El reloj de sol es un instrumento usado desde tiempos muy remotos con el fin de medir el paso de las horas, minutos y segundos (tiempo). En castellano se le denomina también cuadrante solar. Emplea la sombra arrojada por un gnomon o estilo sobre una superficie con una escala para indicar la posición del Sol en el movimiento diurno. Según la disposición del gnomon y la forma de la escala se pueden medir diferentes tipos de tiempo, siendo el más habitual el tiempo solar aparente. La ciencia encargada de elaborar teorías y reunir conocimientos sobre los relojes de sol se denomina gnomónica.

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A sundial is a device that tells the time of day by the apparent position of the Sun in the sky. In the narrowest sense of the word it consists of a flat plate (the dial) and a gnomon which casts a shadow onto the dial. As the sun appears to move across the sky, the shadow aligns with different hour-lines which are marked on the dial to indicate the time of day. The style is the time-telling edge of the gnomon, though a single point or nodus may be used. The gnomon casts a broad shadow; the shadow of the style shows the time. The gnomon may be a rod, a wire or an elaborately decorated metal casting. The style must be parallel to the axis of the Earth's rotation for the sundial to be accurate throughout the year. The style's angle from horizontal is equal to the sundial's geographical latitude. In a broader sense a sundial is any device that uses the sun's altitude or azimuth (or both) to show the time. In addition to their time-telling function, sundials are valued as decorative objects, as literary metaphors and as objects of mathematical study. It is common for inexpensive mass-produced decorative sundials to have incorrectly aligned gnomons and hour-lines, which cannot be adjusted to tell correct time.

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