The lunar day and the types of tide
1The lesson in plain words
The Moon comes back over a place every 24 hours 50 minutes, so high water comes about 50 minutes later each day. Tides can be semi-diurnal, diurnal or mixed.
The lunar day
While the Earth turns once, the Moon moves on along its orbit in the same direction. The Earth needs about 50 more minutes to catch up, so the Moon returns over the same place every 24 hours 50 minutes: the lunar day. With two bulges, high water comes on average every 12 hours 25 minutes, and about 50 minutes later each day.
Semi-diurnal tides
A semi-diurnal tide has two high and two low waters a day, of nearly equal height. This is the tide of the Atlantic coasts of Europe, from Portugal and Galicia to the English Channel and the North Sea.
The Moon’s declination
The Moon does not stay over the equator. Over about a month, its declination, its angle north or south of the equator, swings up to about 28°. The bulges tilt with it, so a place can pass through a high part of one bulge and a low part of the other. The two high waters of the day then differ: this is the diurnal inequality.
Diurnal and mixed tides
Where the inequality is strong, a mixed tide gives two high waters of clearly different height each day, as on the Pacific coast of the United States. In a few places, such as parts of the Gulf of Mexico, one tide almost disappears and there is only one high and one low water a day: a diurnal tide.
Theory versus tables
The astronomy explains the pattern, but each port’s timing depends on its basin and coastline. The Moon being overhead does not reliably tell you when high water occurs. Always take times and heights from official tide tables for that port.
2Key data
- Lunar day
- About 24 h 50 min
- Semi-diurnal tide period
- About 12 h 25 min between high waters
- Daily delay
- High water about 50 min later each day, on average
- Types of tide
- Semi-diurnal, mixed, diurnal
3Key points
- 1
The lunar day lasts about 24 h 50 min.
- 2
High water comes about 50 minutes later each day, on average.
- 3
Semi-diurnal: two nearly equal tides a day, as on Atlantic Europe.
- 4
The Moon’s declination makes the day’s two tides unequal.
- 5
Take times from tide tables, not from the Moon’s position.
4Frequent mistakes
Wrong: High water is always when the Moon is overhead.
Right: The local delay depends on the basin and coast. Use the port’s tide tables.
Wrong: Every coast has two high waters a day.
Right: Mixed tides have unequal highs, and diurnal tides have only one high water a day.
5Practice case & exam keys
In Brittany, high water today is at 09:40. You want to leave on the morning high water tomorrow and the day after, but you have no tide table on board.
Show the answer
Adding the average 50-minute delay suggests about 10:30 tomorrow and 11:20 the following day. These are rough estimates: the local delay varies with the port and date. Get the official times from the harbour office or a tide table before planning your departure.
Exam keys
- Lunar day ≈ 24 h 50 min; semi-diurnal high waters ≈ 12 h 25 min apart; about 50 min later each day.
- Semi-diurnal (Atlantic Europe), mixed (US Pacific coast), diurnal (parts of the Gulf of Mexico).
- The Moon’s declination causes the diurnal inequality.
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