On average, the recurrence of a tide of the same phase and similar rise takes place at an interval of 24 hours 50 minutes after the preceding occurrence. (If it's high at 3pm today then it'll be high tomorrow around 4.) This is, however, only a monthly average. The relative positions of the sun, moon and earth at different times during the month can significantly alter this interval and the "expected" times of high and low tides.

Between the new moon and the 1st quarter and between the full moon and 3rd quarter there is an acceleration in tidal arrival times. Between the 1st quarter and the full moon and between the 3rd quarter and the new noon there is a delay.

One of the occasional results can be two AM or PM high or low tides. This is particularly likely to occur in Buzzards Bay and Narraganset Bay because of the unique coastline configuration, water depth, and ocean-floor topography of that area. However, it can occur at any location.

Here are Newport RI tides for May 24-26, 1999. Notice the two AM low tides on the 26th.

On our tide charts we indicate this AM or PM anomaly with a red asterisk - * - and a note at the bottom of the chart as shown here:

Tide Note: Cape Cod Canal, Buzzards Bay Ent.

Here's what's happening:

Starting on the second, the Canal current (in combination with factors mentioned in the previous section) is beginning to extend the duration of the PM low tide. And by the third the result is a "lower" PM low tide, 2 hours before what you might expect. Of course, our charts give only the moment of lowest tide. Checking the sequence over a few days, however, can give some indication of changes in the duration of highs or lows.