GNOMON

The gnomon is a non-mechanical instrument, whose shadow, cast by the Sun, allows its course across the celestial vault to be tracked; in its simplest form, a vertical rod planted in the ground. Its shadow is shortest, and points exactly north-south, at the instant of true solar noon — the direction of the shadow at any other hour does not, however, indicate the Time passing, contrary to popular belief. In gnomonics, the science of sundials, the term “gnomon” specifically refers to the Indicator of ancient dials with temporary hours; on modern dials with equinoctial hours, the term “style” is used, most often oriented parallel to the Earth’s Axis (polar style). A sundial, originally, is a gnomon paired with a table bearing hour lines; a meridian line reduces it to the noon line alone.

Attested as early as the 2nd millennium BCE in China and Babylon, the gnomon was perfected by the Greeks: in the 6th century BCE, Anaximander of Miletus determined the equinoxes by bisecting the angle formed, on the meridian line, by the solstitial shadows. Eratosthenes used it to measure the obliquity of the ecliptic and then, applying the same method between Alexandria and Syene, the circumference of the Earth. In Rome, the emperor Augustus had an Egyptian obelisk more than twenty meters high erected in 10 BCE, still visible today.