ROGER HARRIS / SCIENCE PHOTO LIBRARY ROGER HARRIS / SCIENCE PHOTO LIBRARY
Sundial. Computer illustration of a spinning armillary type of equinoctial sundial. The celestial sphere is represented by three metal bands, the thickest of which, the equinoctial hour-ring (the large blurred diagonal ring), lies along the celestial equator. The sundial measures time using the shadow cast by the central metal sphere which represents the Earth. This sphere is on a spike known as a gnomon, here aligned with the polar axis. As the Sun moves across the sky the shadow moves around the equinoctial ring, falling on the appropriate hour. This is the most accurate non-adjustable form of sundial.
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