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Updated: May 21, 2025
For the gnomons of dials are instruments and measures of time, not in being moved with the shadows, but in standing still; they being like the earth in closing out the light of the sun when it is down, as Empedocles says that the earth makes night by intercepting light. This therefore may be Plato's meaning.
I have shown how the firmament, and the twelve signs with the constellations arranged to the north and south of them, fly round the earth, so that the matter may be clearly understood. For it is from this revolution of the firmament, from the course of the sun through the signs in the opposite direction, and from the shadows cast by equinoctial gnomons, that we find the figure of the analemma.
Some have maintained that the obelisks which the Egyptians erected served the purpose of gnomons for determining the obliquity of the ecliptic, the altitude of the pole, and the length of the tropical year. It is thought even that the Pyramids, by the position of their sides toward the cardinal points, attest Egyptian acquaintance with a meridional line.
Our gnomons, also, are, among other things, evidence of the revolution of the heavenly bodies, and common-sense at once shows us that if the depth of the earth were infinite such a revolution could not take place." Elsewhere Strabo criticises Eratosthenes for having entered into a long discussion as to the form of the earth.
They constructed sundials, clepsydras, astrolabes, gnomons. Not without interest do we still look on specimens of their method of printing. Upon a revolving roller they engraved, in cuneiform letters, their records, and, running this over plastic clay formed into blocks, produced ineffaceable proofs. From their tile-libraries we are still to reap a literary and historical harvest.
If we represent a unit by one dot and put round it three dots in such a way that the four form the corners of a square, three is the first gnomon. From the series of gnomons for squares we easily deduce a formula for finding square numbers which are the sum of two squares. This is the formula actually attributed to Pythagoras.
Now although I cannot but think that I have already demonstrated, and that abundantly more than was necessary, that our fathers were not originally Egyptians, nor were thence expelled, either on account of bodily diseases, or any other calamities of that sort; yet will I briefly take notice of what Apion adds upon that subject; for in his third book, which relates to the affairs of Egypt, he speaks thus: "I have heard of the ancient men of Egypt, that Moses was of Heliopolis, and that he thought himself obliged to follow the customs of his forefathers, and offered his prayers in the open air, towards the city walls; but that he reduced them all to be directed towards sun-rising, which was agreeable to the situation of Heliopolis; that he also set up pillars instead of gnomons, under which was represented a cavity like that of a boat, and the shadow that fell from their tops fell down upon that cavity, that it might go round about the like course as the sun itself goes round in the other."
And he did prize very high the portraiture of a good doctor a-sitting in his cabinet writing at a table by candle-light. The said cabinet was fully furnished with globes, gnomons, and astrolabes, proper for meting the movements of the orbs of heaven, the which is a right praiseworthy task and one that doth lift the spirit to sublime thoughts and the exceeding pure love of Venus Urania.
"The Peruvians," says Michelet in his admirable Précis d'Histoire Moderne, "handed down the principal facts to posterity by knots, which they made in ropes. They had obelisks and exact gnomons to mark the equinoxes and solstices. Their year consisted of 365 days. They had erected prodigies of architecture, and they carved statues with amazing art.
In Athens, the shadow is equal to three fourths of the length of the gnomon; at Rhodes to five sevenths; at Tarentum, to nine elevenths; at Alexandria, to three fifths; and so at other places it is found that the shadows of equinoctial gnomons are naturally different from one another. Hence, wherever a sundial is to be constructed, we must take the equinoctial shadow of the place.
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