page_title: Sundial
section_title: Universal equinoctial ring dial
hierarchical_section_title: Sundial / Movable-gnomon sundials / Universal equinoctial ring dial
caption_attribution_description: Deutsch: Bild einer Ringsonnenuhr, geöffnet in Komplettansicht. English: Picture of a ring (traveler's) sundial, opened in full view.
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: A universal equinoctial ring dial (sometimes called a ring dial for brevity, although the term is ambiguous), is a portable version of an armillary sundial, or was inspired by the mariner's astrolabe.  It was likely invented by William Oughtred around 1600 and became common throughout Europe.
  In its simplest form, the style is a thin slit that allows the Sun's rays to fall on the hour-lines of an equatorial ring.  As usual, the style is aligned with the Earth's axis; to do this, the user may orient the dial towards true North and suspend the ring dial vertically from the appropriate point on the meridian ring.  Such dials may be made self-aligning with the addition of a more complicated central bar, instead of a simple slit-style.  These bars are sometimes an addition to a set of Gemma's rings.  This bar could pivot about its end points and held a perforated slider that was positioned to the month and day according to a scale scribed on the bar.  The time was determined by rotating the bar towards the Sun so that the light shining through the hole fell on the equatorial ring.  This forced the user to rotate the instrument, which had the effect of aligning the instrument's vertical ring with the meridian.
  When not in use, the equatorial and meridian rings can be folded together into a small disk.
  In 1610, Edward Wright created the sea ring, which mounted a universal ring dial over a magnetic compass.  This permitted mariners to determine the time and magnetic variation in a single step.
caption: Universal ring dial. The dial is suspended from the cord shown in the upper left; the suspension point on the vertical meridian ring can be changed to match the local latitude. The center bar is twisted until a sunray passes through the small hole and falls on the horizontal equatorial ring. See Commons annotations for labels.
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