POLUS
POLUS (
πόλος), a word of various
meanings, all however connected in some way with a sphere of revolution, the
root of the word being the same which appears in
πολέω and
πέλομαι, which
implies motion, especially motion round a centre. It is only the scientific
meanings which will here be noticed, of which the best known is that which
has reference to astronomy; the other, much rarer and derived from the
former, referring to a part of the sun-dial [
HOROLOGIUM], In astronomy, again, by far the most
common meaning of
polus or
πόλος is the heavenly sphere or spheres, or
vault of the sky, originally conceived of as solid: thus in the earliest
passage in which
πόλος occurs (Aesch.
Prom. 427) Atlas is represented as supporting this sphere
on his shoulders. Probably the word was not very ancient in the time of
Aeschylus, for in Aristophanes (
Aristoph.
Birds 179-
182) there is a
formal explanation of it, and it is in his contemporary, Euripides, that it
first becomes frequent (
Orest. 1.685;
Ion, 1154, &c.). The account of the heaven
accepted among Greek philosophers generally (though with variations)
represented it as formed of concentric spheres, the outside sphere being
that which contained the fixed stars, while the inner spheres, each having
its own proper motion, contained the sun, moon, and five principal planets
(which alone are visible to.the naked eye). See Plato,
Timaeus, p. 38, and the second book of Aristotle,
de Caelo; in which book, especially in the latter
half, are to be found acute observations, mixed with obscure reasonings. But
it is to be remarked that both in
Timaeus, 40
B, and
de Caelo, 2.14,
πόλος is used, not for the entire heaven, but for the axis
of heaven and earth, around which the whole revolved. Again, in the
de Caelo, 2.2, the
πόλοι are the poles, north and south, in our sense of the word;
and the same meaning is common in Latin, when the entire heaven is not
intended (
Plin. Nat. 2.63). Another
meaning
[p. 2.443]altogether, the
orbit of a star, is found in [Plat.]
Epin. 986 C. (Cf.
[Plat.]
Axioch. p. 371 B; Alex. ap.
Athen. p. 60 a; Ukert,
Geog. d.
Griech, u. Röm. vol. i. pt. ii. p. 115: and for the
conception of heaven among the Greeks, Whewell,
Hist. of the
Inductive Sciences, vol. i. pp. 153
sqq.; Cornewall Lewis,
Astronomy of the Ancients, which
moreover takes notice of the point next to be mentioned.)
Connected.with the most common astronomical meaning of
πόλος, the revolving heavenly sphere, is the use of the word
to mean a dial. The .first scientific attempt to mark the time of day with
exactitude was by constructing a hollow hemisphere, so placed as to catch
the sun's rays on its interior surface, the axis of the hemisphere being
parallel to the polar axis of the heavens. Then on this interior surface the
path of the sun was marked by means of the shadow of a bead fixed on the
axis of the hemisphere, or (which comes to the same thing) by the extremity
of an index (
γνώμων) reaching to the same
point. The simple index or
μνώμων, in the
sense of an upright rod, had no doubt been used from very early times as a
means of roughly measuring the time. of. day, by the length and direction of
the shadows. But when to the
γνώμων was
added the above-described hemisphere, or
πόλος as it was naturally called, from its being the counterpart
of the heavenly
πόλος, the result was a
scientific sun-dial. Herodotus (
2.109) uses the
two words,
πόλον καὶ ψγώμονα, together,
to describe the compound instrument, and he tells us, no doubt correctly,
that the Greeks derived it from the Babylonians. (See Ideler,
Handbuch der Chronologie, vol. i. p. 233, referred to by
Grote, vol. ii. p. 155, in editions after the first; also Bahr's note on
Herodotus
ad loc., with his, references to Bailly,
Delambre, Letronne, and Creuzer.) Vitruvius (
9.9）
tells us that this form of sup-dial (
hemicyclium) was invented by Berosus, who lived at Babylon at the
end of the fourth, and during the first part of the third, century n.C. But
this, considering the.passage in Herodotus, can hardly be correct; though
Berosus may no doubt have improved the instrument, and his. bust is
represented on the base of a dial found, at Palestrina. Besides Herodotus,
Aristophanes mentions the
πόλος in a
fragment of the Gerytades, where it clearly means a sun-dial, and is
explained as such (
ὡρολόγιον by
Pollux.(9.46), to whom we owe the fragment. Lucian (
Lexiphan.
4) speaks of the
γνώμων as overshadowing
the middle of the
πόλος, which shows
clearly the relation between the two. (See also Alciphron,
Ep. 3.4.) Some interesting remarks on ancient sun-dials, with
pictures in which the
πόλος and its
hour-lines are well illustrated, and another of a different make of dial,
will be found in Mrs. Alfred Gatty's
Book of Sun-dials
(London, 1889), pp. 1-13, 391-404.
[
P.S] [
J.R.M]
