ASTRONOMY.
INTRODUCTORY—HISTORY OF ASTRONOMY—NOMENCLATURE.
[Illustration: Fig. 493.—Celestial globe.]
Astronomy is the science which treats of the heavenly bodies and the
laws which govern them. The term is derived from two Greek words,
_astron_, a star, and _nomos_, a law. It may be included in the study
of Physics, for the motion of the planetary bodies and equilibrium,
gravity, etc., all have something to say to the arrangements and
positions of the stars. The space in which they are set is infinite,
and known as the “Firmament,” or “Heaven.” The number of the heavenly
bodies must therefore be infinite also. We can see a few stars,
comparatively speaking, and there must be numbers whose light has never
yet reached the earth. When we calmly reason upon the immeasurable
distances and the awful rapidity of motion, with the masses of matter
thus in movement, we are constrained to acknowledge that all our
boasted knowledge is as nothing in the wondrous dispensations of Him
“who telleth the number of the stars, and calleth them all by their
names.”
Astronomy, no more than any other of the physical sciences, cannot
stand by itself. We have seen how heat, light, electricity, etc., are
all, in a manner, inter-dependent. So astronomy is dependent upon
mathematics, particularly geometry and trigonometry, for the wondrous
problems to be solved. But in the following sketch we do not propose
to plunge the reader in the slough of calculations. We only desire to
put plainly before him the great phenomena of nature with regard to the
heavens, and the glorious orbs which so thickly stud the space above
us. We need not detail the laborious calculations by which philosophers
have arrived at certain discoveries. We may refer to the results and
explain general principles, thereby indicating the road by which the
student may arrive at the more difficult bypaths in the fields of
scientific discovery.
The history of astronomy is nearly as old as the world itself, or
rather as old as the human race. From the earliest ages we can picture
men gazing upon the “spangled heavens,” and the wandering tribes of the
desert were always very careful observers of the paths of the stars.
To the nomads of the East the planetary system served as compass and
clock, calendar and barometer.
We shall find, therefore, that many observations of the heavenly
bodies were made by the ancients, and have descended to more advanced
generations, and this leads us to remark that the science of astronomy
can be studied without any very special or costly apparatus. In other
branches of science numerous instruments are indispensable before
we can reveal to ourselves the desired results. In astronomy, a
telescope—even a good field glass, such as possessed by any household,
will reveal many interesting facts. We will, by means of more expensive
instruments, and by the aid of large telescopes particularly, enjoy
the sight of the moon and planets. But even with the naked eye a great
variety of phenomena may be observed. With a celestial globe in our
hands upon a fine starry night, we can easily find out the position of
the constellations, and trace their forms in the firmament.
It is to the Chaldeans, Indians, Chinese, and Egyptians, that our
knowledge of astronomy is primarily due. They did much to facilitate
the observation of the stars; they named the planets, grouped the
stars, and marked the sun’s track in the sky. _Astrology_ was
cultivated in very remote ages. The Jews practised it; and the
astrologers of subsequent periods played very important parts in
divining the future of individuals, and casting their _horoscopes_.
Many of these so-called predictions came true, “because,” as was
remarked by Pascal, “as misfortunes are common they” (the astrologers)
“are often right,” as they foretold misfortune oftener than good
fortune. Still the fact remains that occasionally a very startling
prediction was made, and proved true; such, for instance, as the laying
waste of Germany by Gustavus Adolphus, which was foretold by Tycho
Brahé after his consideration of a certain comet, and the date of the
king’s death was also correctly prophesied. Astrology, therefore, held
a very considerable influence over the human race during the Middle
Ages.
We can only give a very brief historical summary of the science. We
know that the destinies of individuals and nations were at a very early
period attributed to the influence of the stars. We read that “the
stars in their courses fought against Sisera,” and many expressions
surviving to the present time serve to remind us that the stars were at
one time paramount in men’s minds. Thus we have the phrases—“unlucky
star,” “born under a lucky star,” “mark my stars,” “moonstruck,”
etc. Even the common term “consider”—to take counsel of the stars—is
thus accounted for, and many men have a habit of looking up to the
ceiling of a room or to the sky when thinking deeply—considering
with the stars. “Contemplate” is another term signifying the same
thing; for _templum_, a temple, was formerly a space marked upon
the sky in imaginary lines, and traced on the ground in accordance
with the supposed diagram. Thus temple became a place for heavenly
“contemplation,” and by an easy transition to a place of worship. In
our old poets’ writings we have many allusions to the influences of the
stars.
“Now glowed the firmament
With living sapphires; Hesperus, that led
The starry host, rode brightest, till the moon,
Riding in clouded majesty, at length
Apparent queen, unveiled her peerless light,
And o’er the dark her silver mantle threw.”—MILTON.
Although from Thales, who lived B.C. 610, the real science of astronomy
may be allowed to date, there can be no doubt that the ancients were
acquainted with many phenomena. The Chaldeans were, doubtless, the
first to place on record the rising and setting of the celestial bodies
and eclipses, and used the water-clock (clepsydra). A list of eclipses
from 2234 B.C. is stated to have been found at Babylon by Alexander the
Great. The Chaldeans also divided the ecliptic into twelve equal parts,
and the day and night into twenty-four hours. The Chinese, again, have
recorded astronomical phenomena as far back as 2857 B.C.; and the
Egyptians also were well versed in the science, although no records
of much importance remain to us, unless the zodiac signs were their
invention.
Thales predicted the eclipse of the sun B.C. 610. Aristarchus and
Eratosthenes also made important observations. Hipparchus (160-125
B.C.) discovered the precession of the equinoxes, calculated eclipses,
determined the length of the year, etc., etc.
Ptolemy, of Alexandria, A.D. 130-150, was the founder of a theory
called the Ptolemaic System, which recognized the earth as the centre
of all—the sun, moon, stars, etc., all revolving in very complicated
courses around it, as figured in the diagram herewith. Even though his
theory turned out to be untenable, he paved the way for his successors
in other ways, and left a valuable collection of observations on
record. In this volume, called the “Almagest,” he reviewed the state of
the science, and gave a catalogue of stars, as well as a description of
the heavens. He discovered the lunar evection.
After his time astronomy, though it was not neglected, appeared to
droop, and it is at a comparatively late period that we again open
the records—viz., in 1543, the year in which Copernicus died. This
philosopher, who was born in 1473, promulgated the true theory of the
solar system. He placed the sun in the centre of the planets, and by
this he explained their motion around the sun, though they appeared to
be carried round the earth. The book in which he explained his theory,
“De Revolutionibus Orbium Celestium,” was not finished till a day or
two before he died.
[Illustration: Fig. 494.—Ptolemaic System.]
The justly celebrated Tycho Brahé was the most important of the
successors of Copernicus, but he opposed the Copernican theory, while
other able philosophers agreed with it. Brahé was a Dane; he died in
1601. He adopted the theory that the sun and moon revolved around the
earth, while the (other) planets moved around the sun. This theory
did not gain much credence, but he, again, though he could not defeat
Copernicus, and though he was wrong in his assumption, made many
important investigations. After him came Kepler, whose observations
upon the planet Mars cleared away many complications, and he laid down
three laws, which are as follows:—
1. Every planet describes an elliptic orbit about the sun, which
occupies one focus of each such ellipse.
2. If a line be drawn from the sun, continually, to any planet, this
line will sweep over equal areas in equal times.
3. The squares of the periodic times of the planets are proportional to
the cubes of their mean distances from the sun.
Kepler also remarked that gravity was a power existing between all
bodies, and reasoned upon the tides being caused by the attraction of
the moon for the waters.
[Illustration: Fig. 495.—Copernican System.]
It was about this time—viz., the beginning of the seventeenth
century—that the telescope was invented, and logarithms came into
use. The actual discoverer of this now almost perfected instrument is
uncertain. Borelli, who wrote in the seventeenth century, ascribes the
discovery to Zachariah Jansen and Hans Lippersheim, spectacle makers of
Middleburg. Baptista Porta, also a spectacle maker, has had the credit
of discovering the magnifying power of the lens, and, so far, the
originator of the telescope.
[Illustration: Fig. 496.—Ellipse.]
[Illustration: Fig. 497.—Radii Vectores.]
[Illustration: Fig. 498.—Ecliptic and Equator.]
But whoever invented it, the telescope did not penetrate into southern
Europe till 1608-9. Galileo then made inquiries concerning the new
instrument, and Kepler made some propositions for their construction.
But Harriot had used the instrument so far back as 1611 or 1612, and
had observed spots upon the sun’s disc. Galileo, in 1610, had also
made observations with the telescope, and discovered the satellites
of Jupiter. He thereby confirmed the Copernican theory;[25] and when
Newton promulgated his immortal discovery of gravitation, after
Picard’s researches, the relations of the sun and planets became
more evident. His researches were published in the _Principia_, and
then one-half the scientific world began to question the principle
of gravitation, which was supported by Newton and his adherents.
Subsequently the researches of Lagrange and Laplace, Adams and
Leverrier, Sir J. Herschel, etc., brought astronomy into prominence
more and more; and the innumerable stars have been indicated as new
planets have been discovered. The spectroscope, which gives us the
analyses of the sun and other heavenly bodies, has, in the able hands
of living astronomers, revealed to us elements existing in the vapours
and composition of the sun, etc. Stars are now known to be suns, some
bearing a great resemblance to our sun, others differing materially.
The nebulæ have been analysed, and found to be stars, or gas, burning
in space—hydrogen and nitrogen being the chief constituents of this
glowing matter. Instruments for astronomical observation have now been
brought to a pitch of perfection scarcely ever dreamed of, and month
by month discoveries are made and recorded, while calculations as to
certain combinations can be made with almost miraculous accuracy. The
transit of Venus, the approaches of comets, eclipses, and the movements
of stars, are now known accurately, and commented upon long before the
event can take place.
We will close this chapter by giving a brief explanation of the various
definitions most usually employed in astronomy.
1. The _Axis_ of the earth is an _imaginary_ line passing through the
centre (north and south); the _poles_ are the extremities of this line.
2. The _Equator_ is an imaginary circle passing round the globe,
dividing it into northern and southern hemispheres. The _equinoctial_
is the plane of the former circle extended to the heavens, and
when the sun appears in that line the days and nights are of equal
duration—twelve hours each.
3. The _Ecliptic_ is the sun’s path through the heavens—though, of
course, the sun does not actually move, and therefore the track, or
supposed circle, is really the earth’s motion observable from the sun.
When the moon is near this circle eclipses happen. The ecliptic cuts
the equinoctial at an angle of about 23°. One half is to the north and
the other to the south of the equinoctial.
[Illustration: Fig. 499.—The Zodiac.]
4. The _Zodiac_ is a girdle extending 8° on each side of the ecliptic,
in which space of 16° the planets move. The zodiac is divided into
twelve parts of 30° each, called the “Signs.” These names are as under
written:—
NORTHERN SIGNS.
_Spring._
Aries, the Ram, March.
Taurus, the Bull, April.
Gemini, the Twins, May.
_Summer._
Cancer, the Crab, June.
Leo, the Lion, July.
Virgo, the Virgin, August.
SOUTHERN SIGNS.
_Autumn._
Libra, the Balance, September.
Scorpio, the Scorpion, October.
Sagittarius, the Archer, November.
_Winter._
Capricornus, the Goat, December.
Aquarius, the Waterbearer, January.
Pisces, the Fishes, February.
5. _Colures_ are two circles dividing the ecliptic into four equal
parts, and making the seasons.
6. The _Horizon_ is the boundary line of our vision, and is called the
sensible (apparent) horizon. The true horizon is the circle—as on a
globe—dividing the heavens into two hemispheres. The sensible horizon
is enlarged according as the eye is elevated above the ground. A man
six feet high can see a distance of three miles when standing on a
plain. We can always find the distance visible when we know the height
at which we stand, or, inversely, we can tell the height of an object
if we know the distance. We have only to increase the height _one half
in feet_, and extract the square root for the distance in miles. On
giving the distance in miles reverse the operation.
[Illustration: Fig. 500.—Right ascension.]
For instance, for the man six feet high, as supposed, add three feet,
being half his height; that makes nine feet. The square root (or number
multiplied by itself to give nine) is three, which is the number of
miles the man can see on a plain. Or, again, suppose we can see a
tower on the level, and we know we are twelve miles away from it. The
square of twelve is one hundred and forty-four feet, one-third of that
is forty-eight feet, which represents the half of the original height
added to the whole tower in feet; so the whole tower is ninety-six feet
high. Reversing, as in the former case, we can prove this by taking the
tower at ninety-six feet high and trying to find the distance we can
see from its summit = 96 + 48 = 144; the square root of 144 = 12, the
distance required.
7. The _Nadir_ and the _Zenith_ are the poles of the horizon. The
zenith is exactly overhead, the nadir exactly under foot. Circles drawn
through these points are azimuth circles.
8. _Meridians_ are circles passing through the poles at right angles
to the equinoctial. Every place is supposed to have a meridian, but
only twenty-four are upon the globe, and they represent the sun’s, or
the planets’, “movements” every hour—15° being one hour, 360° being
twenty-four hours (_see_ fig. 500). One quarter of a degree equals one
minute of time. Parallels of latitude are familiar circles parallel to
the equator. Latitude in astronomy is the distance from the ecliptic at
a right angle north or south. This will be explained as we proceed.
[Illustration: Fig. 501.—Orbit of planet.]
9. _Declination_ is the distance of the heavenly bodies from the
equinoctial measured as a meridian.
The _Tropics_ indicate the limits of the sun’s declination.
10. _Disc_ is the term applied to the apparently flat surface of a
planet, such as the moon, for instance.
11. The _Orbit_ is the path described by a planet revolving round the
sun. The plane of the orbit is an imaginary surface cutting through
the centre of the sun and the planet, and extending to the stars. The
diagram shows the plane of the earth’s orbit. The circle, A B C D
(fig. 501), is the ecliptic. The inclination of an orbit is the plane
of the orbit with reference to the plane of the earth; and, supposing
the shaded part of the illustration to be water, a hoop held _inclined
towards the earth_, with one half in and the other half out of the
water, will describe the planetary orbit.
[Illustration: Fig. 502.—Conjunction of Venus and Saturn.]
12. _Nodes_ are the opposite points of a planet where its orbit cuts
the ecliptic or the earth’s orbit.
13. _Apogee_ is the point of a planet’s orbit farthest from the earth.
Perigee is the nearest point.
14. The terms Culmination, Conjunction, and Opposition require no
special explanation. But planets are in conjunction with each other
when in the same sign and degree. A planet with the sun between it and
the earth is in conjunction with the sun. With the earth between it and
the sun it is in opposition.
15. Latitude and longitude upon a celestial globe are known
respectively as “Declination” and “Right Ascension.”
16. The Radius Vector is a line drawn from a planet to the sun,
wherever the planet may be (_see_ fig. 497).
FOOTNOTES:
[25] He was obliged to recant before the Inquisition, and to repudiate
his researches. He was released on the condition of observing silence
upon the theory he had supported, but again obliged to recant.