THE EARTH
We have already seen (in Chapter I.) how, in very early times, men
naturally enough considered the earth to be a flat plane extending to a
very great distance in every direction; but that, as years went on,
certain of the Greek philosophers suspected it to be a sphere. One or
two of the latter are, indeed, said to have further believed in its
rotation about an axis, and even in its revolution around the sun; but,
as the ideas in question were founded upon fancy, rather than upon any
direct evidence, they did not generally attract attention. The small
effect, therefore, which these theories had upon astronomy, may well be
gathered from the fact that in the Ptolemaic system the earth was
considered as fixed and at the centre of things; and this belief, as we
have seen, continued unaltered down to the days of Copernicus. It was,
indeed, quite impossible to be certain of the real shape of the earth or
the reality of its motions until knowledge became more extended and
scientific instruments much greater in precision.
We will now consider in detail a few of the more obvious arguments which
can be put forward to show that our earth is a sphere.
If, for instance, the earth were a plane surface, a ship sailing away
from us over the sea would appear to grow smaller and smaller as it
receded into the distance, becoming eventually a tiny speck, and fading
gradually from our view. This, however, is not at all what actually
takes place. As we watch a vessel receding, its hull appears bit by bit
to slip gently down over the horizon, leaving the masts alone visible.
Then, in their turn, the masts are seen to slip down in the same manner,
until eventually every trace of the vessel is gone. On the other hand,
when a ship comes into view, the masts are the first portions to appear.
They gradually rise up from below the horizon, and the hull follows in
its turn, until the whole vessel is visible. Again, when one is upon a
ship at sea, a set of masts will often be seen sticking up alone above
the horizon, and these may shorten and gradually disappear from view
without the body of the ship to which they belong becoming visible at
all. Since one knows from experience that there is no _edge_ at the
horizon over which a vessel can drop down, the appearance which we have
been describing can only be explained by supposing that the surface of
the earth is always curving gradually in every direction.
The distance at which what is known as the _horizon_ lies away from us
depends entirely upon the height above the earth's surface where we
happen at the moment to be. A ship which has appeared to sink below the
horizon for a person standing on the beach, will be found to come back
again into view if he at once ascends a high hill. Experiment shows that
the horizon line lies at about three miles away for a person standing at
the water's edge. The curving of the earth's surface is found, indeed,
to be at the rate of eight inches in every mile. Now it can be
ascertained, by calculation, that a body curving at this rate in every
direction must be a globe about 8000 miles in diameter.
Again, the fact that, if not stopped by such insuperable obstacles as
the polar ice and snow, those who travel continually in any one
direction upon the earth's surface always find themselves back again at
the regions from which they originally set out, is additional ground for
concluding that the earth is a globe.
We can find still further evidence. For instance, in an eclipse of the
moon the earth's shadow, when seen creeping across the moon's face, is
noted to be _always_ circular in shape. One cannot imagine how such a
thing could take place unless the earth were a sphere.
Also, it is found from observation that the sun, the planets, and the
satellites are, all of them, round. This roundness cannot be the
roundness of a flat plate, for instance, for then the objects in
question would sometimes present their thin sides to our view. It
happens, also, that upon the discs which these bodies show, we see
certain markings shifting along continually in one direction, to
disappear at one side and to reappear again at the other. Such bodies
must, indeed, be spheres in rotation.
The crescent and other phases, shown by the moon and the inferior
planets, should further impress the truth of the matter upon us, as such
appearances can only be caused by the sunlight falling from various
directions upon the surfaces of spherical bodies.
Another proof, perhaps indeed the weightiest of all, is the continuous
manner in which the stars overhead give place to others as one travels
about the surface of the earth. When in northern regions the Pole Star
and its neighbours--the stars composing the Plough, for instance--are
over our heads. As one journeys south these gradually sink towards the
northern horizon, while other stars take their place, and yet others are
uncovered to view from the south. The regularity with which these
changes occur shows that every point on the earth's surface faces a
different direction of the sky, and such an arrangement would only be
possible if the earth were a sphere. The celebrated Greek philosopher,
Aristotle, is known to have believed in the globular shape of the earth,
and it was by this very argument that he had convinced himself that it
was so.
The idea of the sphericity of the earth does not appear, however, to
have been generally accepted until the voyages of the great navigators
showed that it could be sailed round.
The next point we have to consider is the rotation of the earth about
its axis. From the earliest times men noticed that the sky and
everything in it appeared to revolve around the earth in one fixed
direction, namely, towards what is called the West, and that it made one
complete revolution in the period of time which we know as twenty-four
hours. The stars were seen to come up, one after another, from below the
eastern horizon, to mount the sky, and then to sink in turn below the
western horizon. The sun was seen to perform exactly the same journey,
and the moon, too, whenever she was visible. One or two of the ancient
Greek philosophers perceived that this might be explained, either by a
movement of the entire heavens around the earth, or by a turning motion
on the part of the earth itself. Of these diverse explanations, that
which supposed an actual movement of the heavens appealed to them the
most, for they could hardly conceive that the earth should continually
rotate and men not be aware of its movement. The question may be
compared to what we experience when borne along in a railway train. We
see the telegraph posts and the trees and buildings near the line fly
past us one after another in the contrary direction. Either these must
be moving, or we must be moving; and as we happen to _know_ that it is,
indeed, we who are moving, there can be no question therefore about the
matter. But it would not be at all so easy to be sure of this movement
were one unable to see the objects close at hand displacing themselves.
For instance, if one is shut up in a railway carriage at night with the
blinds down, there is really nothing to show that one is moving, except
the jolting of the train. And even then it is hard to be sure in which
direction one is actually travelling.
The way we are situated upon the earth is therefore as follows. There
are no other bodies sufficiently near to be seen flying past us in turn;
our earth spins without a jolt; we and all things around us, including
the atmosphere itself, are borne along together with precisely the same
impetus, just as all the objects scattered about a railway carriage
share in the forward movement of the train. Such being the case, what
wonder that we are unconscious of the earth's rotation, of which we
should know nothing at all, were it not for that slow displacement of
the distant objects in the heavens, as we are borne past them in turn.
If the night sky be watched, it will be soon found that its apparent
turning movement seems to take place around a certain point, which
appears as if fixed. This point is known as the north pole of the
heavens; and a rather bright star, which is situated very close to this
hub of movement, is in consequence called the Pole Star. For the
dwellers in southern latitudes there is also a point in their sky which
appears to remain similarly fixed, and this is known as the south pole
of the heavens. Since, however, the heavens do not turn round at all,
but the earth does, it will easily be seen that these apparently
stationary regions in the sky are really the points towards which the
axis of the earth is directed. The positions on the earth's surface
itself, known as the North and South Poles, are merely the places where
the earth's axis, if there were actually such a thing, would be expected
to jut out. The north pole of the earth will thus be situated exactly
beneath the north pole of the heavens, and the south pole of the earth
exactly beneath the south pole of the heavens.
We have seen that the earth rotates upon its imaginary axis once in
about every twenty-four hours. This means that everything upon the
surface of the earth is carried round once during that time. The
measurement around the earth's equator is about 24,000 miles; and,
therefore, an object situated at the equator must be carried round
through a distance of about 24,000 miles in each twenty-four hours.
Everything at the equator is thus moving along at the rapid rate of
about 1000 miles an hour, or between sixteen and seventeen times as
fast as an express train. If, however, one were to take measurements
around the earth parallel to the equator, one would find these
measurements becoming less and less, according as the poles were
approached. It is plain, therefore, that the speed with which any point
moves, in consequence of the earth's rotation, will be greatest at the
equator, and less and less in the direction of the poles; while at the
poles themselves there will be practically no movement, and objects
there situated will merely turn round.
The considerations above set forth, with regard to the different speeds
at which different portions of a rotating globe will necessarily be
moving, is the foundation of an interesting experiment, which gives us
further evidence of the rotation of our earth. The measurement around
the earth at any distance below the surface, say, for instance, at the
depth of a mile, will clearly be less than a similar measurement at the
surface itself. The speed of a point at the bottom of a mine, which
results from the actual rotation of the earth, must therefore be less
than the speed of a point at the surface overhead. This can be
definitely proved by dropping a heavy object down a mine shaft. The
object, which starts with the greater speed of the surface, will, when
it reaches the bottom of the mine, be found, as might be indeed
expected, to be a little ahead (_i.e._ to the east) of the point which
originally lay exactly underneath it. The distance by which the object
gains upon this point is, however, very small. In our latitudes it
amounts to about an inch in a fall of 500 feet.
The great speed at which, as we have seen, the equatorial regions of
the earth are moving, should result in giving to the matter there
situated a certain tendency to fly outwards. Sir Isaac Newton was the
first to appreciate this point, and he concluded from it that the earth
must be _bulged_ a little all round the equator. This is, indeed, found
to be the case, the diameter at the equator being nearly twenty-seven
miles greater than it is from pole to pole. The reader will, no doubt,
be here reminded of the familiar comparison in geographies between the
shape of the earth and that of an orange.
In this connection it is interesting to consider that, were the earth to
rotate seventeen times as fast as it does (_i.e._ in one hour
twenty-five minutes, instead of twenty-four hours), bodies at the
equator would have such a strong tendency to fly outwards that the force
of terrestrial gravity acting upon them would just be counterpoised, and
they would virtually have _no weight_. And, further, were the earth to
rotate a little faster still, objects lying loose upon its surface would
be shot off into space.
The earth is, therefore, what is technically known as an _oblate
spheroid_; that is, a body of spherical shape flattened at the poles. It
follows of course from this, that objects at the polar regions are
slightly nearer to the earth's centre than objects at the equatorial
regions. We have already seen that gravitation acts from the central
parts of a body, and that its force is greater the nearer are those
central parts. The result of this upon our earth will plainly be that
objects in the polar regions will be pulled with a slightly stronger
pull, and will therefore _weigh_ a trifle more than objects in the
equatorial regions. This is, indeed, found by actual experiment to be
the case. As an example of the difference in question, Professor Young,
in his _Manual of Astronomy_, points out that a man who weighs 190
pounds at the equator would weigh 191 at the pole. In such an experiment
the weighing would, however, have to be made with a _spring balance_,
and _not with scales_; for, in the latter case, the "weights" used would
alter in their weight in exactly the same degree as the objects to be
weighed.
It used to be thought that the earth was composed of a relatively thin
crust, with a molten interior. Scientific men now believe, on the other
hand, that such a condition cannot after all prevail, and that the earth
must be more or less solid all through, except perhaps in certain
isolated places where collections of molten matter may exist.
The _atmosphere_, or air which we breathe, is in the form of a layer of
limited depth which closely envelops the earth. Actually, it is a
mixture of several gases, the most important being nitrogen and oxygen,
which between them practically make up the air, for the proportion of
the other gases, the chief of which is carbonic acid gas, is exceedingly
small.
It is hard to picture our earth, as we know it, without this atmosphere.
Deprived of it, men at once would die; but even if they could be made to
go on living without it by any miraculous means, they would be like unto
deaf beings, for they would never hear any sound. What we call _sounds_
are merely vibrations set up in the air, which travel along and strike
upon the drum of the ear.
The atmosphere is densest near the surface of the earth, and becomes
less and less dense away from it, as a result of diminishing pressure of
air from above. The greater portion of it is accumulated within four or
five miles of the earth's surface.
It is impossible to determine exactly at what distance from the earth's
surface the air ceases altogether, for it grows continually more and
more rarefied. There are, however, two distinct methods of ascertaining
the distance beyond which it can be said practically not to exist. One
of these methods we get from twilight. Twilight is, in fact, merely
light reflected to us from those upper regions of the air, which still
continue to be illuminated by the sun after it has disappeared from our
view below the horizon. The time during which twilight lasts, shows us
that the atmosphere must be at least fifty miles high.
But the most satisfactory method of ascertaining the height to which the
atmosphere extends is from the observation of meteors. It is found that
these bodies become ignited, by the friction of passing into the
atmosphere, at a height of about 100 miles above the surface of the
earth. We thus gather that the atmosphere has a certain degree of
density even at this height. It may, indeed, extend as far as about 150
miles.
The layer of atmosphere surrounding our earth acts somewhat in the
manner of the glass covering of a greenhouse, bottling in the sun's
rays, and thus storing up their warmth for our benefit. Were this not
so, the heat which we get from the sun would, after falling upon the
earth, be quickly radiated again into space.
It is owing to the unsteadiness of the air that stars are seen to
twinkle. A night when this takes place, though it may please the average
person, is worse than useless to the astronomer, for the unsteadiness is
greatly magnified in the telescope. This twinkling is, no doubt, in a
great measure responsible for the conventional "points" with which Art
has elected to embellish stars, and which, of course, have no existence
in fact.
The phenomena of _Refraction_,[13] namely, that bending which rays of
light undergo, when passing _slant-wise_ from a rare into a dense
transparent medium, are very marked with regard to the atmosphere. The
denser the medium into which such rays pass, the greater is this bending
found to be. Since the layer of air around us becomes denser and denser
towards the surface of the earth, it will readily be granted that the
rays of light reaching our eyes from a celestial object, will suffer the
greater bending the lower the object happens to be in the sky. Celestial
objects, unless situated directly overhead, are thus not seen in their
true places, and when nearest to the horizon are most out of place. The
bending alluded to is upwards. Thus the sun and the moon, for instance,
when we see them resting upon the horizon, are actually _entirely_
beneath it.
When the sun, too, is sinking towards the horizon, the lower edge of its
disc will, for the above reason, look somewhat more raised than the
upper. The result is a certain appearance of flattening; which may
plainly be seen by any one who watches the orb at setting.
In observations to determine the exact positions of celestial objects
correction has to be made for the effects of refraction, according to
the apparent elevation of these objects in the sky. Such effects are
least when the objects in question are directly overhead, for then the
rays of light, coming from them to the eye, enter the atmosphere
perpendicularly, and not at any slant.
A very curious effect, due to refraction, has occasionally been observed
during a total eclipse of the moon. To produce an eclipse of this kind,
_the earth must, of course, lie directly between the sun and the moon_.
Therefore, when we see the shadow creeping over the moon's surface, the
sun should actually be well below the horizon. But when a lunar eclipse
happens to come on just about sunset, the sun, although really sunk
below the horizon, appears still above it through refraction, and the
eclipsed moon, situated, of course, exactly opposite to it in the sky,
is also lifted up above the horizon by the same cause. Pliny, writing in
the first century of the Christian era, describes an eclipse of this
kind, and refers to it as a "prodigy." The phenomenon is known as a
"horizontal eclipse." It was, no doubt, partly owing to it that the
ancients took so long to decide that an eclipse of the moon was really
caused by the shadow cast by the earth. Plutarch, indeed, remarks that
it was easy enough to understand that a solar eclipse was caused by the
interposition of the moon, but that one could not imagine by the
interposition _of what body_ the moon itself could be eclipsed.
In that apparent movement of the heavens about the earth, which men now
know to be caused by the mere rotation of the earth itself, a slight
change is observed to be continually taking place. The stars, indeed,
are always found to be gradually drawing westward, _i.e._ towards the
sun, and losing themselves one after the other in the blaze of his
light, only to reappear, however, on the other side of him after a
certain lapse of time. This is equivalent to saying that the sun itself
seems always creeping slowly _eastward_ in the heaven. The rate at which
this appears to take place is such that the sun finds itself back again
to its original position, with regard to the starry background, at the
end of a year's time. In other words, the sun seems to make a complete
tour of the heavens in the course of a year. Here, however, we have
another illusion, just as the daily movement of the sky around the earth
was an illusion. The truth indeed is, that this apparent movement of the
sun eastward among the stars during a year, arises merely from a
_continuous displacement of his position_ caused by an actual motion of
the earth itself around him in that very time. In a word, it is the
earth which really moves around the sun, and not the sun around the
earth.
The stress laid upon this fundamental point by Copernicus, marks the
separation of the modern from the ancient view. Not that Copernicus,
indeed, had obtained any real proof that the earth is merely a planet
revolving around the sun; but it seemed to his profound intellect that a
movement of this kind on the part of our globe was the more likely
explanation of the celestial riddle. The idea was not new; for, as we
have already seen, certain of the ancient Greeks (Aristarchus of Samos,
for example) had held such a view; but their notions on the subject were
very fanciful, and unsupported by any good argument.
What Copernicus, however, really seems to have done was to _insist_ upon
the idea that the sun occupied the _centre_, as being more consonant
with common sense. No doubt, he was led to take up this position by the
fact that the sun appeared entirely of a different character from the
other members of the system. The one body in the scheme, which performed
the important function of dispenser of light and heat, would indeed be
more likely to occupy a position apart from the rest; and what position
more appropriate for its purposes than the centre!
But here Copernicus only partially solved the difficult question. He
unfortunately still clung to an ancient belief, which as yet remained
unquestioned; _i.e._ the great virtue, one might almost say, the
_divineness_, of circular motion. The ancients had been hag-ridden, so
to speak, by the circle; and it appeared to them that such a perfectly
formed curve was alone fitted for the celestial motions. Ptolemy
employed it throughout his system. According to him the "planets" (which
included, under the ancient view, both the sun and the moon), moved
around the earth in circles; but, as their changing positions in the sky
could not be altogether accounted for in this way, it was further
supposed that they performed additional circular movements, around
peculiarly placed centres, during the course of their orbital
revolutions. Thus the Ptolemaic system grew to be extremely
complicated; for astronomers did not hesitate to add new circular
movements whenever the celestial positions calculated for the planets
were found not to tally with the positions observed. In this manner,
indeed, they succeeded in doctoring the theory, so that it fairly
satisfied the observations made with the rough instruments of
pre-telescopic times.
Although Copernicus performed the immense service to astronomy of boldly
directing general attention to the central position of the sun, he
unfortunately took over for the new scheme the circular machinery of the
Ptolemaic system. It therefore remained for the famous Kepler, who lived
about a century after him, to find the complete solution. Just as
Copernicus, for instance, had broken free from tradition with regard to
the place of the sun; so did Kepler, in turn, break free from the spell
of circular motion, and thus set the coping-stone to the new
astronomical edifice. This astronomer showed, in fact, that if the paths
of the planets around the sun, and of the moon around the earth, were
not circles, but _ellipses_, the movements of these bodies about the sky
could be correctly accounted for. The extreme simplicity of such an
arrangement was far more acceptable than the bewildering intricacy of
movement required by the Ptolemaic theory. The Copernican system, as
amended by Kepler, therefore carried the day; and was further
strengthened, as we have already seen, by the telescopic observations of
Galileo and the researches of Newton into the effects of gravitation.
And here a word on the circle, now fallen from its high estate. The
ancients were in error in supposing that it stood entirely apart--the
curve of curves. As a matter of fact it is merely _a special kind of
ellipse_. To put it paradoxically, it is an ellipse which has no
ellipticity, an oval without any ovalness!
Notwithstanding all this, astronomy had to wait yet a long time for a
definite proof of the revolution of the earth around the sun. The
leading argument advanced by Aristotle, against the reality of any
movement of the earth, still held good up to about seventy years ago.
That philosopher had pointed out that the earth could not move about in
space to any great extent, or the stars would be found to alter their
apparent places in the sky, a thing which had never been observed to
happen. Centuries ran on, and instruments became more and more perfect,
yet no displacements of stars were noted. In accepting the Copernican
theory men were therefore obliged to suppose these objects as
immeasurably distant. At length, however, between the years 1835 and
1840, it was discovered by the Prussian astronomer, Bessel, that a star
known as 61 Cygni--that is to say, the star marked in celestial atlases
as No. 61 in the constellation of the Swan--appeared, during the course
of a year, to perform a tiny circle in the heavens, such as would result
from a movement on our own part around the sun. Since then about
forty-three stars have been found to show minute displacements of a
similar kind, which cannot be accounted for upon any other supposition
than that of a continuous revolution of the earth around the sun. The
triumph of the Copernican system is now at last supreme.
If the axis of the earth stood "straight up," so to speak, while the
earth revolved in its orbit, the sun would plainly keep always on a
level with the equator. This is equivalent to stating that, in such
circumstances, a person at the equator would see it rise each morning
exactly in the east, pass through the _zenith_, that is, the point
directly overhead of him, at midday, and set in the evening due in the
west. As this would go on unchangingly at the equator every day
throughout the year, it should be clear that, at any particular place
upon the earth, the sun would in these conditions always be seen to move
in an unvarying manner across the sky at a certain altitude depending
upon the latitude of the place. Thus the more north one went upon the
earth's surface, the more southerly in the sky would the sun's path lie;
while at the north pole itself, the sun would always run round and round
the horizon. Similarly, the more south one went from the equator the
more northerly would the path of the sun lie, while at the south pole it
would be seen to skirt the horizon in the same manner as at the north
pole. The result of such an arrangement would be, that each place upon
the earth would always have one unvarying climate; in which case there
would not exist any of those beneficial changes of season to which we
owe so much.
The changes of season, which we fortunately experience, are due,
however, to the fact that the sun does not appear to move across the sky
each day at one unvarying altitude, but is continually altering the
position of its path; so that at one period of the year it passes across
the sky low down, and remains above the horizon for a short time only,
while at another it moves high up across the heavens, and is above the
horizon for a much longer time. Actually, the sun seems little by little
to creep up the sky during one half of the year, namely, from mid-winter
to mid-summer, and then, just as gradually, to slip down it again during
the other half, namely, from mid-summer to mid-winter. It will therefore
be clear that every region of the earth is much more thoroughly warmed
during one portion of the year than during another, _i.e._ when the
sun's path is high in the heavens than when it is low down.
Once more we find appearances exactly the contrary from the truth. The
earth is in this case the real cause of the deception, just as it was in
the other cases. The sun does not actually creep slowly up the sky, and
then slowly dip down it again, but, owing to the earth's axis being set
aslant, different regions of the earth's surface are presented to the
sun at different times. Thus, in one portion of its orbit, the northerly
regions of the earth are presented to the sun, and in the other portion
the southerly. It follows of course from this, that when it is summer in
the northern hemisphere it is winter in the southern, and _vice versâ_
(see Fig. 13, p. 176).
[Illustration: FIG. 13.--Summer and Winter.]
The fact that, in consequence of this slant of the earth's axis, the sun
is for part of the year on the north side of the equator and part of the
year on the south side, leads to a very peculiar result. The path of the
moon around the earth is nearly on the same plane with the earth's path
around the sun. The moon, therefore, always keeps to the same regions of
the sky as the sun. The slant of the earth's axis thus regularly
displaces the position of both the sun and the moon to the north and
south sides of the equator respectively in the manner we have been
describing. Were the earth, however, a perfect sphere, such change of
position would not produce any effect. We have shown, however, that the
earth is not a perfect sphere, but that it is bulged out all round the
equator. The result is that this bulged-out portion swings slowly under
the pulls of solar and lunar gravitation, in response to the
displacements of the sun and moon to the north and to the south of it.
This slow swing of the equatorial regions results, of course, in a
certain slow change of the direction of the earth's axis, so that the
north pole does not go on pointing continually to the same region of the
sky. The change in the direction of the axis is, however, so extremely
slight, that it shows up only after the lapse of ages. The north pole of
the heavens, that is, the region of the sky towards which the north pole
of the earth's axis points, displaces therefore extremely slowly,
tracing out a wide circle, and arriving back again to the same position
in the sky only after a period of about 25,000 years. At present the
north pole of the heavens is quite close to a bright star in the tail of
the constellation of the Little Bear, which is consequently known as the
Pole Star; but in early Greek times it was at least ten times as far
away from this star as it is now. After some 12,000 years the pole will
point to the constellation of Lyra, and Vega, the most brilliant star in
that constellation, will then be considered as the pole star. This slow
twisting of the earth's axis is technically known as _Precession_, or
the _Precession of the Equinoxes_ (see Plate XIX., p. 292).
The slow displacement of the celestial pole appears to have attracted
the attention of men in very early times, but it was not until the
second century B.C. that precession was established as a fact by the
celebrated Greek astronomer, Hipparchus. For the ancients this strange
cyclical movement had a mystic significance; and they looked towards the
end of the period as the end, so to speak, of a "dispensation," after
which the life of the universe would begin anew:--
"Magnus ab integro sæclorum nascitur ordo.
Jam redit et Virgo, redeunt Saturnia regna;
. . . . . .
Alter erit tum Tiphys, et altera quæ vehat Argo
Delectos heroas; erunt etiam altera bella,
Atque iterum ad Trojam magnus mittetur Achilles."
We have seen that the orbit of the earth is an ellipse, and that the sun
is situated at what is called the _focus_, a point not in the middle of
the ellipse, but rather towards one of its ends. Therefore, during the
course of the year the distance of the earth from the sun varies. The
sun, in consequence of this, is about 3,000,000 miles _nearer_ to us in
our northern _winter_ than it is in our northern summer, a statement
which sounds somewhat paradoxical. This variation in distance, large as
it appears in figures, can, however, not be productive of much
alteration in the amount of solar heat which we receive, for during the
first week in January, when the distance is least, the sun only looks
about _one-eighteenth_ broader than at the commencement of July, when
the distance is greatest. The great disparity in temperature between
winter and summer depends, as we have seen, upon causes of quite another
kind, and varies between such wide limits that the effects of this
slight alteration in the distance of the sun from the earth may be
neglected for practical purposes.
The Tides are caused by the gravitational pull of the sun and moon upon
the water of the earth's surface. Of the two, the moon, being so much
the nearer, exerts the stronger pull, and therefore may be regarded as
the chief cause of the tides. This pull always draws that portion of the
water, which happens to be right underneath the moon at the time, into a
heap; and there is also a _second_ heaping of water at the same moment
_at the contrary side of the earth_, the reasons for which can be shown
mathematically, but cannot be conveniently dealt with here.
As the earth rotates on its axis each portion of its surface passes
beneath the moon, and is swelled up by this pull; the watery portions
being, however, the only ones to yield visibly. A similar swelling up,
as we have seen, takes place at the point exactly away from the moon.
Thus each portion of our globe is borne by the rotation through two
"tide-areas" every day, and this is the reason why there are two tides
during every twenty-four hours.
The crest of the watery swelling is known as high tide. The journey of
the moon around the earth takes about a month, and this brings her past
each place in turn by about fifty minutes later each day, which is the
reason why high tide is usually about twenty-five minutes later each
time.
The moon is, however, not the sole cause of the tides, but the sun, as
we have said, has a part in the matter also. When it is new moon the
gravitational attractions of both sun and moon are clearly acting
together from precisely the same direction, and, therefore, the tide
will be pulled up higher than at other times. At full moon, too, the
same thing happens; for, although the bodies are now acting from
opposite directions, they do not neutralise each other's pulls as one
might imagine, since the sun, in the same manner as the moon, produces a
tide both under it and also at the opposite side of the earth. Thus both
these tides are actually increased in height. The exceptionally high
tides which we experience at new and full moons are known as _Spring
Tides_, in contradistinction to the minimum high tides, which are known
as _Neap Tides_.
The ancients appear to have had some idea of the cause of the tides. It
is said that as early as 1000 B.C. the Chinese noticed that the moon
exerted an influence upon the waters of the sea. The Greeks and Romans,
too, had noticed the same thing; and Cæsar tells us that when he was
embarking his troops for Britain the tide was high _because_ the moon
was full. Pliny went even further than this, in recognising a similar
connection between the waters and the sun.
From casual observation one is inclined to suppose that the high tide
always rises many feet. But that this is not the case is evidenced by
the fact that the tides in the midst of the great oceans are only from
three to four feet high. However, in the seas and straits around our
Isles, for instance, the tides rise very many feet indeed, but this is
merely owing to the extra heaping up which the large volumes of water
undergo in forcing their passage through narrow channels.
As the earth, in rotating, is continually passing through these
tide-areas, one might expect that the friction thus set up would tend to
slow down the rotation itself. Such a slowing down, or "tidal drag," as
it is called, is indeed continually going on; but the effects produced
are so exceedingly minute that it will take many millions of years to
make the rotation appreciably slower, and so to lengthen the day.
Recently it has been proved that the axis of the earth is subject to a
very small displacement, or rather, "wobbling," in the course of a
period of somewhat over a year. As a consequence of this, the pole
shifts its place through a circle of, roughly, a few yards in width
during the time in question. This movement is, perhaps, the combined
result of two causes. One of these is the change of place during the
year of large masses of material upon our earth; such as occurs, for
instance, when ice and snow melt, or when atmospheric and ocean
currents transport from place to place great bodies of air and water.
The other cause is supposed to be the fact that the earth is not
absolutely rigid, and so yields to certain strains upon it. In the
course of investigation of this latter point the interesting conclusion
has been reached by the famous American astronomer, Professor Simon
Newcomb, that our globe as a whole is _a little more rigid than steel_.
We will bring this chapter to a close by alluding briefly to two strange
appearances which are sometimes seen in our night skies. These are known
respectively as the Zodiacal Light and the Gegenschein.
The _Zodiacal Light_ is a faint cone-shaped illumination which is seen
to extend upwards from the western horizon after evening twilight has
ended, and from the eastern horizon before morning twilight has begun.
It appears to rise into the sky from about the position where the sun
would be at that time. The proper season of the year for observing it
during the evening is in the spring, while in autumn it is best seen in
the early morning. In our latitudes its light is not strong enough to
render it visible when the moon is full, but in the tropics it is
reported to be very bright, and easily seen in full moonlight. One
theory regards it as the reflection of light from swarms of meteors
revolving round the sun; another supposes it to be a very rarefied
extension of the corona.
The _Gegenschein_ (German for "counter-glow") is a faint oval patch of
light, seen in the sky exactly opposite to the place of the sun. It is
usually treated of in connection with the zodiacal light, and one theory
regards it similarly as of meteoric origin. Another theory,
however--that of Mr. Evershed--considers it a sort of _tail_ to the
earth (like a comet's tail) composed of hydrogen and helium--the two
_lightest_ gases we know--driven off from our planet in the direction
contrary to the sun.
[13] Every one knows the simple experiment in which a coin lying at the
bottom of an empty basin, and hidden from the eye by its side, becomes
visible when a certain quantity of water has been poured in. This is an
example of refraction. The rays of light coming from the coin ought
_not_ to reach the eye, on account of the basin's side being in the way;
yet by the action of the water they are _refracted_, or bent over its
edge, in such a manner that they do.