Pole rod
The pole rod (also Polos from ancient Greek πόλος "earth axis") is the most frequently used shadow thrower for sundials . It is usually a thin rod that is arranged parallel to the earth's axis . It points north (to the North Star ) and intersects the horizon plane at the angle of the geographical latitude .
The sun apparently circles the earth once a day. The axis of this movement is also the pole rod with sufficient accuracy, because its distance from the Earth's axis is negligibly small compared to the radius of the sun's orbit , which is assumed to be circular (worst ratio at the equator less than 1: 12000). The hour angle defined in the fixed equatorial coordinate system indicates the declination and the apparent position of a celestial body as it revolves along this axis. In particular, the hour angle of the sun is used as a measure of the time of day. In the case of a sundial with a pole rod as a shadow object, the declination of no interest (measure for the season) is not displayed in favor of the easy readability of the hour angle (time of day) due to the linear shape of the shadow displayed.
He is already mentioned as Polos by Herodotus . Its use has only been known for certain since the late Middle Ages . At the same time, equinox hours came into common use. The previously usual temporal hours can only be displayed on a sundial with a point-shaped shadow object ( Nodus ).
literature
- René RJ Rohr: The sundial. History, theory, function. Callwey, Munich 1982, ISBN 3-7667-0610-1 . Original French title: Les Cadrans solaires
Individual evidence
- ↑ Herodotus 2.109: The Greeks learned the Polos and Gnomon and the 12 hours of the day from the Babylonians. Compare Rohr: The Sundial. Pp. 13–14: It is uncertain whether Herodotus is referring to the pole rod, because he did not take it very carefully with his sources.
- ↑ See Rohr: The Sundial. Pp. 27-28.