Semidiurnal
Two highs and two lows per day, of broadly similar height. Successive highs about 12 h 25 min apart. Four events a day in the table. Common along much of the Australian east coast.
Tide types
Two a day.
Usually.
The standard explanation is right for most of the world and wrong for part of Australia. Both halves are worth knowing.
01 · The basic picture
Two bulges, not one
The moon's gravity raises a bulge of water on the side of the earth facing it. That much is intuitive.
There is a second bulge on the opposite side of the earth, and that is the part that stops most explanations dead. Without it, there would be one high tide a day, not two.
As the earth rotates through both bulges in a day, any given coast passes through two highs and two lows. That is the origin of the twice-daily rhythm.
02 · The part usually fudged
Why the far side bulges too
The common hand-wave is "centrifugal force". The more useful answer is that the tide is caused by a difference in gravity, not by gravity itself.
The moon pulls on every part of the earth, but not equally. It pulls hardest on whatever is nearest and least on whatever is furthest, because gravity weakens with distance.
The earth as a whole accelerates toward the moon at the rate set by the pull at its centre. Now compare each part of the earth against that average:
- On the near side, the moon's pull is stronger than the average. Relative to the earth as a whole, that water is pulled toward the moon. Bulge.
- At the centre, the pull is exactly the average. No relative effect.
- On the far side, the moon's pull is weaker than the average. Relative to the earth as a whole, that water is left behind, which is to say it moves away from the moon. Bulge.
So the two bulges are not two separate effects. They are the two ends of one stretching, produced by the same difference in pull across the width of the earth. The sun does the same thing more weakly, and the interaction between the two produces spring and neap tides.
03 · The clock
Why the interval is about 12 hours 25 minutes
If the moon stood still, the earth would rotate once beneath the bulges every 24 hours and you would get a high tide every 12 hours exactly.
The moon does not stand still. It moves along its orbit in the same direction the earth spins, so by the time a point on earth has completed one full rotation, the moon has moved on a little. The earth has to turn for roughly another 50 minutes to catch up and bring that point back under the moon.
That makes the lunar day about 24 hours and 50 minutes rather than 24 hours. Two tidal cycles fit into it, so successive high waters are about 12 hours 25 minutes apart.
This is why tide times drift later each day by roughly 50 minutes, and why a tide table cannot be a fixed weekly timetable. It is also why "same time tomorrow" is a reliable way to miss the tide you wanted.
04 · Honest limits
Where the two-bulge picture stops being true
Everything above is equilibrium theory, and it explains why tides exist and why they have the rhythm they do. It does not explain the tide at your port, and it is worth being clear about that.
The real ocean is not a smooth shell of water free to follow the moon. It is a set of basins with continents in the way, varying depth, friction and a rotating frame of reference. What actually happens is that the tidal forcing sets up enormous rotating wave systems in each ocean basin, turning around nodal points where the tidal range is near zero.
The practical consequences:
- High tide does not occur when the moon is overhead at most places. The offset is a local property.
- Tidal range is set by basin shape and resonance, not by distance from the equator. This is why the Kimberley has some of the largest tides in the southern hemisphere while parts of the south west coast have very small ones.
- Some places do not fit the two-bulge pattern at all, which brings us to tide types.
This is also why predictions are made by fitting a model to a real tide gauge record at each port, rather than by calculating from astronomy alone. How that works.
05 · Classification
The three tide types
Diurnal
One high and one low per day, about 24 h 50 min apart. Two events a day in the table. Found in parts of northern Australia.
Mixed
Two highs and two lows, but noticeably unequal in height. Four events a day, with the two highs differing, sometimes considerably. Common across southern and western Australia.
Diurnal inequality
In a mixed tide, the difference between the two highs on the same day is called the diurnal inequality. It arises because the once-daily and twice-daily components of the tide are adding on one cycle and partly cancelling on the next.
It varies with the moon's declination, so the inequality itself grows and shrinks over a fortnight. When people report that "the morning tide is always bigger here", they are describing diurnal inequality, and the "always" is usually wrong: it swaps over through the month.
The practical point is simple. If your table shows two highs of different heights, that is not a mistake in the table. Read the one you actually need rather than assuming they are interchangeable.
06 · Australian exception
Where in Australia you get one high tide a day
The Gulf of Carpentaria is the best known Australian region with a predominantly diurnal tide: one high and one low in a day rather than two. Parts of the adjacent northern coast show strongly diurnal or heavily mixed behaviour as well.
This has direct consequences for anyone reading a tide table there:
- Your table has about half the rows. Two events a day, not four.
- The water takes about twelve hours to rise, not six. The rule of twelfths, which assumes a six hour rise, will be badly wrong.
- The fortnightly cycle is driven by the moon's declination, not its phase. The biggest tides do not reliably fall on the full or new moon. Why.
Any tide resource that flatly states "there are two high tides every day" was not written with Australia in mind. It is a useful test to apply to a site before trusting it with anything else.
07 · Practical
How many rows should a day have?
Not always four. Legitimate reasons a day shows a different number:
- Diurnal port: two events, one high and one low.
- The 50 minute drift: because the cycle is 12 h 25 min and the calendar day is 24 h, a high water occasionally falls just before midnight or just after, so a given calendar day can show three events instead of four. Nothing is missing; the fourth landed on the next day.
- Daylight saving changeover: on the day the clocks move, the interval between rows will look an hour wrong. The tide did not change; the clock did.
If a day shows an unexpected count, check these three before concluding the data is broken. In our experience the data is usually fine and the calendar is doing something reasonable.