18 PHYSICAL GEOGRAPHY.

 

25, Laplace’s Nebular Hyp othesis.—The uniformity
in the direction of rotation and revolution of the planets
has led to a very plausible supposition as to the origin of
the solar system, by the celebrated French astronomer La-
place. This supposition, known as Laplace’s nebular hy-
pothesis, assumes that, originally, all the materials of which
the solar system is composed were scattered throughout
space in the form of very tenuous or nebulous matter. It
being granted that this matter began to accumulate around
a centre, and that a motion of rotation was thereby’ ac-
quired, it can be shown, on strict mechanical principles,
that asystem resembling the solar system might be evolved.

As the mass contracted on cooling, the rapidity of its
rotation increased. The equatorial portions bulged out
through the centrifugal force, until ring-like portions
separated, and, collecting in spherical masses, formed the
planets. The planets in a similar manner detached their
satellites. At the time of the separation of Neptune the
nebulous sun must have extended beyond the orbit of this
planet. The temperature requisite for so great an expan-
sion must have been enormous.

Although a mere hypothesis, there are many facts which
tend to sustain it, and it is now generally accepted.

26. The Plane of the Earth’s Orbit is a per-
fectly flat surface so placed as to touch the earth’s
orbit at every point. It may be regarded as an
imaginary plane of enormous extent on which the
earth moves in its journey around the sun.

~~ 27. Causes of the Change of Seasons.—The
change of the earth’s seasons is caused by the
revolution of the earth, together with -the fol-
lowing circumstances:

 

Fig, 14, Inclination of Axis-to Orbit and Ecliptic.

(1.) The inclination of the earth’s axis to the
plane of its orbit. The inclination is equal to
66° 33’.

The ecliptic is the name given to a great circle whose
plane coincides with the plane of the earth’s orbit. Since
the earth’s axis is 90° distant from the equator, the piane
of the ecliptic must be inclined to the plane of the equator
90° minus 66° 33’, or 23° 27’.

The mere revolution of the earth would be unable to
produce a change of seasons, unless the earth’s axis were
inclined to the plane of its orbit. If, for example, the
axis of the earth stood perpendicularly on the plane of its
orbit, the sun’s rays would so illumine the earth that the
great circle of illumination would always be bounded by
some meridian circle. The days and nights would then
be of equal length, and the distribution of heat the same
throughout the year. Under these circumstances there
could be no change of seasons, since the sun’s rays would

 

always fall perpendicularly onthe same part of the earth:
on the equator.

(2.) The Constant Parallelism of the Earth’s
Axis.—During the earth’s revolution its axis
always points nearly to the same place in the
heavens, viz. to the north star. It is therefore
always approximately parallel to any former
position.

Unless the axis were constantly parallel to any former
position, the present change of seasons would not occur.

On account of the spherical form of the earth,
only a small part of its surface can receive the
vertical rays of the sun at the same time. This
part can be regarded as nearly a point; and since
only one-half of the earth is lighted at any one
time, the great circle of illumination must extend
90° in all directions from the point which receives
the vertical rays. By rotation all portions of
the surface situated anywhere within the tropics
in the same latitude, at some time or another
during the day, are turned: so as to receive the
vertical rays of the sun, and consequently, the
portion so illumined has the form of a ring or
zone. Other things being equal, this zone con-
tains the hottest portions of the surface, the heat
gradually diminishing as we pass toward either
pole.

On account of the inclination of its axis, the
earth receives the vertical rays of the sun on new
portions of its surface every day during its revo-
lution; and it is because different portions of the

‘surface are constantly being turned toward the sun

that the change of seasons is to be attributed.

As the earth changes its position in its orbit, the
sun’s rays fall vertically on different parts of the
surface, so that during the year one part or an-
other of the surface within 23° 27’ on either side
of the equator receives the vertical rays.

The astronomical year begins on the 20th
of March, and we shall therefore first consider
the position of the earth in its orbit at that
time.

An inspection of Fig. 15 will show that at this
time the earth is so turned toward the sun that
the vertical rays fall exactly on the equator. The
great circle of illumination, therefore, reaches to
the poles, and the days and nights are of an equal
length all over the earth. This time is called the
March equinox. Spring then begins in the North-
ern Hemisvhere, and autumn in the Southern.
This is shown more clearly in Fig. 16, which
represents the relative positions of the illumined
and non-illumined portions at that time.

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