page_title: Sundial
section_title: Noon marks
hierarchical_section_title: Sundial / Unusual sundials / Noon marks
caption_attribution_description: English: A reconstruction of a long lost mean time Noon-Mark, "unveiled" at the Royal Observatory at Greenwich on 10th October 2012.
context_page_description: A sundial is a device that tells the time of day when there is sunlight by the apparent position of the Sun in the sky. In the narrowest sense of the word, it consists of a flat plate 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, wire, or 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 to show the time. In addition to their time-telling function, sundials are valued as decorative objects, literary metaphors, and objects of intrigue and mathematical study.
context_section_description: The simplest sundials do not give the hours, but rather note the exact moment of 12:00 noon.  In centuries past, such dials were used to correct mechanical clocks, which were sometimes so inaccurate as to lose or gain significant time in a single day.
  In some U.S. colonial-era houses, a noon-mark can often be found carved into a floor or windowsill.  Such marks indicate local noon, and provide a simple and accurate time reference for households that do not possess accurate clocks. In modern times, in some Asian countries, post offices set their clocks from a precision noon-mark.  These in turn provide the times for the rest of the society.  The typical noon-mark sundial was a lens set above an analemmatic plate.  The plate has an engraved figure-eight shape, which corresponds to plotting the equation of time (described above) versus the solar declination.  When the edge of the Sun's image touches the part of the shape for the current month, this indicates that it is 12:00 noon.
caption: Noon mark from the Greenwich Royal Observatory. The analemma is the narrow figure-8 shape, which plots the equation of time (in degrees, not time, 1°=4 minutes) versus the altitude of the Sun at noon at the sundial's location.  The altitude is measured vertically, the equation of time horizontally.
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