The fortnightly cycle

Nothing to do
with springtime.

Spring tides are about alignment, not season. And in northern Australia the fortnightly cycle is not driven by the moon's phase at all.

01 · The mechanism

What makes a spring tide

The tide is raised by the gravitational pull of the moon and, to a lesser degree, the sun. The moon's effect is roughly twice the sun's.

Twice each lunar month, at new moon and at full moon, the sun, earth and moon fall into line. The two tide-raising influences then pull in the same direction and reinforce each other, producing the largest tidal range of the fortnight. That is a spring tide: higher highs and lower lows.

Halfway between those, at first and last quarter, the sun and moon are at right angles as seen from earth and partly cancel. That produces the smallest range of the fortnight: a neap tide, with lower highs and higher lows.

So the cycle runs spring, neap, spring, neap, roughly every seven days between extremes and about every fourteen and a bit days between one spring and the next.

Perigee makes some springs bigger than others

The moon's orbit is not circular. When the moon is nearest the earth (perigee) its pull is stronger. A spring tide that coincides with perigee produces a noticeably larger range than an ordinary spring tide, and these are the ones that end up being called king tides.

02 · The word

Why they are called spring tides

Because the water springs up, in the older sense of springing forth or welling up. It is the same sense as a spring of water.

It has no connection to the season. Spring tides happen twice a month, every month, all year. This is one of the most persistently misunderstood terms in the subject, and it produces the reasonable but wrong assumption that spring tides are a September to November phenomenon.

03 · The part usually left out

The biggest tide arrives after the full moon, not on it

If the tide were a simple, instantaneous response to gravity, the largest range would occur on the day of the new or full moon. It generally does not.

At most places the largest tides of the fortnight arrive one to two days after the astronomical alignment. The delay is called the age of the tide. It arises because the ocean is a physical body of water with inertia, friction and boundaries, and it takes time to respond to the forcing applied to it. The size of the lag depends on the place, and it is effectively a local constant.

This matters practically. If you plan a trip for the day of the full moon because you want the biggest tide, you may well have picked the wrong day by one or two. The tide table for your port already accounts for this, which is the argument for reading the table rather than the moon phase.

04 · What actually changes

Springs and neaps change the range, not the schedule

Going from neaps to springs does not change how many tides you get in a day, and it does not move high water to a wildly different time of day. What changes is the range: the vertical distance between high and low.

At springs the high is higher and the low is lower. More water at the top, less at the bottom, and the water moves faster in between because it has further to travel in the same time.
At neaps everything is compressed toward the middle. The high is lower and the low is higher.

The faster movement at springs is the reason tidal currents are strongest around spring tides, which matters far more for a bar crossing or a channel than the height does. Tides and tidal currents are not the same thing.

There is a smaller timing effect as well: the times of high water shift slightly through the cycle. It is a modest effect compared with the change in range, and again, the table has it built in.

05 · Australian exception

In northern Australia the fortnightly cycle has a different cause

This is the part that generic explainers get wrong for this country.

The spring and neap description above assumes a semidiurnal tide, where two roughly equal highs a day are produced mainly by the twice-daily lunar and solar components. When those two components align you get springs. That holds along much of the Australian east coast.

Where the tide is diurnal, with one high and one low a day, the dominant components are the once-daily ones. Their size is governed not by the moon's phase but by the moon's declination, meaning how far north or south of the equator it sits.

The consequence is a fortnightly cycle that is real but keyed to something else entirely:

The largest diurnal tides occur near maximum lunar declination, when the moon is furthest from the equator. These are sometimes called tropic tides.
The smallest occur when the moon crosses the equator and declination is near zero. These are sometimes called equatorial tides.
Because the moon's declination cycle and its phase cycle have slightly different periods, the biggest tides at a diurnal port do not reliably coincide with the full or new moon.

So the folk rule "big tides at the full moon" is a decent approximation on the east coast and an unreliable one in the Gulf of Carpentaria. If you have moved between those parts of the country and your instincts stopped working, this is why.

Mixed tides, common across southern and western Australia, sit between the two cases and show both influences, which is part of why the two highs on the same day can differ so much. More on tide types.

06 · In practice

Seeing the cycle in a month of predictions

Take any month table for a port and do this:

For each day, subtract the lowest low from the highest high. That is the day's range.
Read down the column of ranges. On a semidiurnal coast you will see a clear wave: a few days of large ranges, a few days of small ones, repeating roughly every fortnight.
Mark the days of new and full moon. The peak ranges should sit one to two days after them, not on them. That gap is the age of the tide at that port.

At a diurnal port the same exercise still produces a fortnightly wave, but the peaks will not line up with the moon phases in the way you expect. That is the declination effect, not an error in the table.