THE EARTH.
FORM OF THE EARTH—MOTION OF THE GLOBE—RATE AND MANNER OF
PROGRESSION—LATITUDE AND LONGITUDE—THE SEASONS.
We have learnt from our books on Geography that the earth is shaped
like an orange,—that is, our globe is round and flattened slightly at
the “poles,” and we can easily see that the earth curves away, if we
only try the experiment mentioned in a foregoing chapter—viz., how far
a person standing (or lying) on the ground can see on a level. Our
power of eyesight is not limited to three or four miles, but a man of
ordinary height standing on the plain cannot see more than three miles,
_because the earth is curving away from him_.
We know that at the seaside we can see ships gradually appear and
disappear. When approaching us the masts and top-sails appear first,
then the main-sails, and then the ship itself. A sailor climbing up the
mast can see farther than the captain on deck, because he can see over
the curve, as it were. When the vessel is at a considerable distance
we see her “hull down” as it is termed,—that is, only her sails are
visible to us, and at last they disappear also. If we want any other
proof that the earth is round we can see when an eclipse takes place
that the shadow on the moon is circular. So we may be certain of one
fact; the earth is round, it is a globe. So much for the rotundity of
the earth.
But the earth appears to us, except in very mountainous districts, as
being almost a plane. This is because of its extent; and even from very
high mountains we can only see a very small portion of the earth, and
so, on a globe sixteen inches in diameter, the highest hills would be
only about 1/100 of an inch, like a grain of sand.
The motion of the earth is known to most people, though as everything
upon the globe passes with it, and a relative fixity is apparent, this
is, of course, not real rest. The earth is moving from west to east
at a tremendous rate,—viz., nearly nineteen miles a second! We think
a train at sixty miles an hour a fast train; but what should we think
of an express going more than 68,000 miles an hour! Yet this is about
the rate at which our globe whirls around the sun. Her fastest pace is
really 18·5 miles a second; the least about one mile per second less.
That is one motion of the earth; the other is its motion on its axis.
If we send a skittle ball rolling we perceive it turns round as it
proceeds. So the earth rotates on its axis, N S, in the accompanying
diagram; the extremities of the axis are called the _poles_. The line
in the middle is the equator, which is divided into 360 equal parts,
each being 69-1/10 miles in length; so there are 180 lines, or rings,
drawn upon the globe from N to S, and these are _meridians_. In England
the degrees are calculated from the Greenwich meridian. We can thus
obtain the distance of localities east or west, as we may briefly show
(fig. 554).
[Illustration: Fig. 553.—Evidence of the spherical form of the earth.]
[Illustration: Fig. 554.—Latitude and Longitude.]
The distance of any meridian from the first meridian is termed the
_longitude_, and it is employed in describing the situation of a place
on the earth’s surface. Suppose L (fig. 554) a city, its longitude will
be 30°, since it lies on a meridian which is 30° from the first. So,
for example, the longitude of Oporto is 8° 37´ west, Paris 2° 22´ east,
Vienna 16° 16´ east, Bagdad 44° 45´ east, reckoned from the meridian of
Greenwich, and so on. At the 180th degree we have proceeded half round
the globe, and reached the farthest distance from the first meridian,
and are now on the opposite side of the earth, and proceeding in a
similar manner in the opposite direction we get west longitude.
It will readily be perceived that a knowledge of the longitude alone
is not sufficient to determine the situation of a place on the earth’s
surface. When we say, for example, that the longitude of a place is
30°, it may lie on any point whatever of the line, N L S, on the whole
hemisphere (fig. 554). This point must therefore be determined more
accurately, and hence the first meridian is divided into 90 equal parts
north and south of the equator towards the poles. These are called
_degrees of latitude_, and the lines drawn through these round the
globe, parallel to the equator, are called circles or _parallels of
latitude_, and diminish as they approach the poles.
Hence, by the _latitude_ of a place we mean its distance from the
equator towards the poles, and we speak of north and south latitude
according as the place is situated in the northern or southern
hemisphere.
So, for example, the point L (fig. 554), which has 30° longitude and
60° N. latitude is in Sweden.
The latitude is also observable by ascertaining the altitude of the
polar star above the horizon when in the northern hemisphere. The
longitude is found by the chronometer; for if we know the time at
Greenwich we can calculate how far we are east or west of it by seeing
whether the local time be an hour (say) earlier or later, and that
difference shows we are 15° to the east or the west as the case may be.
The earth’s rotation, according to sidereal time, is less than solar
time, as we have seen, so we have 365 solar days and 366 sidereal days;
so a person going round the world gains or loses a day as he travels
east or west according to his reckoning, as compared with the reckoning
of his friends at home. We can easily ascertain the earth’s motion by
watching the stars rise and set. Now the path in which the earth moves
is called an _ellipse_,—very nearly a circle,—but it does not always
move at the same rate exactly. We will now look at the relations of the
sun and the earth.
Let us take an example. Suppose we have a rod, at each end of which we
fix a ball (_see_ diagram), and let one ball be three times as large as
the other, the common centre of gravity will be at _c_, at one quarter
of the distance between the centres, and there the bodies will be in
equilibrium. If these masses be set spinning into space they will
revolve at that distance from each other, the attraction of gravitation
and the force in opposition to it equalizing each other.
[Illustration: Fig. 555.—Earth and Sun.]
The earth, as we know, proceeds with a tremendous force around the
sun, not in a circle, remember, but in an ellipse or oval track, from
which it never moves year by year in any appreciable degree. Now what
prevents this earth of ours from rushing off by itself into space? Why
should not the earth fly away in a direct line? The reason is because
the sun holds it back. The force of the sun’s gravitation is just
sufficient, or we may say so enormously great, that it suffices to
retain our globe and all the other planets in their various orbits at
the very same distance, and to counteract the force which launches them
through space. Therefore, as we have already noticed when considering
the sun, it is to that ruler of the day that we are indebted for
everything.
What would happen, then, if the earth were suddenly to increase her
velocity, or the sun to contract his mass?—We should be flung into
infinite space, and in a short time would be frozen up completely.
Our present diurnal course would probably proceed, but all life and
existence would cease as we whirled with distant planets through
infinity.
Suppose, on the contrary, we were to stop suddenly. We have some of
us read in a foregoing part of this volume that heat is the motion
of molecules in ether, and that when a body strikes another heat is
developed by contact and friction. If the earth were to be stopped
suddenly, “an amount of heat would be developed sufficient to raise the
temperature of a globe of lead of the same size as the earth 384,000°
of the Centigrade thermometer. The greater part, if not the whole of
our planet, would be reduced to vapour”, as Professor Tyndall says.
[Illustration: Fig. 556.—Transit of earth across Sun, seen from Mars.]
In the diagram (fig. 546, on page 497) we shall at once find the
explanation of the constantly-recurring seasons, and the amount of our
globe which is illuminated by the sun at various times. It will be
easily understood that the poles have six months day and six months
night. When the earth is at an equinox, one half of the surface is
illuminated and the other half in shade, therefore the days and nights
are equal. But when the north pole turns more and more towards the sun,
the south pole is turning away from it in the same ratio,—the days and
nights respectively are getting longer and longer, and at the north
and south poles day and night are continuous, for the small spaces
round the poles are, during a certain period, wholly in sunshine and
shade respectively.
In March (in the diagram, fig. 546) we see that exactly one half of
the earth is illuminated, and the other is darkened. So in September,
when we have the opposite view. In June the earth is more inclined
apparently to the sun, and more of the surface is exposed to it, so
the days are longer in some parts. The opposite effect is visible in
December.
The summer heat and winter cold are accounted for by the more or less
direct force of the sun’s rays, for the more the angle of incidence
is inclined the fewer rays reach the object; and if the rays fall at
an angle of 60°, the heat is only half what it would be if they came
vertically. When the days are shortest the sun is lowest, and therefore
gives less heat to the earth at certain periods.
The wonderful precision which has adapted the position of the earth on
its axis, will be apparent from the illustration (fig. 557). Here we
have a table and some bottles, a candle to represent the sun, and a
ball of worsted and a knitting-needle to represent the earth and its
axis. Suppose we place the ball in the position at _a_, with its axis
perpendicular to the plane of the orbit. As the earth would turn and
go round the sun in this supposed case, we should find the days and
nights equal, and the sun would quickly scorch up the tropics, and
the other portions would have a never-changing spring or winter all
the year for ever. This would not be so pleasant, for variety is the
charm of nature, and the salt of life. So we may put _a_ aside, as
the earth would be scarcely habitable under the supposed conditions,
and try _b_. Here we find the poles directed to the sun. The whole
northern hemisphere would thus be illuminated one half year, and the
southern similarly; such rapid changes from heat to cold and back again
would not suit us. So we fall back upon _c_, the actual appearance of
the position of the earth, and here we find all the most favourable
circumstances existing for us. This inclination gives rise to all the
varied phenomena of the pleasant gradations of heat and cold, summer
and winter, the charming changes of season, and the wonderful results
of the ever-recurring days and nights, months and years, as the earth
spins round. So we see that the sun does not really rise and set upon
the earth; the globe rotates, and brings us into view of the sun, and
as we turn we lose his light.
[Illustration: Fig. 557.—Inclination of Axis.]
In the foregoing brief description we have learnt some few facts
concerning our earth. We have ascertained that the planet we inhabit
is round; we have also seen that the earth moves around the sun and
around its own axis, and also that it moves at a tremendous rate; we
know that that rate is just counterbalanced by the attraction of
gravitation, and the course round the sun gives us varying seasons, day
and night. There are many subjects relating to the earth which will be
more properly included under Physical Geography. We may here just add
the diameter of the earth, and proceed to inquire concerning the moon.
The polar diameter of our globe is 7,899 miles; the equatorial diameter
7,925 miles. It is distant from the moon 238,500 miles. We will close
this chapter with the letters and characters of the Greek alphabet used
in astronomical works to designate the stars.
Α α = Alpha
Β β = Beta
Γ γ = Gamma
Δ δ = Delta
Ε ε = Epsilon
Ζ ζ = Zeta
Ê ê = Eta
Θ θ = Theta
Ι ι = Iota
Κ κ = Kappa
Λ λ = Lambda
Μ μ = Mu
Ν ν = Nu
Ξ ξ = Xi
Ο ο = Omicron
Π π = Pi
Ρ ρ = Rho
Σ ς = Sigma
Τ τ = Tau
Υ υ = Upsilon
Φ φ = Phi
Χ χ = Chi
Ψ ψ = Psi
Ω ω = Omega
[Illustration: Fig. 558.—A ship disappearing below the horizon.]