Tide dial

A tide dial, also known as a Mass or scratch dial, is a sundial marked with the canonical hours rather than or in addition to the standard hours of daylight. Such sundials were particularly common between the 7th and 14th centuries in Europe, at which point they began to be replaced by mechanical clocks. There are more than 3,000 surviving tide dials in England and at least 1,500 in France.

For devices indicating the current state of local ocean tides, see Tide clock.
A 7th-century Saxon tide dial on the porch of St Andrew's in Bishopstone, in East Sussex in England, with larger crosses marking the canonical hours.
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