[1] _Latine loqui elegantissime_.
LATINUS, in Roman legend, king of the aborigines in Latium, and
eponymous hero of the Latin race. In Hesiod (_Theogony_, 1013) he is the
son of Odysseus and Circe, and ruler of the Tyrsenians; in Virgil, the
son of Faunus and the nymph Marica, a national genealogy being
substituted for the Hesiodic, which probably originated from a Greek
source. Latinus was a shadowy personality, invented to explain the
origin of Rome and its relations with Latium, and only obtained
importance in later times through his legendary connexion with Aeneas
and the foundation of Rome. According to Virgil (_Aeneid_, vii.-xii.),
Aeneas, on landing at the mouth of the Tiber, was welcomed by Latinus,
the peaceful ruler whose seat of government was Laurentum, and
ultimately married his daughter Lavinia.
Other accounts of Latinus, differing considerably in detail, are to be
found in the fragments of Cato's _Origines_ (in Servius's commentary
on Virgil) and in Dionysius of Halicarnassus; see further authorities
in the article by J. A. Hild, in Daremberg and Saglio, _Dictionnaire
des antiquités_.
LATITUDE (Lat. _latitudo_, _latus_, broad), a word meaning breadth or
width, hence, figuratively, freedom from restriction, but more generally
used in the geographical and astronomical sense here treated. The
latitude of a point on the earth's surface is its angular distance from
the equator, measured on the curved surface of the earth. The direct
measure of this distance being impracticable, it has to be determined by
astronomical observations. As thus determined it is the angle between
the direction of the plumb-line at the place and the plane of the
equator. This is identical with the angle between the horizontal planes
at the place and at the equator, and also with the elevation of the
celestial pole above the horizon (see ASTRONOMY). Latitude thus
determined by the plumb-line is termed _astronomical_. The _geocentric
latitude_ of a place is the angle which the line from the earth's centre
to the place makes with the plane of the equator. _Geographical
latitude_, which is used in mapping, is based on the supposition that
the earth is an elliptic spheroid of known compression, and is the
angle which the normal to this spheroid makes with the equator. It
differs from the astronomical latitude only in being corrected for local
deviation of the plumb-line.
The latitude of a celestial object is the angle which the line drawn
from some fixed point of reference to the object makes with the plane of
the ecliptic.
_Variability of Terrestrial Latitudes._--The latitude of a point on the
earth's surface, as above defined, is measured from the equator. The
latter is defined by the condition that its plane makes a right angle
with the earth's axis of rotation. It follows that if the points in
which this axis intersects the earth's surface, _i.e_. the poles of the
earth, change their positions on the earth's surface, the position of
the equator will also change, and therefore the latitudes of places will
change also. About the end of the 19th century research showed that
there actually was a very minute but measurable periodic change of this
kind. The north and south poles, instead of being fixed points on the
earth's surface, wander round within a circle about 50 ft. in diameter.
The result is a variability of terrestrial latitudes generally.
[Illustration]
To show the cause of this motion, let BQ represent a section of an
oblate spheroid through its shortest axis, PP. We may consider this
spheroid to be that of the earth, the ellipticity being greatly
exaggerated. If set in rotation around its axis of figure PP, it will
continue to rotate around that axis for an indefinite time. But if,
instead of rotating around PP, it rotates around some other axis, RR,
making a small angle, POR, with the axis of figure PP; then it has
been known since the time of Euler that the axis of rotation RR, if
referred to the spheroid regarded as fixed, will gradually rotate
round the axis of figure PP in a period defined in the following
way:--If we put C = the moment of momentum of the spheroid around the
axis of figure, and A = the corresponding moment around an axis
passing through the equator EQ, then, calling one day the period of
rotation of the spheroid, the axis RR will make a revolution around PP
in a number of days represented by the fraction C/(C - A). In the case
of the earth, this ratio is 1/0.0032813 or 305. It follows that the
period in question is 305 days.
Up to 1890 the most careful observations and researches failed to
establish the periodicity of such a rotation, though there was strong
evidence of a variation of latitude. Then S. C. Chandler, from an
elaborate discussion of a great number of observations, showed that
there was really a variation of the latitude of the points of
observation; but, instead of the period being 305 days, it was about 428
days. At first sight this period seemed to be inconsistent with
dynamical theory. But a defect was soon found in the latter, the
correction of which reconciled the divergence. In deriving a period of
305 days the earth is regarded as an absolutely rigid body, and no
account is taken either of its elasticity or of the mobility of the
ocean. A study of the figure will show that the centrifugal force round
the axis RR will act on the equatorial protuberance of the rotating
earth so as to make it tend in the direction of the arrows. A slight
deformation of the earth will thus result; and the axis of figure of the
distorted spheroid will no longer be PP, but a line P´P´ between PP and
RR. As the latter moves round, P´P´ will continually follow it through
the incessant change of figure produced by the change in the direction
of the centrifugal force. Now the rate of motion of RR is determined by
the actual figure at the moment. It is therefore less than the motion in
an absolutely rigid spheroid in the proportion RP´ : RP. It is found
that, even though the earth were no more elastic than steel, its
yielding combined with the mobility of the ocean would make this ratio
about 2 : 3, resulting in an increase of the period by one-half, making
it about 457 days. Thus this small flexibility is even greater than
that necessary to the reconciliation of observation with theory, and the
earth is shown to be more rigid than steel--a conclusion long since
announced by Kelvin for other reasons.
Chandler afterwards made an important addition to the subject by showing
that the motion was represented by the superposition of two harmonic
terms, the first having a period of about 430 days, the other of one
year. The result of this superposition is a seven-year period, which
makes 6 periods of the 428-day term (428^d × 6 = 2568^d = 7 years,
nearly), and 7 periods of the annual term. Near one phase of this
combined period the two component motions nearly annul each other, so
that the variation is then small, while at the opposite phase, 3 to 4
years later, the two motions are in the same direction and the range of
variation is at its maximum. The coefficient of the 428-day term seems
to be between 0.12´´ and 0.16´´; that of the annual term between 0.06´´
and 0.11´´. Recent observations give smaller values of both than those
made between 1890 and 1900, and there is no reason to suppose either to
be constant.