The terrestrial globe is a member of the Solar system, the third in
distance from the Sun, and the largest within the orbit of Jupiter. In
the wider sense it may be regarded as composed of a gaseous atmosphere
(see METEOROLOGY), which encircles the crust or lithosphere (see
GEOGRAPHY), and surface waters or hydrosphere (see OCEAN AND
OCEANOGRAPHY). The description of the surface features is a branch of
Geography, and the discussions as to their origin and permanence belongs
to Physiography (in the narrower sense), physiographical geology, or
physical geography. The investigation of the crust belongs to geology
and of rocks in particular to petrology.
In the present article we shall treat the subject matter of the Earth as
a planet under the following headings:--(1) Figure and Size, (2) Mass
and Density, (3) Astronomical Relations, (4) Evolution and Age. These
subjects will be treated summarily, readers being referred to the
article ASTRONOMY and to the cross-references for details.
1. _Figure and Size._--To primitive man the Earth was a flat disk with
its surface diversified by mountains, rivers and seas. In many
cosmogonies this disk was encircled by waters, unmeasurable by man and
extending to a junction with the sky; and the disk stood as an island
rising up through the waters from the floor of the universe, or was
borne as an immovable ship on the surface. Of such a nature was the
cosmogony of the Babylonians and Hebrews; Homer states the same idea,
naming the encircling waters [Greek: Okeanos]; and Hesiod regarded it as
a disk midway between the sky and the infernal regions. The theory that
the Earth extended downwards to the limit of the universe was subjected
to modification when it was seen that the same sun and stars reappeared
in the east after their setting in the west. But man slowly realized
that the earth was isolated in space, floating freely as a balloon, and
much speculation was associated about that which supported the Earth.
Tunnels in the foundations to permit the passage of the sun and stars
were suggested; the Greeks considered twelve columns to support the
heavens, and in their mythology the god Atlas appears condemned to
support the columns; while the Egyptians had the Earth supported by four
elephants, which themselves stood on a tortoise swimming on a sea.
Earthquakes were regarded as due to a movement of these foundations; in
Japan this was considered to be due to the motion of a great spider, an
animal subsequently replaced by a cat-fish; in Mongolia it is a hog; in
India, a mole; in some parts of South America, a whale; and among some
of the North American Indians, a giant tortoise.
The doctrine of the spherical form has been erroneously assigned to
Thales; but he accepted the Semitic conception of the disk, and regarded
the production of springs after earthquakes as due to the inrushing of
the waters under the Earth into fissures in the surface. His pupil,
Anaximander (610-547), according to Diogenes Laertius, believed it to be
spherical (see _The Observatory_, 1894, P. 208); and Anaximenes probably
held a similar view. The spherical form is undoubtedly a discovery of
Pythagoras, and was taught by the Pythagoreans and by the Eleatic
Parmenides. The expositor of greatest moment was Aristotle; his
arguments are those which we employ to-day:--the ship gradually
disappearing from hull to mast as it recedes from the harbour to the
horizon; the circular shadow cast by the Earth on the Moon during an
eclipse, and the alteration in the appearance of the heavens as one
passes from point to point on the Earth's surface.[1] He records
attempts made to determine the circumference; but the first scientific
investigation in this direction was made 150 years later by
Eratosthenes. The spherical form, however, only became generally
accepted after the Earth's circumnavigation (see GEOGRAPHY).
The historical development of the methods for determining the figure of
the Earth (by which we mean a theoretical surface in part indicated by
the ocean at rest, and in other parts by the level to which water freely
communicating with the oceans by canals traversing the land masses would
rise) and the mathematical investigation of this problem are treated in
the articles EARTH, FIGURE OF THE, and GEODESY; here the results are
summarized. Sir Isaac Newton deduced from the mechanical consideration
of the figure of equilibrium of a mass of rotating fluid, the form of an
oblate spheroid, the ellipticity of a meridian section being {1/231},
and the axes in the ratio 230 : 231. Geodetic measurements by the
Cassinis and other French astronomers pointed to a prolate form, but the
Newtonian figure was proved to be correct by the measurement of
meridional arcs in Peru and Lapland by the expeditions organized by the
French Academy of Sciences. More recent work points to an elliptical
equatorial section, thus making the earth pear-shaped. The position of
the longer axis is somewhat uncertain; it is certainly in Africa, Clarke
placing it in longitude 8 deg. 15' W., and Schubert in longitude 41 deg. 4' E.;
W.J. Sollas, arguing from terrestrial symmetry, has chosen the position
lat. 6 deg. N., long. 28 deg. E., i.e. between Clarke's and Schubert's
positions. For the lengths of the axes and the ellipticity of the Earth,
see EARTH, FIGURE OF THE.
2. _Mass and Density._--The earliest scientific investigation on the
density and mass of the Earth (the problem is really single if the
volume of the Earth be known) was made by Newton, who, mainly from
astronomical considerations, suggested the limiting densities 5 and 6;
it is remarkable that this prophetic guess should be realized, the mean
value from subsequent researches being about 51/2, which gives for the
mass the value 6 x 10^21 tons. The density of the Earth has been
determined by several experimenters within recent years by methods
described in the article GRAVITATION; the most probable value is there
stated to be 5.527.
3. _Astronomical Relations._--The grandest achievements of astronomical
science are undoubtedly to be associated with the elucidation of the
complex motion of our planet. The notion that the Earth was fixed and
immovable at the centre of an immeasurable universe long possessed the
minds of men; and we find the illustrious Ptolemy accepting this view in
the 2nd century A.D., and rejecting the notion of a rotating Earth--a
theory which had been proposed as early as the 5th century B.C. by
Philolaus on philosophical grounds, and in the 3rd century B.C. by the
astronomer Aristarchus of Samos. He argued that if the Earth rotated
then points at the equator had the enormous velocity of about 1000 m.
per hour, and as a consequence there should be terrific gales from the
east; the fact that there were no such gales invalidated, in his
opinion, the theory. The Ptolemaic theory was unchallenged until 1543,
in which year the _De Revolutionibus orbium Celestium_ of Copernicus was
published. In this work it was shown that the common astronomical
phenomena could be more simply explained by regarding the Earth as
annually revolving about a fixed Sun, and daily rotating about itself. A
clean sweep was made of the geocentric epicyclic motions of the planets
which Ptolemy's theory demanded, and in place there was substituted a
procession of planets about the Sun at different distances. The
development of the Copernican theory--the corner-stone of modern
astronomy--by Johann Kepler and Sir Isaac Newton is treated in the
article ASTRONOMY: _History_; here we shall summarily discuss the
motions of our planet and its relation to the solar system.
The Earth has two principal motions--revolution about the Sun, rotation
about its axis; there are in addition a number of secular motions.
_Revolution._--The Earth revolves about the Sun in an elliptical orbit
having the Sun at one focus. The plane of the orbit is termed the
ecliptic; it is inclined to the Earth's equator at an angle termed the
obliquity, and the points of intersection of the equator and ecliptic
are termed the equinoctial points. The major axis of the ellipse is the
line of apsides; when the Earth is nearest the Sun it is said to be in
perihelion, when farthest it is in aphelion. The mean distance of the
Earth from the Sun is a most important astronomical constant, since it
is the unit of linear measurement; its value is about 93,000,000 m., and
the difference between the perihelion and aphelion distances is about
3,000,000 m. The eccentricity of the orbit is 0.016751. A tabular
comparison of the orbital constants of the Earth and the other planets
is given in the article PLANET. The period of revolution with regard to
the Sun, or, in other words, the time taken by the Sun apparently to
pass from one equinox to the same equinox, is the tropical or
equinoctial year; its length is 365 d. 5 hrs. 48 m. 46 secs. It is about
20 minutes shorter than the true or sidereal year, which is the time
taken for the Sun apparently to travel from one star to it again. The
difference in these two years is due to the secular variation termed
precession (see below). A third year is named the _anomalistic year_,
which is the time occupied in the passage from perihelion to perihelion;
it is a little longer than the sidereal.
_Rotation._--The Earth rotates about an axis terminating at the north
and south geographical poles, and perpendicular to the equator; the
period of rotation is termed the day (q.v.), of which several kinds are
distinguished according to the body or point of reference. The rotation
is performed from west to east; this daily rotation occasions the
_diurnal_ motion of the celestial sphere, the rising of the Sun and
stars in the east and their setting in the west, and also the phenomena
of day and night. The inclination of the axis to the ecliptic brings
about the presentation of places in different latitudes to the more
direct rays of the sun; this is revealed in the variation in the length
of daylight with the time of the year, and the phenomena of seasons.
Although the rotation of the Earth was an accepted fact soon after its
suggestion by Copernicus, an experimental proof was wanting until 1851,
when Foucault performed his celebrated pendulum experiment at the
Pantheon, Paris. A pendulum about 200 ft. long, composed of a flexible
wire carrying a heavy iron bob, was suspended so as to be free to
oscillate in any direction. The bob was provided with a style which
passed over a table strewn with fine sand, so that the style traced the
direction in which the bob was swinging. It was found that the
oscillating pendulum never retraced its path, but at each swing it was
apparently deviated to the right, and moreover the deviations in equal
times were themselves equal. This means that the floor of the Pantheon
was moving, and therefore the Earth was rotating. If the pendulum were
swung in the southern hemisphere, the deviation would be to the left; if
at the equator it would not deviate, while at the poles the plane of
oscillation would traverse a complete circle in 24 hours.
The rotation of the Earth appears to be perfectly uniform, comparisons
of the times of transits, eclipses, &c., point to a variation of less
than {1/100}th of a second since the time of Ptolemy. Theoretical
investigations on the phenomena of tidal friction point, however, to a
retardation, which may to some extent be diminished by the accelerations
occasioned by the shrinkage of the globe, and some other factors
difficult to evaluate (see TIDE).
We now proceed to the secular variations.
_Precession._--The axis of the earth does not preserve an invariable
direction in space, but in a certain time it describes a cone, in much
the same manner as the axis of a top spinning out of the vertical. The
equator, which preserves approximately the same inclination to the
ecliptic (there is a slight variation in the obliquity which we shall
mention later), must move so that its intersections with the ecliptic,
or equinoctial points, pass in a retrograde direction, i.e. opposite to
that of the Earth. This motion is termed the precession of the
equinoxes, and was observed by Hipparchus in the 2nd century B.C.;
Ptolemy corrected the catalogue of Hipparchus for precession by adding
2 deg. 40' to the longitudes, the latitudes being unaltered by this motion,
which at the present time is 50.26" annually, the complete circuit being
made in about 26,000 years. Owing to precession the signs of the zodiac
are traversing paths through the constellations, or, in other
words, the constellations are continually shifting with regard to the
equinoctial points; at one time the vernal equinox Aries was in the
constellations of that name; it is now in Pisces, and will then pass
into Aquarius. The pole star, i.e. the star towards which the Earth's
axis points, is also shifting owing to precession; in about 2700 B.C.
the Chinese observed [alpha] Draconis as the pole star (at present
[alpha] Ursae minoris occupies this position and will do so until 3500);
in 13600 Vega ([alpha] Lyrae) the brightest star in the Northern
hemisphere, will be nearest.
Precession is the result of the Sun and the Moon's attraction on the
Earth not being a single force through its centre of gravity. If the
Earth were a homogeneous sphere the attractions would act through the
centre, and such forces would have no effect upon the rotation about the
centre of gravity, but the Earth being spheroidal the equatorial band
which stands up as it were beyond the surface of a sphere is more
strongly attracted, with the result that the axis undergoes a tilting.
The precession due to the Sun is termed the _solar precession_ and that
due to the Moon the _lunar precession_; the joint effect (two-thirds of
which is due to the Moon) is the _luni-solar_ precession. Solar
precession is greatest at the solstices and zero at the equinoxes; the
part of luni-solar precession due to the Moon varies with the position
of the Moon in its orbit. The obliquity is unchanged by precession (see
PRECESSION OF THE EQUINOXES).
_Nutation._--In treating precession we have stated that the axis of the
Earth traces a cone, and it follows that the pole describes a circle
(approximately) on the celestial sphere, about the pole of the ecliptic.
This is not quite true. Irregularities in the attracting forces which
occasion precession also cause a slight oscillation backwards and
forwards over the mean precessional path of the pole, the pole tracing a
wavy line or nodding. Both the Sun and Moon contribute to this effect.
Solar nutation depends upon the position of the Sun on the ecliptic; its
period is therefore 1 year, and in extent it is only 1.2"; lunar
nutation depends upon the position of the Moon's nodes; its period is
therefore about 18.6 years, the time of revolution of the nodes, and its
extent is 9.2". There is also given to the obliquity a small oscillation
to and fro. Nutation is one of the great discoveries of James Bradley
(1747).
_Planetary Precession._--So far we have regarded the ecliptic as
absolutely fixed, and treated precession as a real motion of the
equator. The ecliptic (q.v.), however, is itself subject to a motion,
due to the attractions of the planets on the Earth. This effect also
displaces the equinoctial points. Its annual value is 0.13". The term
General Precession in longitude is given to the displacement of the
intersection of the equator with the apparent ecliptic on the latter.
The standard value is 50.2453", which prevailed in 1850, and the value
at 1850 + t, i.e. the constant of precession, is 50.2453" + 0.0002225"
t. This value is also liable to a very small change. The nutation of the
obliquity at time 1850 + t is given by the formula 23 deg. 27' 32.0" - 0.47"
t. Complete expressions for these functions are given in Newcomb's
_Spherical Astronomy_ (1908), and in the _Nautical Almanac_.
The variation of the _line of apsides_ is the name given to the motion
of the major axis of the Earth's orbit along the ecliptic. It is due to
the general influence of the planets, and the revolution is effected in
21,000 years.
The variation of the eccentricity denotes an oscillation of the form of
the Earth's orbit between a circle and ellipse. This followed the
mathematical researches of Lagrange and Leverrier. It was suggested by
Sir John Herschel in 1830 that this variation might occasion great
climatic changes, and James Croll developed the theory as affording a
solution of the glacial periods in geology (q.v.).
_Variation of Latitude._--Another secular motion of the Earth is due to
the fact that the axis of rotation is not rigidly fixed within it, but
its polar extremities wander in a circle of about 50 ft. diameter. This
oscillation brings about a variability in terrestrial latitudes, hence
the name. Euler showed mathematically that such an oscillation existed,
and, making certain assumptions as to the rigidity of the Earth, deduced
that its period was 305 days; S.C. Chandler, from 1890 onwards, deduced
from observations of the stars a period of 428 days; and Simon Newcomb
explained the deviation of these periods by pointing out that Euler's
assumption of a perfectly rigid Earth is not in accordance with fact.
For details of this intricate subject see the articles LATITUDE and
EARTH, FIGURE OF THE.
4. _Evolution and Age._--In its earliest history the mass now
consolidated as the Earth and Moon was part of a vast nebulous
aggregate, which in the course of time formed a central nucleus--our
Sun--which shed its outer layers in such a manner as to form the solar
system (see NEBULAR THEORY). The moon may have been formed from the
Earth in a similar manner, but the theory of tidal friction suggests the
elongation of the Earth along an equatorial axis to form a pear-shaped
figure, and that in the course of time the protuberance shot off to form
the Moon (see TIDE). The age of the Earth has been investigated from
several directions, as have also associated questions related to
climatic changes, internal temperature, orientation of the land and
water (permanence of oceans and continents), &c. These problems are
treated in the articles GEOLOGY and GEOGRAPHY.