The span from one passage of the Moon over a meridian to its next, averaging about 24.84 hours. A semidiurnal tide fits two highs and two lows into each lunar day, which is why the tide arrives roughly 50 minutes later from one day to the next. A tidal day in the datum sense is the same span, and datum calculations pick one higher high and one lower low from each.
High water was 0700 this morning, so plan on about 0750 tomorrow, that's the lunar day for you.
Also heard as
- tidal day
Related terms
M2
The principal lunar semidiurnal constituent, period 12.42 hours, usually the largest part of the tide.
Lunitidal interval
How long a place waits after the Moon crosses its meridian before the next high or low water arrives.
Semidiurnal tide
A tide with two high waters and two low waters of similar height in each lunar day.
Higher high water
The taller of a tidal day's two high waters, the one that matters most on mixed-tide coasts.