Lunitidal Interval

Lunitidal Interval Web App Calculator.

Specific clocks and wristwatches to allow for simple tidal predictions use the Lunitidal Interval to calibrate the results.
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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, averaging high water interval on days of the new and full moon.

This interval is also known as the "common establishment" or "vulgar establishment" to distinguish it from the "corrected establishment," the latter being the mean of all the high water intervals. The "corrected establishment" 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. Still, there is usually a delay before high tide that depends mainly 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 semi-diurnal component of the tide is dominant and should not be used to estimate tides for any critical purpose.

Step 1


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Step 2

Moon Transit

Step 3

Next High Tide