Glossary term: Sundial

Description: The Sun's position in the sky has long been used to define time, with local noon at the point of the Sun's greatest height above the horizon and the day defined in the narrow sense as the period between sunrise and sunset, and in the broader sense as the length of time between one local noon and the next. A sundial is a device that projects the direction of the Sun onto a surface with markings, usually by tracking the shadow of an elongated pointer known as the "gnomon". When that shadow points at the marking designated "12", for instance, one would read off the time as 12 o'clock. Simple sundials only show apparent solar time, which is directly linked to the Sun's sky position. More elaborate versions have provisions for allowing the determination of mean solar time.

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