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Updated: May 31, 2025
Some writers maintain that they were as memorials, pillars, or altars consecrated to the sun; others, that they served as a kind of gnomon for astronomical observations; that they were built to gratify the vanity and tyranny of kings, or for the celebration of religious mysteries; according to Diderot, for the transmission and preservation of historical information; and to others, for sepulchres for the kings, which last was the common opinion of the ancients.
The little town of Settle has a most distinctive feature in the possession of Castleberg, a steep limestone hill, densely wooded except at the very top, that rises sharply just behind the market-place. Before the trees were planted there seems to have been a sundial on the side of the hill, the precipitous scar on the top forming the gnomon.
With the aid of this implement, Eratosthenes carefully noted the longest and the shortest shadows cast by the gnomon that is to say, the shadows cast on the days of the solstices. He found that the distance between the tropics thus measured represented 47 degrees 42' 39" of arc.
Another of the great obelisks of Rome is that which stands on Monte Citorio, in front of the present Parliament House. It was brought to Rome by Augustus, who dedicated it anew to the sun, and placed it as the gnomon of a meridian in the midst of the Campus Martius. Originally it had been erected at Heliopolis in honour of Psammeticus I., who reigned about seven hundred years before Christ.
The treatment was mostly geometrical, the numbers being represented by dots filling up geometrical figures of the various kinds. The laws of formation of the various figured numbers were established. In this investigation the gnomon played an important part.
His geographical studies had taught him that the town of Syene lay directly south of Alexandria, or, as we should say, on the same meridian of latitude. He had learned, further, that Syene lay directly under the tropic, since it was reported that at noon on the day of the summer solstice the gnomon there cast no shadow, while a deep well was illumined to the bottom by the sun.
Now the easiest way in which angles can be measured, when solar angles are in question, is to pay attention, not to the sun itself, but to the shadow that it casts. We saw that Thales made some remarkable measurements with the aid of shadows, and we have more than once referred to the gnomon, which is the most primitive, but which long remained the most important, of astronomical instruments.
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.
We may be sure, however, that the gnomon, which is the rudimentary sundial, had been known and used from remote periods in the Orient, and the most that is probable is that Anaximander may have elaborated some special design, possibly the bowl-shaped sundial, through which the shadow of the gnomon would indicate the time.
Then, from the line in the plane, let the line of the gnomon be divided off by the compasses into nine parts, and take the point designating the ninth part as a centre, to be marked by the letter A. Then, opening the compasses from that centre to the line in the plane at the point B, describe a circle. This circle is called the meridian.
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