Lunitidal interval

The lunitidal interval, measures the time lag from the moon passing overhead, to the next high. It is also called the high water interval (HWI) or establishment of the port.

Average high water interval on days of the new and full moon. This interval is also sometimes called the common or vulgar establishment to distinguish it from the corrected establishment, the latter being the mean of all the high water intervals. The latter is usually 10 to 15 minutes less than the common establishment.

Tides are known to be mainly caused by the moon's gravity. Theoretically, peak tidal forces at a given location occur when the moon is at the meridian, but there is usually a delay before high tide that depends largely on the shape of the coastline, and the sea floor, therefore, the lunitidal interval varies from place to place. The lunitidal interval further varies within about +/- 30 minutes according to the lunar phase.

The Lunitidal interval can only be used to estimate the tides at places where the semidiurnal component of the tide is dominant.

The Lunitidal interval constants should not be used to estimate the tides for any critical purpose.