepler’s tutor)
must have possessed wonderfully sharp eyesight. The Hyades form a
V-shaped figure, and contain the bright reddish star Aldebaran.
South of Taurus and Gemini will be found the splendid constellation
of Orion, perhaps the most brilliant group of stars visible in
either hemisphere. A remarkable quadrilateral figure is formed by
its four stars, Betelgeuse (Alpha) and Gamma[18] on the north, and
Rigel (Beta) and Kappa on the south. Of these Betelgeuse and Rigel
are bright stars of the first magnitude. Betelgeuse is distinctly
reddish and also slightly variable in its light. Rigel is a beautiful
white star. In the middle of the quadrilateral are three stars of the
second magnitude, nearly in a straight line, known as Delta, Epsilon,
and Zeta, Delta being the northern of the three. These form Orion’s
“belt.” South of these are three faint stars, also in a straight
line, forming the “sword” of Orion. Surrounding the central star
of the “sword” is “the great nebula of Orion,” one of the finest
objects in the heavens. It is barely visible to the naked eye, but
may be seen with a good opera-glass.
To the southeast of Orion will be found Sirius, the brightest star
in the heavens. It is the chief star of the constellation Canis
Major, or the Great Dog, and has been well termed “the monarch of the
skies,” from its great brilliancy.
The bright star Regulus, referred to above, is situated in a
remarkable group of stars shaped like a sickle, and known as “the
Sickle in Leo.” Regulus lies at the extremity of the handle. Leo is
well placed for observation in April and May.
The famous group called the Southern Cross forms a conspicuous object
in the southern heavens. It has formed a subject of interest since
the earliest ages of antiquity. Its component stars, are, however,
not so brilliant as some suppose, the two brightest being between the
first and second magnitudes, the next of the second, and one between
the third and fourth magnitudes. Near the Southern Cross are two
bright stars known as Alpha[19] and Beta of the Centaur.
Among the stars are many objects known as “double stars.” These
consist of two stars very close together, but which appear to
the naked eye only as single stars. Some are triple, and even
quadruple. Of these double stars there are now about 10,000 known
to astronomers, but they are only visible with a telescope. Some,
indeed, are so close that the highest powers of the very largest
telescopes are necessary to see them as anything but single stars.
Of the naked-eye stars there are, however, some apparently so close
that they present very much the appearance of real double stars as
seen in a telescope. These, although not recognized by astronomers
as double stars, have been termed “naked-eye doubles.” Houzeau found
that the brighter the stars are the easier it is to separate them;
and that for small stars, about 15′ of arc, or half the moon’s
apparent diameter, is about the limit below which the naked eye can
not see a faint star double.
[Illustration: Fig. 15.—Constellation of the Great Bear]
Of the “naked-eye doubles,” perhaps the most remarkable is Mizar,
the middle star in the “tail” of the Great Bear. Close to it is a
small star, sometimes called “Jack on the Middle Horse.” It was
known to the ancient astronomers as Alcor, or “the test,” as it was
_then_ considered a test of excellent eyesight. Whether it has really
brightened seems doubtful, but at present it is perhaps visible to
_ordinary_ eyesight. Some, however, fail to see it, while to others
with keener vision it seems as plain as the proverbial “pike-staff.”
The star Alpha Capricorni consists of two stars which, although
closer than Mizar and Alcor, are more equal in brightness, and may
be easily seen with the naked eye on a clear night. Nu Sagittarii
may also be seen double in this way. Theta Tauri, in the Hyades, is
another object which some eyes can see distinctly double; also Kappa
Tauri, a little to the north of the Hyades; Omicron Cygni, a little
to the west of Alpha Cygni (Deneb), is another example. On a very
fine night two stars may be seen in Iota Orionis, the most southern
star in the “sword.” Near Gamma Leonis, one of the brightest stars in
the “sickle,” is a star of the sixth magnitude, which some can see
without optical aid.
The most severe test is, however, Epsilon Lyræ, the northern of two
small stars which form a little triangle with the brilliant Vega.
This, to some eyes, appears double. The famous German astronomer
Bessel is said to have seen it at thirteen years of age. To most
people, however, it will perhaps appear only elongated. This is a
very remarkable star, as each of the components is seen to be a close
double when examined with a good telescope; and between the pairs are
several fainter stars.
Among those interesting objects, the variable stars, are several
which may be well observed without optical assistance. Of these
may be mentioned Algol, of which all the fluctuations of light
may be easily observed with the naked eye; Mira Ceti, which may be
well observed when at its brightest; Lambda Tauri, a variable star
of the Algol type; Betelgeuse (Alpha Orionis), which is slightly
variable; Zeta Geminorum, a fourth magnitude star, which varies
about three-quarters of a magnitude in a period of about ten days;
R. Hydræ, which is visible to the naked eye at maximum; Beta Lyræ,
period about thirteen days; Eta Aquilæ, period about seven days;
and Delta Cephei, which varies about one magnitude in a period of a
little over five days. Of all these stars useful observations may be
made without optical assistance of any sort.
Observations, and even discoveries, of new or “temporary” stars
may also be made with the naked eye. This occurred in the case of
the “temporary” stars of 1572, 1604, 1670, 1866, and 1870, but, of
course, these were bright objects at the time of their discovery.
Hind’s “new star” of 1848 in Ophiuchus was, however, only of the
fifth magnitude when it appeared, and it might have escaped detection
with the naked eye. A star of this magnitude might, however, be
easily detected by an observer who is familiar with the principal
stars of a constellation.
The Milky Way may, perhaps, be better seen with the naked eye than
with any instrument, although an opera-glass brings out well, in some
places, its more delicate details. A mere passing glance might lead
a casual observer to suppose that the Galaxy stretched as a band of
nearly uniform brightness across the heavens. But good eyesight,
careful attention, and a clear sky will soon disclose numerous
details previously unsuspected; streams and rays of different
brightness, intersected by rifts of darkness, and interspersed with
spots and channels of comparatively starless spaces. An excellent
drawing of the Milky Way—the result of five years’ observations
with the naked eye alone—has recently been completed by Dr. Otto
Boeddicker at Lord Rosse’s observatory in Ireland. This beautiful
picture is exquisitely drawn, and shows a wonderful amount of
detail. A writer in the _Saturday Review_ of November 30, 1889,
says: “His maps are in many respects a completely new disclosure.
Features barely suspected before come out in them as evident and
persistent; every previous representation appears, by comparison,
_structureless_.” This shows what can be done with the naked eye in
the study of this wonderful zone.
Among the nebulæ and clusters there are not many objects visible to
the naked eye. A hazy appearance about the middle star in Orion’s
“sword” indicates the presence of the “great Nebula,” one of the
finest objects in the heavens. The “great Nebula in Andromeda,” aptly
termed “the Queen of the Nebulæ,” is distinctly visible to the naked
eye on a very clear night. It lies near the four and a half magnitude
star, Nu Andromedæ (a few degrees north of Beta Andromedæ), and may
be well seen in the early evening hours in the month of January, when
it is high in the sky. It somewhat resembles a small comet. This
nebula was known long before the invention of the telescope, and it
was described by one of the earlier astronomers as resembling “a
candle shining through horn,” a not inapt description.
Of star clusters visible without optical aid may be mentioned the
double cluster Chi Persei, which appears to the eye as a luminous
spot in the Milky Way; the cluster known as 35 Messier, a little
north of Eta Geminorum, just visible to the naked eye on a very clear
night; and there are others in the Southern Hemisphere, notably the
globular cluster known as Omega in the Centaur, which shines as a
hazy star of the fourth magnitude. Among the clusters may perhaps
be included the Præsepe, or the “Beehive,” in Cancer, which has a
nebulous appearance to the naked eye.
Coming now to the Solar System, the sun and moon, of course, first
attract attention. Cases of sun-spots visible to the naked eye are
recorded, but, of course, spots of such enormous size are of rare
occurrence. Of lunar detail little can be seen without a telescope of
some sort, but the larger markings are sufficiently distinct to good
eyesight to convince the observer that they do not alter perceptibly,
thus showing clearly that the moon always turns the same side to the
earth.
Of the planets, nothing of their appearance in the telescope can,
of course, be seen with the naked eye, but it is easy to identify
the brighter planets. Mercury, owing to its proximity to the sun,
is rarely visible in Europe and North America, but when favorably
situated, it may sometimes be detected near the sun shortly after
sunset or a little before sunrise. Notwithstanding the difficulty of
seeing it, it was well known to the ancients, an observation of the
planet dating back to 264 B. C. It is easier, however, to see in more
southern latitudes, and I have frequently observed it as bright as
a star of the first magnitude in the clear air of the Punjab sky. I
have also seen it on several occasions in Ireland, and the Rev. S. S.
Johnson, F.R.A.S., tells me he has seen it with the naked eye no less
than one hundred times in the south of England. The brilliant planet
Venus can hardly be mistaken when seen in the morning or evening sky.
When at its brightest it considerably exceeds Jupiter and Mars, and
far surpasses Sirius, the brightest star in the heavens.
If a very bright planet is seen rising at sunset, it can not be
Venus, which is never seen beyond a limited distance from the sun.
The observer may, therefore, conclude with certainty that the
planet is either Jupiter or Mars. The latter, which occasionally
rivals Jupiter in brilliancy, may be easily distinguished from the
“giant planet” by its distinctly reddish color. Saturn shines with
a yellowish light, and is never so bright as Mars or Jupiter when
at their brightest. The planet Uranus is just visible to the naked
eye, and may be found without optical assistance when its position is
accurately known.
Some observers think that they can see the crescent of Venus with
the naked eye when the planet is in that phase, but this seems
very doubtful. Cases have been recorded of one or two satellites
of Jupiter having been seen with the unaided eyesight, but few are
gifted with such keen vision.
Occultations of bright stars may be well seen with the naked eye,
especially when they pass behind the moon’s _dark_ limb, and as the
disappearance of a star is practically instantaneous, really valuable
observations may be made without a telescope, by merely noting the
exact time at which the star vanishes.
Most of the comets discovered by astronomers are small and faint, and
only visible in good telescopes. At intervals, however, a brilliant
visitor appears on the scene, and its path among the stars may be
watched from night to night with the naked eye. Before the invention
of the telescope, bright comets were watched in this way, and their
course recorded so carefully that it has been found possible to
calculate their orbits with some approach to accuracy. In these days
of large telescopes and instruments of almost mathematical precision,
such a method of observation is, of course, superseded; but we may
still watch the movements of a bright comet with interest, and note
its apparent path across the sky with pleasure and profit. Shooting
stars and fire-balls may be best observed with the naked eye, and
the excellent work done in this way by Mr. W. F. Denning, F.R.A.S.,
should encourage others to take up this interesting branch of
astronomy.
Another object which may be well seen with the naked eye—indeed, it
may best be observed in this way—is the Zodiacal Light. This is a
lenticular or cone-shaped beam of light, which makes its appearance
at certain times of the year, above the eastern horizon before the
dawn, and above the western horizon after sunset, when the sky is
clear and the moon absent. In the tropics it is much more easily
seen, the twilight being shorter, and I have often observed it in
India shining with great brilliancy.
From the above sketch my readers will see how much may be learned of
astronomy without optical assistance of any kind, and I hope that
those who do not possess a telescope will use their eyes instead, and
thus gain some knowledge of the wonders and beauties of the starry
heavens. The knowledge thus gained will stimulate their curiosity and
will give them keener interest in reading books which describe the
still greater wonders revealed by the telescope.
FOOTNOTES:
[16] Also known as the Dipper and Charles’s Wain.—E. S.
[17] The Arabian names Dubhe (Alpha), Merak (Beta), Phecda (Gamma),
Megrez (Delta), Alioth (Epsilon), Mizar (Zeta), and Alkaid (Eta).
—E. S.
[18] Bellatrix.
[19] This is the nearest star to the earth.—E. S.
THE MILKY WAY.—RICHARD A. PROCTOR
To those who rightly appreciate its meaning the Milky Way is the most
magnificent of all astronomical phenomena. However opinions may vary
as to the configuration of the star-streams composing this object, no
doubt now exists among astronomers that the Milky Way consists really
of suns, some doubtless falling short of our own sun in brilliancy,
but many probably surpassing it. Around these suns, we may fairly
conceive, there revolve systems of dependent orbs, each supporting
its myriads of living creatures. We have afforded to us a noble theme
for contemplation in the consideration of the endless diversities of
structure, and of arrangement, which must prevail throughout this
immensity of systems.
The Galaxy traverses the constellation Cassiopeia. Thence it throws
off a branch toward Alpha Persei (Mirfak), prolonged faintly toward
the Pleiades. The main stream, here faint, passes on through Auriga,
between the feet of Gemini and the Bull’s horns, over Orion’s club
to the neck of Monoceros. Thence, growing gradually brighter, the
stream passes over the head of Canis Major, in a uniform stream,
until it enters the prow of Argo, where it subdivides. One stream
continues to Gamma Argus, the other diffuses itself broadly, forming
a fan-like expanse of interlacing branches, which terminate abruptly
on a line through Lambda and Gamma Argus. Here there is a gap beyond
which the Milky Way commences in a similar fan-shaped grouping,
converging on the brilliant (and in other respects remarkable)
star Eta Argus. Thence, it enters the Cross by a narrow neck, and
then directly expands into a broad, bright mass, extending almost
to Alpha Centauri. Within this mass is a singular cavity known as
the Coal-Sack. At Alpha Centauri the Milky Way again subdivides, a
branch running off at an angle of 20°, and losing itself in a narrow
streamlet. The main stream increases in breadth, until, “making
an abrupt elbow,” it subdivides into one continuous but irregular
stream, and a complicated system of interlacing streams covering the
region around the tail and following claw of Scorpio. A wide interval
separates this part of the Galaxy from the great branch on the
northern side, terminating close on Beta Ophiuchi.
The main stream, after exhibiting several very remarkable
condensations, passes through Aquila, Sagitta, and Vulpecula to
Cygnus. In Cygnus there is a “confused and patchy” region marked by a
broad vacancy, not unlike the Coal-Sack. From this region there is
thrown off the offset to Beta Ophiuchi, already mentioned; the main
stream is continued to Cassiopeia.
[Illustration: Fig. 16.—The Midnight Sky, with Milky Way]
There only remains to be noticed “a considerable offset or
protuberant appendage,” thrown from the head of Cepheus directly
toward the pole. Galileo was the first to prove, though earlier
astronomers had entertained the notion, that the Milky Way was
composed of a vast number of stars crowded closely together. But no
attempt was made to offer a theory of its structure until, in 1754
Thomas Wright, in his _Theory of the Universe_, propounded views
closely according with those entertained later by Sir W. Herschel.
Wright, having examined a portion of the Galaxy with a reflecting
telescope, only one foot in focal length, came to the conclusion that
our sun is in the midst of a vast stratum of stars; that it is when
we look along the direction in which this stratum extends that we see
the zone of light constituting the Milky Way; and that as the line of
sight is inclined at a greater and greater angle to the mean plane
of the stratum, the apparent density of the star-grouping gradually
diminishes.
But it is to Sir W. Herschel, and the supplementary labors of Sir
J. Herschel, that we owe the more definite views now commonly
entertained respecting the Via Lactea. The elder Herschel, whose
nobly speculative views of nature were accompanied by practical
common-sense, and a wonderful power of patient observation, applied
to the heavens his celebrated method of gauging. He assumed as a
first principle, to be modified by the results of observation, that
there is a tolerable uniformity in the distribution of stars through
space. Directing his twenty-foot reflector successively toward
different parts of the heavens, he counted the number of stars which
were visible at any single view. The field of view of this reflector
was fifteen minutes in diameter, so that the portion of the sky
included in any one view was less than one-fourth of that covered by
the moon. He found the number of stars visible in different parts
of the heavens in a field of view of this size to be very variable.
Sometimes there were but two or three stars in the field;[20]
indeed, on one occasion he counted only three stars in four fields.
In other parts of the heavens the whole field was crowded with
stars. In the richer parts of the Galaxy as many as four hundred or
five hundred stars would be visible at once, and on one occasion he
saw as many as five hundred and eighty-eight. He calculated that
in one-quarter of an hour 116,000 stars traversed the field of his
telescope, when the richest part of the Galaxy was under observation.
Now, on the assumption above named, the number of stars visible when
the telescope was pointed in any given direction was a criterion of
the depth of the bed of stars in that direction. Thus, by combining
a large number of observations, a conception—rough, indeed, but
instructive—might be formed of the figure of that stratum of stars
within which our sun is situated.
Sir J. Herschel, during his residence at the Cape of Good Hope,
carried out an extensive series of observations of the southern
heavens. Applying his father’s methods of gauging with a telescope
of equal power, he obtained a result agreeing, in a most remarkable
manner, with those obtained by Sir William Herschel. It appeared,
however, that the Southern Hemisphere is somewhat richer in stars
than the Northern—a result which has been accepted as indicating that
our system is probably somewhat nearer the southern than the northern
part of the galactic nebula. Moreover, Sir J. Herschel was led to
believe that the sidereal system forms a cloven flat ring rather than
a disk.
I think no one who has attentively examined the glories of Orion,
the richly jeweled Taurus, the singular festoon of stars in Perseus,
and the closely set stars of Cassiopeia, but must have felt that
the association of splendor along this streak of the heavens is
not wholly accidental. The stars here seem to form a system, and a
system which one can hardly conceive to be wholly unconnected with
the neighboring stream of the Milky Way. But in the southern portion
the arrangement is yet more remarkable and significant. From Scorpio,
over the feet of the Centaur, over the keel of Argo, to Canis Major,
there is a clustering of brilliant stars, which it seems wholly
impossible not to connect with the background of nebulous light.
It is noteworthy, also, that this stream of stars merges into the
stream commencing with the group of Orion, already noticed. Nor is
this all. It is impossible not to be struck by the marked absence of
bright stars in the region of the heavens between Algol, Crux, and
Corvus. One has the impression that the stars have been attracted
toward the region of the stream indicated, so as to leave this space
comparatively bare.
Now, this last circumstance would appear less remarkable if the
paucity of stars here noticed were common also in parts of the
heavens far removed from the Milky Way. But this is not the case.
Beyond this very region, which we find so bare of stars, we come to a
region in which stars are clustered in considerable density, a region
including Crater, Corvus, and Virgo, with the conspicuous stars
Algores, Alkes, and Spica. But what is very remarkable, while we can
trace a connection between the stream of bright stars over the Milky
Way and the stream of nebulous light in the background, it is obvious
that the two streams are not absolutely coincident in direction.
The stream lies on one side of the Milky Way near Scorpio, crosses
it in the neighborhood of Crux, and passes to the other side along
Canis Major, Orion, and Taurus. Does the stream return to the Milky
Way? It seems to me that there is clear evidence of a separation
near Aldebaran, one branch curving through Auriga, Perseus, and
Cassiopeia, the other proceeding (more nearly in the direction
originally observed) through Aries (throwing out an outlier along the
band of Pisces), over the Square of Pegasus, and along the streams
which the ancients compared to water from the urn of Aquarius (but
which in our modern maps are divided between Aquarius and Grus).
The stream-formation here is very marked, as is evident from the
phenomenon having attracted the notice of astronomers so long ago.
But modern travels have brought within our ken the continuation of
the stream over Toucan, Hydrus, and Reticulum (the two latter names
being doubtless suggested by the convolutions of the stream in this
neighborhood). Here the stream seems to end in a sort of double loop,
and it is not a little remarkable that the Nubecula Major lies within
one loop, the Nubecula Minor within the other. It is also noteworthy
that from the foot of Orion there is another remarkable stream
of stars, recognized by the ancients under the name of the River
Eridanus, which proceeds in a sinuous course toward this same region
of the Nubeculæ.