The difference between these two times is the lunitidal interval.

The lunitidal interval further varies within about +/- 30 minutes according to the lunar phase.

The lunitidal interval, measures the time lag from the moon passing overhead, to the next high or low tide.

The phase p of the first harmonic for the moon term is called the lunitidal interval or high water interval.

The approximate lunitidal interval can be calculated if the moon-rise, moon-set and high tide times are known for a location.

Lunitidal interval

High Water Interval, see Lunitidal interval

The lunitidal interval is 45 min, the average duration of rise is 5 h 40 min, and the average duration of fall is 6 h 53 min.

Tides have an inherent lead or lag, known as the lunitidal interval, that is different at every location, so tidal clocks are set for the time when the local lunar high tide occurs.

However, for a given location the relationship between lunar altitude and the time of high or low tide (the lunitidal interval) is relatively constant and predictable, as is the time of high or low tide relative to other points on the same coast.

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.