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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]

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