How does the Moon affect the tides?

The Moon’s pull is not the same everywhere on Earth

Gravity weakens with distance, and the Earth is wide enough for that to matter. The Moon pulls hardest on the water on the side facing it, a little less hard on the solid body of the planet, and least of all on the water on the far side. It is that difference across the width of the Earth — not the pull itself — that raises a tide.

The distinction matters, because the Moon’s gravity is nowhere near strong enough to lift an ocean against the Earth’s own. What it can do is nudge water sideways, along the surface, towards the two points where the Moon is directly overhead and directly underfoot. Countless small sideways nudges, over an area the size of an ocean, pile the water up at those two points. That is the tide: not water being lifted, but water being gathered.

Two bulges, two tides a day

The near-side bulge is easy: the water closest to the Moon is pulled towards it harder than the Earth as a whole is, so it gathers under the Moon. The far-side bulge is the one that catches people out, and it is the same effect read backwards — the Moon pulls the Earth’s centre harder than it pulls the water on the far side, so the planet is drawn out from under that water and it is left standing proud. The ocean ends up stretched along the Earth–Moon line, high at both ends.

The Earth then turns inside that shape. A coast passes under one bulge, into the low between them, under the second bulge and back — two high waters and two low waters in a single lunar day, with about 12 hours 25 minutes between one high water and the next.

That is the textbook picture and no coast obeys it exactly. Continents get in the way, so the tide is not a bulge sliding freely around the planet but a wave running around the edges of each ocean basin, arriving early here and late there. It is why high water does not arrive at the moment the Moon is overhead, and why some coasts get two tides a day, some get one, and some get two of unequal size.

Why high tide is 50 minutes later each day

None of this part is about water. It is bookkeeping between two clocks: the one the Earth’s spin keeps, and the one the Moon keeps as it works its way round the orbit.

The Moon moves about 12° further along its orbit each day, so the Earth has to turn about 12° more than a full circle to bring the same coast back under it — roughly 50 extra minutes. That is a lunar day: 24 hours 50 minutes. The tide keeps the Moon’s time rather than the clock’s, which is why every printed high water slides later day after day, and why a tide chart has to be read as a moving target rather than a timetable you can memorise.

The Moon’s phase: spring and neap tides

The Moon sets the rhythm; the Sun decides how big each turn of it is, and the Moon’s phase is simply the record of where the two are relative to each other.

At new Moon and at full Moon the Sun, the Earth and the Moon lie roughly in line and their two tides add: high water higher, low water lower, currents faster. At the first and last quarters the Sun pulls at right angles to the Moon and cancels part of its tide, leaving the smallest range of the fortnight. Those are spring and neap tides, and they alternate every seven days or so because the whole cycle takes 14.77 days.

A full Moon does not mean the water is high tonight — it means the difference between high and low is large this week. On most coasts the biggest tide of the cycle comes one to two days after the exact phase, because an ocean basin takes time to answer.

The Moon’s distance: perigee and apogee

The Moon’s orbit is an ellipse, not a circle. It averages 384,400 km away, comes as close as about 356,000 km at perigee and goes out to about 406,700 km at apogee, once each orbit. Tide-raising force falls off as the cube of the distance, so those differences count for more than they look: a perigee Moon raises a tide roughly a fifth bigger than an average one, and an apogee Moon one about a sixth smaller.

Perigee comes round about every 28 days and the phases repeat every 29.53, so the two cycles drift against each other through the year. When perigee lands close to a new or full Moon, the biggest predicted tides of the year follow — a perigean spring tide, or as most people say, a king tide.

The Moon’s declination: why the two daily tides differ

The Moon does not track the equator. Its declination — its angle north or south of the equator — swings out to about ±28.6° at the extreme of an 18.6-year cycle, and when the Moon is well north or south the two bulges are no longer symmetrical about the equator. One of them sits north of it, the other south.

A coast at mid-latitude then passes through the thick part of one bulge and only clips the other, so the day’s two high waters are no longer equal. That is diurnal inequality — the higher high water and the lower low water printed on a mixed coast’s tide table. The effect is strongest when the Moon is at its maximum declination (tropic tides) and smallest when it crosses the equator (equatorial tides), so on many coasts the two daily tides pull apart and come back together twice a month.

How much of it you see depends on where you are. At San Francisco the two high waters of a day are plainly unequal; at Pensacola, on the Gulf of Mexico, one of the two tides all but disappears and the coast runs on a single high and a single low a day. Neither is unusual — see types of tides for what decides it.

The 18.6-year cycle is also the reason chart datums are defined over such long periods: the lowest astronomical tide is worked out across about 19 years so that one nodal cycle is contained in it (what is chart datum?).

What about the Sun?

The Sun raises a tide of its own, about 46 per cent as strong as the Moon’s. Mass is not what decides it — the Sun has plenty — but tide-raising force depends on the cube of the distance, and the Sun is very far away. Take the two together and roughly two thirds of the tide is the Moon’s and a third the Sun’s.

That split is why the Moon gets the credit. The Sun is not strong enough to set the timing on its own, but it is easily strong enough to change the size of what the Moon does, twice a month, in both directions. Every spring tide and every neap tide is the Sun’s 46 per cent being added or taken away.

How this site uses the Moon

Every station page is built on a real ephemeris rather than a phase approximation. We compute the Moon’s rise, set, transit and anti-transit, its phase, its distance and its declination for the station’s own coordinates from JPL’s DE421 ephemeris (how we compute), which is the same data an almanac is built from, and none of it needs a network call at the moment you ask for it.

Two of the things you see come straight out of that. The solunar windows are built from the Moon’s daily events — about two hours around its transit and anti-transit, about one hour around its rise and set — and we present them as the rule of thumb they are, not as a law of fishing (how solunar theory works). The tidal coefficient, the 20-to-120 number beside every date, comes from the Moon’s and the Sun’s phase, distance and declination together, which is why it rises at a spring tide and rises further still when perigee joins in (understanding tidal coefficients).

The heights themselves take a longer route. They come from a global harmonic model, which resolves all of this astronomy into a set of fixed frequencies — one for the Moon’s twice-daily pull, one for the Sun’s, one for the ellipse of the orbit, and so on — and measures how strongly your patch of ocean answers each. The Moon in the sky and the number on the chart are the same thing, expressed twice.

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