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Last edited 18 Sep 2022

Sundial

A sundial is an instrument used to show the time of day. It does so using a spike or triangle (a gnomon) that protrudes from a horizontal or vertical plate, casting a shadow which indicates the time. It is one of the earliest devices relating to the study of time (horology) and was used by the ancient Egyptians and Babylonians (1500 BC). The art or science of constructing sundials is called this gnomonics, named after the central part used to cast the shadow.

Solar time is derived from the Sun's position, by counting one hour of time every 15° of hour angle, as Earth is rotating around its axis once every 24 hours, which gives 360°/24 h = 15°/hour. On a sundial, the noon line corresponds to the moment of culmination of the Sun, when it crosses the local meridian. This sort of time depends on the location or longitude. The designer must calculate a longitude correction for a given location, by comparing the longitude to the reference meridian (CET meridian is 15° East). For example, the time in Exeter, Devon might be up to 15 minutes later than that in Greenwich. So sundials are based on the equation of time plus a longitudinal correction.

In the UK, the world heritage site, Stonehenge, built in the Neolithic period (2500 BC), is thought to be a form of sundial with each of the 30 upright stones representing one day within a repeating 30-day month. Twelve rotations of the Circle equates to 12 months, forming a solar year (which is 365 days, not 360).

The oldest sundials measuring shorter time cycles to be found in the UK are the tide dial of Bewcastle Cross, Cumbria, dating from the 7th century, and the Boyne Valley sundial in Ireland, a sundial stone believed to be over 5000 years old.

A tide, mass or scratch dial is a type of sundial that has the canonical hours instead or in addition to the standard hours of daylight marked.

Today sundials can be found as features of historical buildings and landscapes, often as single horizontal standalone features in gardens or incorporated into building facades as vertical sundials. Sundials can also be added as interesting features to modern homes or as home educational design projects.