Spring tides and neap tides

What is a spring tide?

A spring tide is the large tide that comes round at new Moon and at full Moon. At those two phases the Sun, the Earth and the Moon lie roughly on one line, so the tide the Sun raises and the tide the Moon raises arrive in the same place at the same time and add together. High water reaches further up the beach, low water drops further down it, and the currents running between the two are faster.

The Sun’s tide-raising force is about 46 per cent of the Moon’s — not enough to set the rhythm, but easily enough to change the size of it. Adding nearly half of the Moon’s tide, and then taking it away again a week later, is the whole of the fortnightly swing between big tides and small ones. A spring tide is not a different kind of tide: it is the ordinary twice-daily tide with both forces pulling the same way.

The name has nothing to do with the season. It is the older sense of the word — springing forth, as in a spring of water — and NOAA is explicit that spring tides come twice each lunar month, all year, regardless of season.

What is a neap tide?

A neap tide is the small tide around the first and last quarter Moon. Seen from the Earth the Sun and the Moon are then about 90° apart, so the Sun is pulling water across the Moon’s bulge rather than into it, and part of the lunar tide is cancelled. Highs are lower, lows are higher, and the day moves less water in total.

On a neap the sea genuinely does very little. A shallow bar that opens at the bottom of a spring low may not uncover at all; a mark that fishes on a hard run can sit in near-slack water for most of the ebb; the strandline barely moves. Nothing is wrong with the tide table — that is what a neap looks like.

Neaps matter for the same reasons springs do, in reverse. If you need clearance over a sill, a bar or a slipway, a neap high water may not give it to you. If you would rather work a boat, dive a wreck or fish a channel without fighting the current, the neap week is the one to pick. Leave a margin either way: every height on this site is astronomy only, and pressure and wind can move the real water level by half a metre or more in either direction.

Spring and neap tides diagram

The mechanism is one piece of geometry: where the Moon stands relative to the Sun, and what shape that leaves the ocean in.

The spring panel shows syzygy — Sun, Earth and Moon on one line. It happens twice a month: at new Moon, with the Moon between us and the Sun, and at full Moon, with the Earth in the middle. The tide does not care which of the two it is, because a bulge is raised at both ends of the line either way. Both phases give a spring tide, and that is why springs come fortnightly rather than monthly.

The neap panel is quadrature, at the quarter Moons. The dashed outline is the tide the Sun would raise on its own; the solid one is the ocean as it actually stands — stretched towards the Moon, but flattened because the Sun is now pulling across that bulge at right angles instead of into it. Neither force has weakened; they are simply working against each other, and the ocean is left close to round.

When do spring tides occur?

Twice in every lunar month, at new Moon and at full Moon, about 14.77 days apart. Because the cause is astronomy and nothing else, the dates are settled years ahead: there is no forecast to revise and no weather in them.

The water lags the sky, though. On most coasts the biggest tide of the cycle arrives one to two days after the exact new or full Moon — the age of the tide, the time an ocean basin takes to answer a force that has already moved on. The lag is a property of the coast, so it is consistent at your own station and different from one on the other side of the sea. It is also why a trip planned for the night of the full Moon often misses the peak by a day.

The next spring tide peaks on Thursday 10 September (coefficient 104, new Moon) — a king tide.

When do neap tides occur?

Seven days after a spring tide, at the first quarter and at the last quarter Moon. Neaps keep the same 14.77-day spacing as springs and interleave with them, so a coast runs spring, neap, spring, neap through the year without a break.

Nothing switches over on the day itself. The range shrinks with each tide from the spring peak, is smallest at the quarter Moon, then grows again — so the two or three days either side of a neap are nearly as small, and the two or three days either side of a spring nearly as big. Choosing a day is picking a position on a slope, not a side of a line.

How often do spring and neap tides occur?

The Moon takes 29.53 days to go from new Moon back to new Moon. Springs happen at both ends of the Sun–Earth–Moon line, so they arrive at half that interval — every 14.77 days, which comes to roughly 24 or 25 spring tides in a calendar year, with as many neaps between them.

Spring tides occur twice each lunar month, year-round, regardless of season. — NOAA

Three rhythms run at once here, and they are easy to confuse. The tide itself repeats every 12 hours 25 minutes on a semidiurnal coast. The pair of tides slides about 50 minutes later each day, because a lunar day is 24 hours 50 minutes long. On top of both, the range swells and shrinks on the 14.77-day spring–neap cycle. Only the third of those is what people mean by spring tides.

Spring tide vs neap tide

The same coast and the same tide table, one week apart:

Neither is the better tide; they suit different days out. What the table cannot tell you is how much water either one means where you are, because that is set by the shape of your coast rather than by the Moon — a spring tide is a few centimetres in the Baltic and about 16 metres in the Bay of Fundy.

Perigean springs and king tides

Springs are not all the same size, and the reason is the Moon’s distance. Its orbit is an ellipse: 384,400 km on average, as close as about 356,000 km at perigee and as far as about 406,700 km at apogee. Tide-raising force falls off as the cube of the distance, so a perigee Moon raises a tide roughly a fifth bigger than an average one, and an apogee Moon about a sixth smaller.

Perigee comes round about every 28 days, close to the 29.53-day phase cycle but not equal to it, so the two drift in and out of step over the year. When perigee falls near a new or full Moon the result is a perigean spring tide — NOAA puts these at often between six and eight times a year, running more than a foot higher than the springs that land near apogee, and three feet or more somewhere like Anchorage, where the range is enormous to begin with. Those are the tides most people mean by king tides.

How to see it on this site

You do not have to follow the Moon yourself. Every date on this site carries a tidal coefficient between 20 and 120, worked out from the phase, distance and declination of the Moon and the Sun (how we compute). It is a single number for the size of the tide worldwide that day: springs push it up, neaps pull it down, and a perigean spring pushes it past 100.

Four bands split the scale, and they are the same four colours on every station page:

The coefficient is location-free, and that is its one catch: it tells you the tide is a big one, not how many metres of water that is at your coast. Compare a spring week at Boston, where the mean range is about 2.9 m, with the same week at Plymouth: the coefficients are identical to within a point or two between time zones, and the metres are not. For the metres, read the month table on the station page, which prints the coefficient beside every date so you can pick your day rather than take the day you are given. The tidal coefficient guide goes through the number in more detail.

Questions people ask