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The lunitidal interval[1], is also called the high water interval (HWI)[2][3], it is the length of time from when the moon passes over a meridian and the next high tide at that meridian. Tides are known to be mainly caused by the moon's gravity. Theoretically, high tide happens when the moon is at meridian. However, there is actually a delay that depends on many complicated factors.

The length of lunitidal interval differs on all shores in the world. For any particular shore it is, on average, a constant value. Knowing the lunitidal interval, it is possible to predict how long after the moon moves to meridian until high tide will occur.

The approximate lunitidal interval can be calculated if the moon rise, moon set and high tide timings are known. These timings can be obtained if the coordinates for your region are known or even from local media.

First you will have to find the time at which the moon is at its highest point (meridian) at the specific longitude of interest. In the northern hemisphere, this will be the time at which it is at its southernmost point. These data are available from a number of sources (e.g. http://www.timeanddate.com). Next use tide tables to find the time at which the next high water occurs at the same area of interest. Again tide data are widely available (e.g. http://www.pol.ac.uk/ntslf/tidalp.html and http://tidesandcurrents.noaa.gov/tide_predictions.shtml). Be careful to make sure that all timings used are expressed in GMT. The difference between these two times is the lunitidal interval. This value can be used to program certain watches to allow tidal predictions, but the accuracy is limited by the fact that this time delay is somewhat variable.

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