own axis, in the succession of days and seasons, and in all the
phenomena governed by the great law of attraction, the earth becomes
the representative of all the other planets; in studying it, we study
all the heavenly bodies.
Our planet is a spheroid; that is, a sphere flattened at the
two poles and enlarged at the equator, so that all the circles
passing through the extremity of the polar axis form ellipses. The
presumed depression of each pole is about thirteen miles, nearly
a three-hundredth part of the radius of the earth; but it is not
altogether certain that the two poles are equally flattened. Perhaps
a contrast exists between the two hemispheres, not only in the
features of their continents and the distribution of seas, but also
in their geometrical shape. Be this as it may, it appears to be
proved that the curvature is not exactly the same at all points of
the earth at an equal distance from the poles; the meridians appear
without exception to be irregular ellipses.
The dimensions of the earth, as we have already seen, are almost as
nothing compared with the larger celestial bodies, and especially
with the extent of space which can be explored by the telescope. If
light, the speed of which has been adopted in astronomy as a term
of comparison, could be diffused in a curved line, it would travel
seven times round the globe in a second of time; this standard of
measurement, therefore, the only one suited to the stellary field, is
completely inapplicable to the surface of our globe.
The isolated globule in the immensity of space which we call the
earth is not motionless, as the ancients necessarily supposed,
looking upon it, as they did, as the immovable base of the firmament
of heaven. Hurried on in the vortex of universal vitality, our globe
is ever actuated by ceaseless motion, describing in ether a series
of elliptic spirals so complicated that astronomers have not yet
been able to calculate their various curves. Besides rotating on
its own axis, the earth describes an ellipse round the sun, and,
under the influence of this body, is drawn along from one heaven to
another toward distant constellations. It also oscillates and rocks
on its axis, and deviates more or less from its path, to salute, as
it were, every heavenly body which meets it. It is probable that
it never passes a second time through the same regions of the air;
yet, if it has again to traverse the spiral line of ellipses it has
already described, it would be after a cycle of so many thousands
of millions of years, that the earth itself, completely transformed,
would be no longer the same planet.
The motion of the earth, the immediate effects of which are the
most obvious to the notice of men, is the daily rotation which
takes place round an ideal axis passing through the two poles. The
globe turns from right to left, or from west to east—that is, in
a contrary direction to the apparent motion of the sun and stars,
which seem to rise in the east and to set in the west. As the earth’s
axis terminates at each pole, there is least surface-motion at
those points, and the motion is the more rapid in any part of the
surface of the globe the further it is from the central axis. At St.
Petersburg, in 60° latitude, the speed of rotation is about nine
miles a minute; in Paris, it exceeds eleven and a half miles during
the same brief time; on the equatorial line, which may be looked upon
as the ring of an immense wheel, the speed of the earth is twice as
great as it is at 60° of latitude—that is, about eighteen miles a
minute, or 528 yards a second—a rapidity equal to the flight of a
26-pound cannon-ball impelled by thirteen pounds of powder. By means
of this rotatory motion, the earth presents toward the sun each of
its faces alternately, and each also in turn toward the comparatively
darker regions of space; the succession of day and night is thus
constituted. In addition to this, the rotation of the earth is an
important fact which must always be taken into account in determining
the direction of fluids in motion on the surface of the globe, such
as streams and rivers, also marine and atmospheric currents.
The annual revolution which the earth performs round the sun follows
the line of an ellipse, one of the _foci_ of which is occupied by
the central star; the eccentricity of the ellipse is nearly equal
to 17/1000th of the great axis. The distance between the sun and
the earth always varies according to the particular point of its
orbit which the latter is traveling over. At its _aphelion_, that
is, at its greatest remoteness, this distance is about 93¾ millions
of miles; at the period of its _perihelion_, when the two heavenly
bodies are nearest to each other, it is approximately 90,259,000
miles. The mean distance, as estimated by astronomers since the
corrections of Encke, Hansen, Foucault, and Hind, is 91,839,000
miles. This extent of space is traversed by the solar rays in 8
minutes, 16 seconds; sound would take fifteen years in passing
through the same distance.
As Kepler has laid down in his celebrated laws, our planet moves with
an increased rapidity as it approaches nearer to the sun and travels
more slowly in proportion to its distance from that luminary; but
its mean speed may be estimated at nearly nineteen miles a second,
or sixty times the rapidity of a ball from the cannon’s mouth. This
speed, which makes one dizzy to think of, is to be added, as regards
each point in the surface of the earth, to the rotatory motion which
impels it round the polar axis.
After having turned round 366 times on its axis, our planet has
terminated its orbicular course, and is in the same position
relatively to the sun as at its starting-point; it has then
accomplished its _year_.
This daily rotation of the earth round its axis produces the
succession of days and nights, and, in the same way, its annual
revolution round the sun causes the alternations of the seasons. If
the axis of the earth, that is the ideal line which passes through
its two poles, were perpendicular to the plane of its annual orbit,
it is evident that the portion of the globe lighted by the sun would
invariably extend from one pole to the other, and that in both
hemispheres the days and nights would always consist of twelve hours
each. But this is not the case. The earth performs its revolutionary
movements in an inclined position; its ideal polar axis is sloped
about 23° 28′ from a perpendicular to its plane, and this position is
so far maintained that as regards the comparatively rapid succession
of days and seasons it may be looked upon as invariable. This
obliquity of axis causes continued changes in the phase presented
to the sun. The portion of the earth illumined by the rays of the
sun varies every day; for, although the planetary axis may appear
to maintain its extremity in a fixed position as regards some point
in infinite space, in respect to the sun it presents a constantly
varying degree of inclination, in consequence of the continual motion
of the earth. Twice during the course of the year it so happens that
the solar rays fall perpendicularly upon the equator of the earth; at
every other period in the annual revolution, sometimes the Northern
and sometimes the Southern Hemisphere receives the greatest amount of
light.
The astronomical year commences on the 20th of March, at the exact
moment when the sun illumines the equator in a vertical direction,
and the line of separation between light and shade passes through
the two poles. The period of darkness is then equal to that of light,
and admits of exactly twelve hours at all points of the earth. Hence
the name of “equinox” (equality of nights). But after this day, which
in the Northern Hemisphere serves as the starting-point of spring,
the earth continues its translatory movement. In consequence of the
inclination of its axis, the Northern Hemisphere, being turned toward
the sun, receives a greater quantity of light, while the southern
half of the globe is less vividly lighted. The vertical rays of the
sun now fall more and more to the north of the equator, and the
circle of light, far from arresting its progress at the poles, where
the day of six months’ duration is commencing to dawn, extends far
beyond it over the regions of the north. On the 21st of June, the day
of the first solstice, the axis of the earth being deeply inclined
toward the sun, this luminary shines on the zenith of the tropic
of Cancer at 23½° north of the equator, and its light illumines
the whole of the arctic zone, that is, the portion of the earth’s
surface extending to 23½° round the North Pole. Then spring ceases
and summer begins as regards the Northern Hemisphere. In the Southern
Hemisphere, on the contrary, autumn is giving place to winter. Above
the equator long days are prevailing, interrupted by short nights;
while in the south it is the nights which last the longest. In the
arctic zone the sun performs its apparent course of diurnal rotation
entirely above the horizon. The six months’ day, which spring
inaugurated at the North Pole, attains its high noon on the first
day of summer. At the same moment midnight arrives in the darkness
which is oppressing its antipodes.
Immediately after the 21st of June all the phenomena which took place
during the preceding season are directly reversed. The sun appears
to retrograde toward the southern horizon; its vertical rays cease
to fall on the line of the northern tropic, and constantly approach
the equator. The zone of light in the northern pole and of shade in
the southern equally diminish, and the days shorten in the Northern
Hemisphere in the same proportion as they lengthen in the Southern;
an equilibrium is gradually being re-established between the two
halves of the earth. On the 22d of September the position of the
sun is again exactly above the equator, and its light just reaches
both poles. The equinox, or the absolute equality of day and night
in every part of the globe, occurs for the second time in the year;
but this moment of equilibrium is, so to speak, but a mathematical
point between the two seasons. The axis of the earth which, during
the six months past, turned the North Pole toward the sun, now
presents to him the South Pole; the vertical rays of the central
luminary fall to the south of the earth’s equator, and the Southern
Hemisphere, in its turn, is the best endowed of the two halves of
the globe in the amount of light it receives and in the length of
its days. In the Southern Hemisphere spring is commencing; in the
Northern, autumn. Three months afterward, on the 21st of December,
the sun comes directly over the southern tropic, or the tropic of
Capricorn, 23½° south of the equator, and the whole of the antarctic
zone is presented to the solar rays. Summer has begun in the Southern
Hemisphere, and at the same time winter commences in that of the
north. Then, as the globe moves on, these two seasons follow each
other in their course, until at length the earth attains a position
similar to that from which it started; the March equinox, the first
day of spring in Europe, and the first day of autumn in Australia,
commences anew the astronomical year.
The elliptical form of the earth’s orbit and the unequal pace of the
globe in the various points of its course cause some considerable
variations in the duration of the seasons. In fact, from the 20th of
March to the 22d of September, that is, during the spring and summer
of the Northern Hemisphere, the earth takes 186 days to travel over
the first and largest half of its orbit, while during the winter
period, from the 22d of September to the 20th of March, only 179
days are required to accomplish the second half of its journey. The
summer period of the Northern Hemisphere actually exceeds by seven
or eight days, or about 187 hours, the corresponding period in the
southern half of the globe; added to this, in consequence of the
longer space of time during which the Arctic Pole remains inclined
toward the sun in the regions north of the equator, the hours of
daylight exceed the hours of night, while in the south the hours of
darkness predominate. This is, however, to some extent compensated
for; as, although in the southern regions of the earth the summer
lasts a shorter time, our planet is then closer to the sun; it is
at its perihelion, and consequently receives a larger proportion of
heat. There is, however, no doubt about the fact—as it is proved by a
direct observation, both of the winds and currents, and also of their
various temperatures—that, taking an equal distance from the equator,
the southern regions are colder than those of the north.
If an equality of seasons between the two halves of the world does
not at present exist, it will not fail to be established after a long
series of centuries by means of a slow terrestrial movement, which
has been known by the name of the _precession of the equinoxes_.
Just as a top (if we may be allowed to avail ourselves of so old an
illustration) turns round on the ground and bends over successively
in every direction, thus describing with its axis an ideal cone, so
the earth revolves in space, and slowly sways the line of its poles.
This line, which is always sloped at an angle of 66° 32′ to the
plane of the terrestrial orbit, turns round with a slight lateral
motion, so as always to point to a new region of the sky; if it were
prolonged indefinitely it would describe a circle amid the distant
stars. As the axis of the earth is constantly changing its direction
in this way, the plane of the equator must vary exactly to the same
extent in its position as regards the sun. In fact, every year
the exact moment of the March equinox anticipates by about twenty
minutes the time at which the corresponding equinox fell in the
year preceding. Each revolution of the earth round the sun brings a
fresh advance of twenty minutes in the determination of the equinox;
and as, during the long course of ages, the axis of the earth does
not intermit in this swaying motion, the time must come, after a
period of 12,900 years, that the conditions of the seasons will be
altogether changed. The hemisphere which hitherto received the larger
proportion of heat will receive the lesser share, and that half of
the globe which has endured the larger number of wintry days will
now, in its turn, enjoy the more lengthened period of summer. Then,
after a second period of 12,900 years, during which the relation
between the seasons of the two hemispheres is being gradually
modified, the axis of the earth completes its round of swaying,
which has lasted for 258 centuries, and the position of the globe in
respect to the sun being nearly the same as at its starting-point, a
second cycle of seasons will then commence.
We might call this period _the earth’s great year_, if, at the end of
it, the earth were in an identical position to that which it occupied
at the commencement; but this is not the case. The attraction of the
moon, and the disturbances caused by the vicinity of certain planets,
are incessantly modifying the curve described in the starry fields
of space by the earth’s axis, and complicate it with a multitude of
spirals, the various periods of which do not coincide with the great
period of the swaying of the axis. The successive undulations form a
continuous system of interwoven spirals. “It is a manifestation of
the infinite.”
But even this is not all. In addition to all the motions of the
globe which we have already pointed out—its diurnal rotation, its
annual revolution round the sun, the rhythmical swaying of its axis,
proved by the precession of the equinoxes, the nutation or more rapid
swaying which is caused by the attraction of the moon—we must now
notice the enormous translatory movement which is dragging it through
endless tracks of space in the train of the sun. Not many years ago,
this motion was entirely unknown to astronomers, and yet it is going
on with inconceivable rapidity—a rapidity more than double that of
the course of the planet round its central luminary. In one second
of time the earth moves about forty-four miles toward the point of
the heavens where we find the constellation of Hercules. During one
year only she travels 1,382 millions of miles in this direction.
Our own little earth itself is carried on from space to space, and
never closes the cycle of its revolutions. Ever since the time when
its particles were first grouped together, it has been describing in
space the infinite spiral of its ellipses, and thus will it go on
turning and oscillating in ether until the moment when it will exist
no longer as an independent planet. For the earth, too, must have an
end; like every other body in the universe, it comes into existence,
and lives only to die when its turn comes. Already its annual motion
of rotation is diminishing in speed; certainly this slackening of
pace is not very observable, since no astronomer from Hipparchus to
Laplace has yet exactly defined it. But, unless some cosmical force
acting in a contrary direction compensates for the loss of speed
caused by the friction of the tides against the bed and the shores of
the ocean, the impetus of our planet will every century diminish.
After various catastrophes which it is impossible to foresee, the
earth will eventually completely change its course of action, and
lose its independent existence, either uniting itself with other
planetary bodies or breaking up into fragments; or it will perhaps
terminate its course by falling like a mere aerolite upon the surface
of the sun.
THE MOON.—THOMAS GWYN ELGER
We know, both by tradition and published records, that from the
earliest times the faint gray and light spots which diversify the
face of our satellite excited the wonder and stimulated the curiosity
of mankind, giving rise to superstitions more or less crude and
erroneous as to their actual nature and significance. It is true
that Anaxagoras, five centuries before our era, and probably other
philosophers preceding him—certainly Plutarch at a much later
date—taught that these delicate markings and differences of tint,
obvious to every one with normal vision, point to the existence of
hills and valleys on her surface; the latter maintaining that the
irregularities of outline presented by the “terminator,” or line
of demarcation between the illumined and unillumined portion of
her spherical superficies, are due to mountains and their shadows;
but more than fifteen centuries elapsed before the truth of this
sagacious conjecture was unquestionably demonstrated. Selenography,
as a branch of observational astronomy, dates from the spring of
1609, when Galileo directed his “optic tube” to the moon, and in the
following year, in the _Sidereus Nuncius_, or the “Intelligencer of
the Stars,” gave to an astonished and incredulous world an account of
the unsuspected marvels it revealed.
The bright and dusky areas, so obvious to the unaided sight, were
found by Galileo to be due to a very manifest difference in the
character of the lunar surface, a large portion of the Northern
Hemisphere, and no inconsiderable part of the southeastern quadrant,
being seen to consist of large gray monotonous tracts, often
bordered by lofty mountains, while the remainder of the superficies
was much more conspicuously brilliant, and, moreover, included
by far the greater number of those curious ring-mountains and
other extraordinary features whose remarkable aspect and peculiar
arrangement first attracted his attention.
Before the close of the century when selenography first became
possible, Hevel of Dantzig, Scheiner, Langrenus (cosmographer to
the King of Spain), Riccioli, the Jesuit astronomer of Bologna, and
Dominic Cassini, the celebrated French astronomer, greatly extended
the knowledge of the moon’s surface, and published drawings of
various phases and charts, which, though very rude and incomplete,
were a clear advance upon what Galileo, with his inferior optical
means, had been able to accomplish. Langrenus, and after him Hevel,
gave distinctive names to the various formations, mainly derived
from terrestrial physical features, for which Riccioli subsequently
substituted those of philosophers, mathematicians, and other
celebrities; and Cassini determined by actual measurement the
relative position of many of the principal objects on the disk, thus
laying the foundation of an accurate system of lunar topography;
while the labors of T. Mayer and Schröter in the Eighteenth Century,
and of Lohrmann, Mädler, Neison (Nevill), Schmidt, and other
observers in the Nineteenth, have been mainly devoted to the study of
the minuter detail of the moon and its physical characteristics.
As was manifest to the earliest telescopic observers, its visible
surface is clearly divisible into strongly contrasted areas,
differing both in color and structural character. Somewhat less
than half of what we see of it consists of comparatively level dark
tracts, some of them many thousands of square miles in extent, the
monotony of whose dusky superficies is often unrelieved for great
distances by any prominent object; while the remainder, everywhere
manifestly brighter, is not only more rugged and uneven, but is
covered to a much greater extent with numbers of quasi-circular
formations differing widely in size, classed as walled-plains,
ring-plains, craters, craterlets, crater-cones, etc. (the latter
bearing a great outward resemblance to some terrestrial volcanoes),
and mountain ranges of vast proportions, isolated hills and other
features.
Though nothing resembling sheets of water, either of small or large
extent, has ever been detected on the surface of the moon, the
superficial resemblance, in small telescopes, of the large gray
tracts to the appearance which we may suppose our terrestrial lakes
and oceans would present to an observer on the moon, naturally
induced the early selenographers to term them Maria, or “seas”—a
convenient name, which is still maintained, without, however,
implying that these areas, as we now see them, are, or ever were,
covered with water.
There are twenty-three of these dusky areas which have received
distinctive names; seventeen of them are wholly, or in great part,
confined to the northern and to the southeastern quarter of the
Southern Hemisphere—the southwestern quadrant being to a great extent
devoid of them. By far the largest is the vast Oceanus Procellarum,
extending from a high northern latitude to beyond latitude 10° in the
southeastern quadrant, and, according to Schmidt, with its bays and
inflections, occupying an area of nearly two million square miles, or
more than that of all the remaining Maria put together. Next in order
of size come the Mare Nubium, or about one-fifth the superficies,
covering a large portion of the southeastern quadrant, and extending
considerably north of the equator, and the Mare Imbrium, wholly
confined to the northeastern quadrant, and including an area of about
340,000 square miles. These are by far the largest lunar “seas”. The
Mare Fœcunditatis, in the Western Hemisphere, the greater part of
it lying in the southwestern quadrant, is scarcely half so big as
the Mare Imbrium; while the Maria Serenitatis and Tranquilitatis,
about equal in area (the former situated wholly north of the equator
and the latter only partially extending south of it), are still
smaller. The arctic Mare Frigoris, some 100,000 square miles in
extent, is the only remaining large sea; the rest, such as the Mare
Vaporum, the Sinus Medii, the Mare Crisium, the Mare Humorum, and
the Mare Humboldtianum, are of comparatively small dimensions, the
Mare Crisium not greatly exceeding 70,000 square miles, the Mare
Humorum (about the size of England) 50,000 square miles, while the
Mare Humboldtianum, according to Schmidt, includes only about 42,000
square miles, an area which is approached by some formations not
classed with the Maria.