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Updated: May 31, 2025
Peter's, between two beautiful fountains that are continually showering high in the air their radiant sunlit spray. It is meant to serve as the gnomon of a gigantic dial, traced in lines of white marble in the pavement of the square. Its rosy surface glistening in the rays of the sun, and its long shadow cast before it on the ground, make it a very impressive object.
They invented the gnomon and the polos, two kinds of sundial, by means of which they were able to measure time during the day, and to fix the true length of the solar day, with sufficient accuracy. They determined correctly within a small fraction the length of the synodic revolution of the moon. They knew that the true length of the solar year was 365 days and a quarter, nearly.
The shadow of the sun roughly gave those who were familiar with astronomy the lay of the land and the time, approximately. When the dial and the gnomon were understood, dialling became a popular science, and ere long the sundial on the church tower, in a public place, or in a private garden, told the time.
It is seventy-two feet high, and is surmounted by a globe and a small pyramid of bronze, which, along with its pedestal, increases its height to one hundred and thirty-four feet. A portion of the lines of the celebrated sun-dial, whose gnomon it formed, was brought to light under the sacristy of San Lorenzo in Lucina in 1463.
By means of a gnomon he observed, at the summer solstice at Marseilles, that the length of the shadow was to the height of the gnomon as 120 to 41-1/5; or, in other words, that the obliquity of the ecliptic was 23:50.
It mattered little, however, that only the stump of a gnomon was left, seeing the hedge around it had grown to such a height in relation to the diameter of the circle, that it was only for a very brief hour or so in the middle of a summer's day, when, of all periods, the passage of Time seems least to concern humanity, that it could have served to measure his march.
And he had found his daughter standing at the threshold of his house, having risen from her couch when the gnomon had marked the third hour, the same moment when he had made his supplication to Jesus. The Pharisees admitted that certain mysterious arts and powerful herbs existed that would heal the sick.
At about the fifth hour in the morning, take the end of the shadow cast by this gnomon, and mark it with a point. Then, opening your compasses to this point which marks the length of the gnomon's shadow, describe a circle from the centre.
You may rely on it, that it is all a mistake to consider the regular Roman nose, with a curve like a shoemaker's paring knife, or the straight Grecian, with a thin transparent ridge, that you can see through, or the Deutsch meerschaum, or the Saxon pump handle, or the Scotch mull, or any other nose, that can be taken hold of, as the standard gnomon.
Thales, born 640 B.C., taught the rotundity of the earth, and that the moon shines with reflected light. He also predicted eclipses. Anaximander, born 610 B.C., invented the gnomon, and constructed geographical charts. But the Greeks, after all, were the only people of antiquity who elevated astronomy to the dignity of a science.
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