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For as touching the sun-dial and the gnomon and the twelve divisions of the day, they were learnt by the Hellenes from the Babylonians.
The first astronomical instrument was the gnomon. This was not only early in use in the East, but it was found also among the Mexicans; the sole astronomical observations of the Peruvians were made by it; and we read that 1100 B.C., the Chinese found that, at a certain place, the length of the sun's shadow, at the summer solstice, was to the height of the gnomon as one and a half to eight.
Remembering, however, that Eratosthenes of Cyrene, employing mathematical theories and geometrical methods, discovered from the course of the sun, the shadows cast by an equinoctial gnomon, and the inclination of the heaven that the circumference of the earth is two hundred and fifty-two thousand stadia, that is, thirty-one one million five hundred thousand paces, and observing that an eighth part of this, occupied by a wind, is three million nine hundred and thirty-seven thousand five hundred paces, they should not be surprised to find that a single wind, ranging over so wide a field, is subject to shifts this way and that, leading to a variety of breezes.
Exactly as the gnomon of the official dial up in the citadel pointed the second hour half gone, the legion, in full panoply, and with all its standards on exhibit, descended from Mount Sulpius; and when the rear of the last cohort disappeared in the bridge, Antioch was literally abandoned not that the Circus could hold the multitude, but that the multitude was gone out to it, nevertheless.
He constructed geographical charts, and attempted to delineate the celestial sphere, and to measure time with a gnomon, or time-pillar, by the motion of its shadow upon a dial. Ahaz erected a similar altar at Jerusalem, and also a sun-dial, the same one mentioned in the account of the miraculous cure of his son Hezekiah. "This," says Dr.
He merely measured the angle of the shadow which his perpendicular gnomon at Alexandria cast at mid-day on the day of the solstice, when, as already noted, the sun was directly perpendicular at Syene.
Here, one can hardly doubt, are things older even than Rome. Scholars have talked, indeed, of a Greek origin or of an Etruscan origin, and the technical term for the Roman surveying instrument, groma, has been explained as the Greek word 'gnomon', borrowed through an Etruscan medium.
The angle actually measured by Eratosthenes is KFA, as determined by the shadow cast by the gnomon AF. This angle is equal to the opposite angle JFL, which measures the sun's distance from the zenith; and which is also equal to the angle AES to determine the Size of which is the real object of the entire measurement.
It is Pytheas, we may add, who was cited by Hipparchus as having made the mistaken observation that the length of the shadow of the gnomon is the same at Marseilles and Byzantium, hence that these two places are on the same parallel.
Here again it is observable, not only that the instrument is found ready made, but that Nature is perpetually performing the process of measurement. Any fixed, erect object a column, a dead palm, a pole, the angle of a building serves for a gnomon; and it needs but to notice the changing position of the shadow it daily throws to make the first step in geometrical astronomy.
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