earth's, equal to it, or less. It could not have been greater, for then
the two bodies could not have been together again at the end of the
year, since the meteoric body would not have completed its revolution in
a year. Its period might obviously be the same as the earth's, for then
they might easily come together again after one revolution of each;
although their orbits might differ so much in shape as to prevent their
being together at any intermediate point. But the period of the body
might also be less than that of the earth, provided it were some
_aliquot part of a year_, so as to revolve just twice, or three times,
for example, while the earth revolves once. Let us suppose that the
period is one third of a year. Then, since we have given the periodic
times of the two bodies, and the major axis of the orbit of one of them,
namely, of the earth, we can, by Kepler's law, find the major axis of
the other orbit; for the square of the earth's periodic time 1^2 is to
the square of the body's time (1/3)^2 as the cube of the major axis of
the earth's orbit is to the cube of the major axis of the orbit in
question. Now, the three first terms of this proportion are known, and
consequently, it is only to solve a case in the simple rule of three, to
find the term required. On making the calculation, it is found, that the
supposition of a periodic time of only one third of a year gives an
orbit of insufficient length; the whole major axis would not reach from
the sun to the earth; and consequently, a body revolving in it could
never come near to the earth. On making trial of six months, we obtain
an orbit which satisfies the conditions, being such as is represented by
the diagram on page 362, Fig. 69', where the outer circle denotes the
earth's orbit, the sun being in the centre, and the inner ellipse
denotes the path of the meteoric body. The two bodies are together at
the top of the figure, being the place of the meteoric body's aphelion
on the thirteenth of November, and the figures 10, 20, &c., denote the
relative positions of the earth and the body for every ten days, for a
period of six months, in which time the body would have returned to its
aphelion.
[Illustration Fig. 69'.]
Such would be the relation of the body that affords the meteoric shower
of November, provided its revolution is accomplished in six months; but
it is still somewhat uncertain whether the period be half a year or a
year; it must be one or the other.
If we inquire, now, why the meteors always appear to radiate from a
point in the constellation Leo, recollecting that this is the point to
which the body is projected among the stars, the answer is, that this
is the very point towards which the earth is moving in her orbit at that
time; so that if, as we have proved, the earth passed through or near a
nebulous body on the thirteenth of November, that body must necessarily
have been projected into the constellation Leo, else it could not have
lain directly in her path. I consider it therefore as established by
satisfactory proof, that the meteors of November thirteenth emanate from
a nebulous or cometary body, revolving around the sun, and coming so
near the earth at that time that the earth passes through its _skirts_,
or extreme portions, and thus attracts to itself some portions of its
matter, giving to the meteors a greater velocity than could be imparted
by gravity alone, in consequence of passing rapidly by them.
All these conclusions were made out by a process of reasoning strictly
inductive, without supposing that the meteoric body itself had ever been
seen. But there are some reasons for believing that we do actually see
it, and that it is no other than that mysterious appearance long known
under the name of the _zodiacal light_. This is a faint light, which at
certain seasons of the year appears in the west after evening twilight,
and at certain other seasons appears in the east before the dawn,
following or preceding the track of the sun in a triangular figure, with
its broad base next to the sun, and its vertex reaching to a greater or
less distance, sometimes more than ninety degrees from that luminary.
You may obtain a good view of it in February or March, in the west, or
in October, in the morning sky. The various changes which this light
undergoes at different seasons of the year are such as to render it
probable, to my mind, that this is the very body which affords the
meteoric showers; its extremity coming, in November, within the sphere
of the earth's attraction. But, as the arguments for the existence of a
body in the planetary regions, which affords these showers, were drawn
without the least reference to the zodiacal light, and are good, should
it finally be proved that this light has no connexion with them, I will
not occupy your attention with the discussion of this point, to the
exclusion of topics which will probably interest you more.
It is perhaps most probable, that the meteoric showers of August and
December emanate from the same body. I know of nothing repugnant to this
conclusion, although it has not yet been distinctly made out. Had the
periods of the earth and of the meteoric body been so adjusted to each
other that the latter was contained an exact even number of times in the
former; that is, had it been _exactly_ either a year or half a year;
then we might expect a similar recurrence of the meteoric shower every
year; but only a slight variation in such a proportion between the two
periods would occasion the repetition of the shower for a few years in
succession, and then an intermission of them, for an unknown length of
time, until the two bodies were brought into the same relative situation
as before. Disturbances, also, occasioned by the action of Venus and
Mercury, might wholly subvert this numerical relation, and increase or
diminish the probability of a repetition of the phenomenon. Accordingly,
from the year 1830, when the meteoric shower of November was first
observed, until 1833, there was a regular increase of the exhibition; in
1833, it came to its maximum; and after that time it was repeated upon a
constantly diminishing scale, until 1838, since which time it has not
been observed. Perhaps ages may roll away before the world will be again
surprised and delighted with a display of celestial fire-works equal to
that of the morning of November 13, 1833.
LETTER XXVIII.
FIXED STARS.
----"O, majestic Night!
Nature's great ancestor! Day's elder born,
And fated to survive the transient sun!
By mortals and immortals seen with awe!
A starry crown thy raven brow adorns,
An azure zone thy waist; clouds, in heaven's loom
Wrought, through varieties of shape and shade,
In ample folds of drapery divine,
Thy flowing mantle form; and heaven throughout
Voluminously pour thy pompous train."--_Young._
SINCE the solar system is but one among a myriad of worlds which
astronomy unfolds, it may appear to you that I have dwelt too long on so
diminutive a part of creation, and reserved too little space for the
other systems of the universe. But however humble a province our sun and
planets compose, in the vast empire of Jehovah, yet it is that which
most concerns us; and it is by the study of the laws by which this part
of creation is governed, that we learn the secrets of the skies.
Until recently, the observation and study of the phenomena of the solar
system almost exclusively occupied the labors of astronomers. But Sir
William Herschel gave his chief attention to the _sidereal heavens_, and
opened new and wonderful fields of discovery, as well as of speculation.
The same subject, has been prosecuted with similar zeal and success by
his son, Sir John Herschel, and Sir James South, in England, and by
Professor Struve, of Dorpat, until more has been actually achieved than
preceding astronomers had ventured to conjecture. A limited sketch of
these wonderful discoveries is all that I propose to offer you.
The fixed stars are so called, because, to common observation, they
always maintain the same situations with respect to one another. The
stars are classed by their apparent _magnitudes_. The whole number of
magnitudes recorded are _sixteen_, of which the first six only are
visible to the naked eye; the rest are _telescopic stars_. These
magnitudes are not determined by any very definite scale, but are merely
ranked according to their relative degrees of brightness, and this is
left in a great measure to the decision of the eye alone. The brightest
stars, to the number of fifteen or twenty, are considered as stars of
the first magnitude; the fifty or sixty next brightest, of the second
magnitude; the next two hundred, of the third magnitude; and thus the
number of each class increases rapidly, as we descend the scale, so that
no less than fifteen or twenty thousand are included within the first
seven magnitudes.
The stars have been grouped in _constellations_ from the most remote
antiquity; a few, as Orion, Bootes, and Ursa Major, are mentioned in the
most ancient writings, under the same names as they bear at present. The
names of the constellations are sometimes founded on a supposed
resemblance to the objects to which they belong; as the Swan and the
Scorpion were evidently so denominated from their likeness to those
animals; but in most cases, it is impossible for us to find any reason
for designating a constellation by the figure of the animal or hero
which is employed to represent it. These representations were probably
once blended with the fables of pagan mythology. The same figures,
absurd as they appear, are still retained for the convenience of
reference; since it is easy to find any particular star, by specifying
the part of the figure to which it belongs; as when we say, a star is in
the neck of Taurus, in the knee of Hercules, or in the tail of the Great
Bear. This method furnishes a general clue to its position; but the
stars belonging to any constellation are distinguished according to
their apparent magnitudes, as follows: First, by the Greek letters,
Alpha, Beta, Gamma, &c. Thus, _Alpha Orionis_ denotes the largest star
in Orion; _Beta Andromedæ_ the second star in Andromeda; and _Gamma
Leonis_, the third brightest star in the Lion. When the number of the
Greek letters is insufficient to include all the stars in a
constellation, recourse is had to the letters of the Roman alphabet, a,
b, c, &c.; and in all cases where these are exhausted the final resort
is to numbers. This is evidently necessary, since the largest
constellations contain many hundreds or even thousands of stars.
_Catalogues_ of particular stars have also been published, by different
astronomers, each author numbering the individual stars embraced in his
list according to the places they respectively occupy in the catalogue.
These references to particular catalogues are sometimes entered on large
celestial globes. Thus we meet with a star marked 84 H., meaning that
this is its number in Herschel's catalogue; or 140 M., denoting the
place the star occupies in the catalogue of Mayer.
The earliest catalogue of the stars was made by Hipparchus, of the
Alexandrian school, about one hundred and forty years before the
Christian era. A new star appearing in the firmament, he was induced to
count the stars, and to record their positions, in order that posterity
might be able to judge of the permanency of the constellations. His
catalogue contains all that were conspicuous to the naked eye in the
latitude of Alexandria, being one thousand and twenty-two. Most persons,
unacquainted with the actual number of the stars which compose the
visible firmament, would suppose it to be much greater than this; but it
is found that the catalogue of Hipparchus embraces nearly all that can
now be seen in the same latitude; and that on the equator, where the
spectator has both the northern and southern hemispheres in view, the
number of stars that can be counted does not exceed three thousand. A
careless view of the firmament in a clear night gives us the impression
of an infinite number of stars; but when we begin to count them, they
appear much more sparsely distributed than we supposed, and large
portions of the sky appear almost destitute of stars.
By the aid of the telescope, new fields of stars present themselves, of
boundless extent; the number continually augmenting, as the powers of
the telescope are increased. Lalande, in his 'Histoire Celeste,' has
registered the positions of no less than fifty thousand; and the whole
number visible in the largest telescopes amounts to many millions.
When you look at the firmament on a clear Autumnal or Winter evening, it
appears so thickly studded with stars, that you would perhaps imagine
that the task of learning even the brightest of them would be almost
hopeless. Let me assure you, this is all a mistake. On the contrary, it
is a very easy task to become acquainted with the names and positions of
the stars of the first magnitude, and of the leading constellations. If
you will give a few evenings to the study, you will be surprised to
find, both how rapidly you can form these new acquaintances, and how
deeply you will become interested in them. I would advise you, at first,
to obtain, for an evening or two, the assistance of some friend who is
familiar with the stars, just to point out a few of the most conspicuous
constellations. This will put you on the track, and you will afterwards
experience no difficulty in finding all the constellations and stars
that are particularly worth knowing; especially if you have before you a
map of the stars, or, what is much better, a celestial globe. It is a
pleasant evening recreation for a small company of young astronomers to
go out together, and learn one or two constellations every favorable
evening, until the whole are mastered. If you have a celestial globe,
_rectify_ it for the evening; that is, place it in such a position, that
the constellations shall be seen on it in the same position with respect
to the horizon, that they have at that moment in the sky itself. To do
this, I first elevate the north pole until the number of degrees on the
brass meridian from the pole to the horizon corresponds to my latitude,
(forty-one degrees and eighteen minutes.) I then find the sun's place in
the ecliptic, by looking for the day of the month on the broad horizon,
and against it noting the corresponding sign and degree. I now find the
same sign and degree on the ecliptic itself, and bring that point to the
brass meridian. As that will be the position of the sun at noon, I set
the hour-index at twelve, and then turn the globe westward, until the
index points to the given hour of the evening. If I now inspect the
figures of the constellations, and then look upward at the firmament, I
shall see that the latter are spread over the sky in the same manner as
the pictures of them are painted on the globe. I will point out a few
marks by which the leading constellations may be recognised; this will
aid you in finding them, and you can afterwards learn the individual
stars of a constellation, to any extent you please, by means of the
globes or maps. Let us begin with the _Constellations of the Zodiac_,
which, succeeding each other, as they do, in a known order, are most
easily found.
_Aries_ (_the Ram_) is a small constellation, known by two bright stars
which form his head, _Alpha_ and _Beta Arietis_. These two stars are
about four degrees apart; and directly south of Beta, at the distance of
one degree, is a smaller star, _Gamma Arietis_. It has been already
intimated that the Vernal equinox probably was near the head of Aries,
when the signs of the zodiac received their present names.
_Taurus_ (_the Bull_) will be readily found by the seven stars, or
_Pleiades_, which lie in his neck. The largest star in Taurus is
_Aldebaran_, in the Bull's eye, a star of the first magnitude, of a
reddish color, somewhat resembling the planet Mars. Aldebaran and four
other stars, close together in the face of Taurus, compose the _Hyades_.
_Gemini_ (_the Twins_) is known by two very bright stars, _Castor and
Pollux_, five degrees asunder. Castor (the northern) is of the first,
and Pollux of the second, magnitude.
_Cancer_ (_the Crab_.) There are no large stars in this constellation,
and it is regarded as less remarkable than any other in the zodiac. It
contains, however, an interesting group of small stars, called
_Præsepe_, or the nebula of Cancer, which resembles a comet, and is
often mistaken for one, by persons unacquainted with the stars. With a
telescope of very moderate powers this nebula is converted into a
beautiful assemblage of exceedingly bright stars.
_Leo_ (_the Lion_) is a very large constellation, and has many
interesting members. _Regulus_ (_Alpha Leonis_) is a star of the first
magnitude, which lies directly in the ecliptic, and is much used in
astronomical observations. North of Regulus, lies a semicircle of bright
stars, forming a _sickle_, of which Regulus is the handle. _Denebola_, a
star of the second magnitude, is in the Lion's tail, twenty-five degrees
northeast of Regulus.
_Virgo_ (_the Virgin_) extends a considerable way from west to east, but
contains only a few bright stars. _Spica_, however, is a star of the
first magnitude, and lies a little east of the place of the Autumnal
equinox. Eighteen degrees eastward of Denebola, and twenty degrees north
of Spica, is _Vindemiatrix_, in the arm of Virgo, a star of the third
magnitude.
_Libra_ (_the Balance_) is distinguished by three large stars, of which
the two brightest constitute the beam of the balance, and the smallest
forms the top or handle.
_Scorpio_ (_the Scorpion_) is one of the finest of the constellations.
His head is formed of five bright stars, arranged in the arc of a
circle, which is crossed in the centre by the ecliptic nearly at right
angles, near the brightest of the five, _Beta Scorpionis_. Nine degrees
southeast of this is a remarkable star of the first magnitude, of a
reddish color, called _Cor Scorpionis_, or _Antares_. South of this, a
succession of bright stars sweep round towards the east, terminating in
several small stars, forming the tail of the Scorpion.
_Sagittarius_ (_the Archer_.) Northeast of the tail of the Scorpion are
three stars in the arc of a circle, which constitute the _bow_ of the
Archer, the central star being the brightest, directly west of which is
a bright star which forms the _arrow_.
_Capricornus_ (_the Goat_) lies northeast of Sagittarius, and is known
by two bright stars, three degrees apart, which form the head.
_Aquarius_ (_the Water-Bearer_) is recognised by two stars in a line
with _Alpha Capricorni_, forming the shoulders of the figure. These two
stars are ten degrees apart; and three degrees southeast is a third
star, which, together with the other two, make an acute triangle, of
which the westernmost is the vertex.
_Pisces_ (_the Fishes_) lie between Aquarius and Aries. They are not
distinguished by any large stars, but are connected by a series of small
stars, that form a crooked line between them. _Piscis Australia_, the
Southern Fish, lies directly below Aquarius, and is known by a single
bright star far in the south, having a declination of thirty degrees.
The name of this star is _Fomalhaut_, and it is much used in
astronomical measurements.
The constellations of the zodiac, being first well learned, so as to be
readily recognised, will facilitate the learning of others that lie
north and south of them. Let us, therefore, next review the principal
_Northern Constellations_, beginning north of Aries, and proceeding from
west to east.
_Andromeda_ is characterized by three stars of the second magnitude,
situated in a straight line, extending from west to east. The middle
star is about seventeen degrees north of Beta Arietis. It is in the
girdle of Andromeda, and is named _Mirach_. The other two lie at about
equal distances, fourteen degrees west and east of Mirach. The western
star, in the head of Andromeda, lies in the equinoctial colure. The
eastern star, _Alamak_, is situated in the foot.
_Perseus_ lies directly north of the Pleiades, and contains several
bright stars. About eighteen degrees from the Pleiades is _Algol_, a
star of the second magnitude, in the head of Medusa, which forms a part
of the figure; and nine degrees northeast of Algol is _Algenib_, of the
same magnitude, in the back of Perseus. Between Algenib and the Pleiades
are three bright stars, at nearly equal intervals, which compose the
right leg of Perseus.
_Auriga_ (_the Wagoner_) lies directly east of Perseus, and extends
nearly parallel to that constellation, from north to south. _Capella_, a
very white and beautiful star of the first magnitude, distinguishes this
constellation. The feet of Auriga are near the Bull's horns.
The _Lynx_ comes next, but presents nothing particularly interesting,
containing no stars above the fourth magnitude.
_Leo Minor_ consists of a collection of small stars north of the sickle
in Leo, and south of the Great Bear. Its largest star is only of the
third magnitude.
_Coma Berenices_ is a cluster of small stars, north of Denebola, in the
tail of the Lion, and of the head of Virgo. About twelve degrees
directly north of Berenice's hair, is a single bright star, called _Cor
Caroli_, or Charles's Heart.
_Bootes_, which comes next, is easily found by means of _Arcturus_, a
star of the first magnitude, of a reddish color, which is situated near
the knee of the figure. Arcturus is accompanied by three small stars,
forming a triangle a little to the southwest. Two bright stars, _Gamma_
and _Delta Bootis_, form the shoulders, and _Beta_, of the third
magnitude, is in the head, of the figure.
_Corona Borealis_, (_the Crown_,) which is situated east of Bootes, is
very easily recognised, composed as it is of a semicircle of bright
stars. In the centre of the bright crown is a star of the second
magnitude, called _Gemma_: the remaining stars are all much smaller.
_Hercules_, lying between the Crown on the west and the Lyre on the
east, is very thickly set with stars, most of which are quite small.
This constellation covers a great extent of the sky, especially from
north to south, the head terminating within fifteen degrees of the
equator, and marked by a star of the third magnitude, called _Ras
Algethi_, which is the largest in the constellation.
_Ophiucus_ is situated directly south of Hercules, extending some
distance on both sides of the equator, the feet resting on the Scorpion.
The head terminates near the head of Hercules, and, like that, is marked
by a bright star within five degrees of _Alpha Herculis_ Ophiucus is
represented as holding in his hands the _Serpent_, the head of which,
consisting of three bright stars, is situated a little south of the
Crown. The folds of the serpent will be easily followed by a succession
of bright stars, which extend a great way to the east.
_Aquila_ (_the Eagle_) is conspicuous for three bright stars in its
neck, of which the central one, _Altair_, is a very brilliant white star
of the first magnitude. _Antinous_ lies directly south of the Eagle, and
north of the head of Capricornus.
_Delphinus_ (_the Dolphin_) is a small but beautiful constellation, a
few degrees east of the Eagle, and is characterized by four bright stars
near to one another, forming a small rhombic square. Another star of the
same magnitude, five degrees south, makes the tail.
_Pegasus_ lies between Aquarius on the southwest and Andromeda on the
northeast. It contains but few large stars. A very regular square of
bright stars is composed of _Alpha Andromedæ_ and the three largest
stars in Pegasus; namely, _Scheat_, _Markab_, and _Algenib_. The sides
composing this square are each about fifteen degrees. Algenib is
situated in the equinoctial colure.
We may now review the _Constellations which surround the north pole_,
within the circle of perpetual apparition.
_Ursa Minor_ (_the Little Bear_) lies nearest the pole. The pole-star,
_Polaris_, is in the extremity of the tail, and is of the third
magnitude. Three stars in a straight line, four degrees or five degrees
apart, commencing with the pole-star, lead to a trapezium of four stars,
and the whole seven form together a _dipper_,--the trapezium being the
body and the three stars the handle.