Its sign [Symbol].
DISTANCE FROM THE SUN, 92,500,000 MILES. DIAMETER, POLAR, 7899
MILES; EQUATORIAL, 7925-1/2 MILES. AXIAL REVOLUTION, 23H. 56M.
4.09S.; ORBITAL, 365.86. ORBITAL VELOCITY PER MINUTE, 1152.8 MILES.
Let us lift ourselves up a thousand miles from the earth. We see it
as a ball hung upon nothing in empty space. As the drop of falling
water gathers itself [Page 142] into a sphere by its own inherent
attraction, so the earth gathers itself into a ball. Noticing
closely, we see forms of continents outlined in bright relief, and
oceanic forms in darker surfaces. We see that its axis of revolution
is nearly perpendicular to the line of light from the sun. One-half
is always dark. The sunrise greets a new thousand miles every hour;
the glories of [Page 143] the sunset follow over an equal space,
180° behind. We are glad that the darkness never overtakes the
morning.
[Illustration: Fig. 54.--Earth and Moon in Space.]
_The Aurora Borealis._
While east and west are gorgeous with sunrise and sunset, the north
is often more glorious with its aurora borealis. We remember that
all worlds have weird and inexplicable appendages. They are not
limited to their solid surfaces or their circumambient air. The
sun has its fiery flames, corona, zodiacal light, and perhaps a
finer kind of atmosphere than we know. The earth is
[Page 144]
not without its inexplicable surroundings. It has not only its
gorgeous eastern sunrise, its glorious western sunset, high above
its surface in the clouds, but it also has its more glorious northern
dawn far above its clouds and air. The realm of this royal splendor
is as yet an unconquered world waiting for its Alexander. There are
certain observable facts, viz., it prevails mostly near the arctic
circle rather than the pole; it takes on various forms--cloud-like,
arched, straight; it streams like banners, waves like curtains in
the wind, is inconstant; is either the cause or result of electric
disturbance; it is often from four hundred to six hundred miles
above the earth, while our air cannot be over one hundred miles.
It almost seems like a revelation to human eyes of those vast,
changeable, panoramic pictures by which the inhabitants of heaven
are taught.
[Illustration: Fig. 55.--The Aurora as Waving Curtains.]
Investigation has discovered far more mysteries than it has explained.
It is possible that the same cause that produces sun-spots produces
aurora in all space, visible in all worlds. If so, we shall see
more abundant auroras at the next maximum of sun-spot, between
1880-84.
_The Delicate Balance of Forces._
A soap-bubble in the wind could hardly be more flexible in form
and sensitive to influence than is the earth. On the morning of
May 9th, 1876, the earth's crust at Peru gave a few great throbs
upward, by the action of expansive gases within. The sea fled,
and returned in great waves as the land rose and fell. Then these
waves fled away over the great mobile surface, and in less than
five hours they had covered a space equal to half of Europe. The
waves ran out to the Sandwich Islands, six [Page 145] thousand
miles, at the rate of five hundred miles an hour, and arrived there
thirty feet high. They not only sped on in straight radial lines,
but, having run up the coast to California, were deflected away into
the former series of waves, making the most complex undulations.
Similar beats of the great heart of the earth have sent its pulses
as widely and rapidly on previous occasions.
The figure of the earth, even on the ocean, is irregular, in consequence
of the greater preponderance of land--and hence greater density--in the
northern hemisphere. These irregularities are often very perplexing
in making exact geodetic measurements. The tendency of matter to
fly from the centre by reason of revolution causes the equatorial
diameter to be twenty-six, miles longer than the polar one. By this
force the Mississippi River is enabled to run up a hill nearly
three miles high at a very rapid rate. Its mouth is that distance
farther from the centre of the earth than its source, when but
for this rotation both points would be equally distant.
If the water became more dense, or if the world were to revolve
faster, the oceans would rush to the equator, burying the tallest
mountains and leaving polar regions bare. If the water should become
lighter in an infinitesimal degree, or the world rotate more slowly,
the poles would be submerged and the equator become an arid waste.
No balance, turning to 1/1000 of a grain, is more delicate than
the poise of forces on the world. Laplace has given us proof that
the period of the earth's axial rotation has not changed 1/100
of a second of time in two thousand years.
[Page 146]
_Tides._
But there is an outside influence that is constantly acting upon
the earth, and to which it constantly responds. Two hundred and
forty thousand miles from the earth is the moon, having 1/81 the
mass of the world. Its attractive influence on the earth causes the
movable and nearer portions to hurry away from the more stable and
distant, and heap themselves up on that part of the earth nearest
the moon. Gravitation is inversely as the square of the distance;
hence the water on the surface of the earth is attracted more than
the body of the earth, some parts of which are eight thousand miles
farther off; hence the water rises on the side next the moon. But
the earth, as a whole, is nearer the moon than the water on the
opposite side, and being drawn more strongly, is taken away from
the water, leaving it heaped up also on the side opposite to the
moon.
A subsidiary cause of tides is found in the revolution of the earth
and moon about their common centre of gravity. Revolution about
an axis through the centre of a sphere enlarges the equator by
centrifugal force. Revolution about an axis touching the surface
of a flexible globe converts it into an egg-shaped body, with the
longer axis perpendicular to the axis of revolution. In Fig. 56 the
point of revolution is seen at the centre of gravity at G; hence,
in the revolution of earth and moon as one, a strong centrifugal
force is caused at D, and a less one at C. This gives greater height
to the tides than the attraction of the moon alone could produce.
[Page 147]
[Illustration: Fig. 56.]
If the earth had no axial revolution, the attractive point where
the tide rises would be carried around the earth once in twenty-seven
days by the moon's revolution about the earth. But since the earth
revolves on its axis, it presents a new section to the moon's attraction
every hour. If the moon were stationary, that would bring two high
tides in exactly twenty-four hours; but as the moon goes forward,
we need nearly twenty-five hours for two tides.
The attractive influence of the sun also gives us a tide four-tenths
as great as that of the moon. When these two influences of the sun
and moon combine, as they do, in conjunction--when both bodies
are on one side of the earth; or in opposition, sun and moon being
on opposite sides of the earth--we have spring or increased tides.
When the moon is in its first or third quarter, _i. e._, when a
line from the moon to the earth makes a right angle with one from
the sun to the earth, these influences antagonize one another,
and we have the neap or low tides.
It is easy to see that if, when the moon was drawing its usual
tide, the sun drew four-tenths of the water in a tide at right
angles with it, the moon's tide must be by so much lower. Because
of the inertia of the water [Page 148] it does not yield instantly
to the moon's influence, and the crest of the tide is some hours
behind the advancing moon.
The amount of tide in various places is affected by almost innumerable
influences, as distance of moon at its apogee or perigee; its position
north, south, or at the equator; distance of earth from sun at
perihelion and aphelion; the position of islands; the trend of
continents, etc. All eastern shores have far greater tides than
western. As the earth rolls to the east it leaves the tide-crest
under the moon to impinge on eastern shores, hence the tides of
from seventy-five to one hundred feet in the Bay of Fundy. Lakes and
most seas are too small to have perceptible tides. The spring-tides
in the Mediterranean Sea are only about three inches.
This constant ebb and flow of the great sea is a grand provision for
its purification. Even the wind is sent to the sea to be cleansed.
The sea washes every shore, purifies every cove, bay, and river
twice every twenty-four hours. All putrescible matter liable to
breed a pestilence is carried far from shore and sunk under fathoms
of the never-stagnant sea. The distant moon lends its mighty power
to carry the burdens of commerce. She takes all the loads that
can be floated on her flowing tides, and cheerfully carries them
in opposite directions in successive journeys.
It must be conceded that the profoundest study has not mastered
the whole philosophy of tides. There are certain facts which are
apparent, but for an explanation of their true theory such men as
Laplace, Newton, and Airy have labored in vain. There are plenty
of other worlds still to conquer.
[Page 150]
[Illustration: Fig. 57.--Lunar Day.]
[Page 151]
THE MOON.
New moon, [Symbol]; first quarter, [Symbol]; full moon, [Symbol];
last quarter, [Symbol].
EXTREME DISTANCE FROM THE EARTH, 259,600 MILES; LEAST, 221,000
MILES; MEAN, 240,000 MILES. DIAMETER, 2164.6 MILES [2153, LOCKYER].
REVOLUTION ABOUT THE EARTH, 29-1/2 DAYS. AXIAL REVOLUTION, SAME
TIME.
When the astronomer Herschel was observing the southern sky from
the Cape of Good Hope, the most clever hoax was perpetrated that
ever was palmed upon a credulous public. Some new and wonderful
instruments were carefully described as having been used by that
astronomer, whereby he was enabled to bring the moon so close that
he could see thereon trees, houses, animals, and men-like human
beings. He could even discern their movements, and gestures that
indicated a peaceful race. The extent of the hoax will be perceived
when it is stated that no telescope that we are now able to make
reveals the moon more clearly than it would appear to the naked
eye if it was one hundred or one hundred and fifty miles away.
The distance at which a man can be seen by the unaided eye varies
according to circumstances of position, background, light, and
eye, but it is much inside of five miles.
Since, however, the moon is our nearest neighbor, a member of our
own family in fact, it is a most interesting object of study.
A glance at its familiar face reveals its unequal illumination.
All ages and races have seen a man in the moon. All lovers have
sworn by its constancy, and only part of them have kept their oaths.
Every twenty-nine or thirty days we see a silver crescent in the
west, and are glad if it comes over the right shoulder--so [Page
152] much tribute does habit pay to superstition. The next night it
is thirteen degrees farther east from the sun. We note the stars it
occults, or passes by, and leaves behind as it broadens its disk,
till it rises full-orbed in the east when the sun sinks in the west.
It is easy to see that the moon goes around the earth from west to
east. Afterward it rises later and smaller each night, till at
length, lost from sight, it rises about the same time as the sun,
and soon becomes the welcome crescent new moon again.
The same peculiarities are always evident in the visible face of
the moon; hence we know that it always presents the same side to
the earth. Obviously it must make just one axial to one orbital
revolution. Hold any body before you at arm's-length, revolve it
one-quarter around you until exactly overhead. If it has not revolved
on an axis between the hands, another quarter of the surface is
visible; but if in going up it is turned a quarter over, by the
hands holding it steady, the same side is visible. Three causes
enable us to see a little more than half the moon's surface: 1. The
speed with which it traverses the ellipse of its orbit is variable.
It sometimes gets ahead of us, sometimes behind, and we see farther
around the front or back part. 2. The axis is a little inclined to
the plane of its orbit, and its orbit a little inclined to ours;
hence we see a little over its north pole, and then again over
the south pole. 3. The earth being larger, its inhabitants see
a little more than half-way around a smaller body. These causes
combined enable us to see 576/1000 of the moon's surface. Our eyes
will never see the other side of the moon. If, now, being solid,
her axial revolution could [Page 153] be increased enough to make
one more revolution in two or three years, that difference between
her axial and orbital revolution would give the future inhabitants
of the earth a view of the entire circumference of the moon. Yet if
the moon were once in a fluid state, or had oceans on the surface,
the enormous tide caused by the earth would produce friction enough,
as they moved over the surface, to gradually retard the axial
revolution till the two tidal elevations remained fixed toward and
opposite the earth, and then the axial and orbital revolutions would
correspond, as at present. In fact, we can prove that the form of
the moon is protuberant toward the earth. Its centre of gravity is
thirty-three miles beyond its centre of magnitude, which is the same
in effect as if a mountain of that enormous height rose on the earth
side. Hence any fluid, as water or air, would flow round to the
other side.
The moon's day, caused by the sun's light, is 29-1/2 times as long
as ours. The sun shines unintermittingly for fifteen days, raising a
temperature as fervid as boiling water. Then darkness and frightful
cold for the same time succeed, except on that half where the earth
acts as a moon. The earth presents the same phases--crescent, full,
and gibbous--to the moon as the moon does to us, and for the same
causes. Lord Rosse has been enabled, by his six-foot reflector, to
measure the difference of heat on the moon under the full blaze
of its noonday and midnight. He finds it to be no less than five
hundred degrees. People not enjoying extremes of temperature should
shun a lunar residence. The moon gives us only 1/6180000 as much
light as the sun. A sky full of moons would scarcely make daylight.
[Page 154]
[Illustration: Fig. 58.--View of the Moon near the Third Quarter.
From a Photograph by Professor Henry Draper.]
There are no indications of air or water on the moon. When it occults
a star it instantly shuts off the light and as instantly reveals
it again. An atmosphere would gradually diminish and reveal the
light, and by refraction [Page 155] cause the star to be hidden in
much less time than the solid body of the moon would need to pass
over it. If the moon ever had air and water, as it probably did,
they are now absorbed in the porous lava of its substance.
_Telescopic Appearance._
[Illustration: Fig. 59.--Illumination of Craters and Peaks.]
Probably no one ever saw the moon by means of a good telescope
without a feeling of admiration and awe. Except at full-moon, we
can see where the daylight struggles with the dark along the line
of the moon's sunrise or sunset. This line is called the terminator.
It is broken in the extreme, because the surface is as rough as
possible. In consequence of the small gravitation of the moon, utter
absence of the expansive power of ice shivering the cliffs, or the
levelling power of rains, precipices can stand in perpendicularity,
mountains shoot up like needles, and cavities three miles deep
remain unfilled. The light of the sun falling on the rough body
of the moon, shown in section (Fig. 59), illuminates the whole
cavity at _a_, part of the one at _b_, casts a long shadow from the
mountain at _c_, and touches the tip of the one at _d_, which appears
to a distant observer as a point of light beyond the terminator,
As the moon revolves the conical cavity, _a_ is illuminated on
the forward side only; the light creeps down the backward side
of cavity _b_ to the bottom; mountain _c_. comes directly under
the sun and casts no shadow, and mountain _d_ casts its long shadow
over the plain. Knowing the time of revolution, and observing the
change of [Page 156] illumination, we can easily measure the height
of mountain and depth of crater. An apple, with excavations and
added prominences, revolved on its axis toward the light of a
candle, admirably illustrates the crescent light that fills either
side of the cavities and the shadows of the mountains on the plain.
Notice in Fig. 58 the crescent forms to the right, showing cavities
in abundance.
[Illustration: Fig. 60.--Lunar Crater "Copernicus," after Secchi.]
The selenography of one side of the moon is much better known to
us than the geography of the earth. Our maps of the moon are far
more perfect than those of the earth; and the photographs of lunar
objects by Messrs. Draper and De la Rue are wonderfully perfect,
[Page 157] and the drawings of Padre Secchi equally so (Fig. 60).
The least change recognizable from the earth must be speedily
detected. There are frequently reports of discoveries of volcanoes
on the moon, but they prove to be illusions. The moon will probably
look the same to observers a thousand years hence as it does to-day.
This little orb, that is only 1/81 of the mass of the earth, has
twenty-eight mountains that are higher than Mont Blanc, that "monarch
of mountains," in Europe.
_Eclipses._
[Illustration: Fig. 61.--Eclipses; Shadows of Earth and Moon.]
It is evident that if the plane of the moon's orbit were to correspond
with that of the earth, as they all lie in the plane of the page
(Fig. 61), the moon must pass between the centres of the earth
and sun, and exactly behind the earth at every revolution. Such
successive and total darkenings would greatly derange all affairs
dependent on light. It is easily avoided. Venus does [Page 158] not
cross the disk of the sun at every revolution, because of the
inclination of the plane of its orbit to that of the earth (see Fig.
41, p. 107). So the plane of the orbit of the moon is inclined to
the orbit of the earth 5° 8' 39"; hence the full-moon is often above
or below the earth's shadow, and the earth is below or above the
moon's shadow at new moon. It is as if the moon's orbit were pulled
up one-quarter of an inch from the page behind the earth, and
depressed as much below it between the earth and the sun. The point
where the orbit of the moon penetrates the plane of the ecliptic is
called a node. If a new moon occur when the line of intersection of
the planes of orbits points to the sun, the sun must be eclipsed; if
the full-moon occur, the moon must be eclipsed. In any other
position the sun or moon will only be partially hidden, or no
eclipse will occur.
If the new moon be near the earth it will completely obscure the
sun. A dime covers it if held close to the eye. It may be so far
from the earth as to only partially hide the sun; and, if it cover
the centre, leave a ring of sunlight on every side. This is called
an annular eclipse. Two such eclipses will occur this year (1879).
If the full-moon passes near the earth, or is at perigee, it finds
the cone of shadow cast by the earth larger, and hence the eclipse
is greater; if it is far from the earth, or near apogee, the earth's
shadow is smaller, and the eclipse less, or is escaped altogether.
There is a certain periodicity in eclipses. Whenever the sun, moon,
and earth are in a line, as in the total eclipse of July 29th,
1878, they will be in the same position after the earth has made
about eighteen revolutions, [Page 159] and the moon two hundred and
sixteen--that is, eighteen years after. This period, however, is
disregarded by astronomers, and each eclipse calculated by itself to
the accuracy of a second.
How terrible is the fear of ignorance and superstition when the sun
or moon appear to be in the process of destruction! how delightful
are the joys of knowledge when its prophesies in regard to the
heavenly bodies are being fulfilled!