How Does the Moon Affect Tides?

The tide is the ocean answering the pull of the moon and the sun. The moon does most of the work: it is far smaller than the sun, but much closer, and tidal forces fall off very quickly with distance.

Two bulges, two high tides

The moon pulls on the whole Earth, but not equally: the ocean on the side facing the moon is pulled a little more than the centre of the Earth, and the ocean on the far side a little less. The result is that the ocean is stretched into two bulges, one under the moon and one on the opposite side. As the Earth turns, a coast passes through both bulges, which is why most places have two high tides and two low tides a day.

Why high tide is about 50 minutes later each day

While the Earth turns once, the moon moves along its own orbit in the same direction. The Earth needs about 24 hours and 50 minutes to face the moon again: this is the lunar day. So the two high tides are about 12 hours 25 minutes apart, and each high tide comes roughly 50 minutes later than the day before.

The sun's part

The sun raises its own, smaller tide, a bit less than half of the moon's. When the sun and moon are in line, at new and full moon, the two add up and the range is large: spring tides. At the quarter moons they work against each other: neap tides. When the moon is also at its closest point to the Earth, the result is a king tide.

Why some places have one tide a day

The moon's orbit is tilted, so the bulges are often off the equator. A coast then passes through one bulge near its centre and the other near its edge: the two high tides of the day are unequal. In some seas, like the Gulf of Mexico, the shape of the basin also favours the daily rhythm. The result is a mixed tide, with two very different highs or lows, or a diurnal tide, with only one high and one low on some days. Marinalo's tide pages flag these days in the 7-day table.

The shape of the coast changes everything

The bulges are only half of the story. Real oceans are broken up by continents, and the tide travels around each ocean basin as a very long wave. Bays and estuaries can amplify it, as in the Bay of Fundy or the Bristol Channel, where the range exceeds 12 m, or damp it, as in the Mediterranean, where it is often a few tens of centimetres. That is why tide times are calculated station by station from measurements, not from the moon's position alone.

See it in real places: Auckland, Folly Beach, Plymouth, San Diego, Worthing, Townsville, Swansea, Sydney.

The moon and fishing

Many anglers time their trips with the moon: the "solunar" theory says fish feed more when the moon is overhead or underfoot (major periods) and around moonrise and moonset (minor periods). Evidence is mixed, but these periods often line up with strong tidal currents, which do concentrate bait. Every Marinalo tide page has a 7-day table of solunar periods next to the tides.

Questions

Why are there two high tides a day?

The moon's pull is stronger on the side of the Earth facing it and weaker on the far side, which stretches the ocean into two bulges. As the Earth turns, most coasts pass through both bulges each day.

Why are high tides about 50 minutes later each day?

The moon moves along its orbit while the Earth turns, so the Earth needs about 24 hours and 50 minutes to face the moon again.

Does the sun affect tides too?

Yes, at a bit less than half the strength of the moon. When the sun and moon line up the tides are larger (spring tides); at right angles they are smaller (neap tides).

More tide guides

King tides Spring and neap tides Tide times worldwide How we calculate tides

Sources: NOAA Tides & Currents education pages and standard oceanography textbooks; tide dates and heights on this page are calculated by Marinalo from the stations listed on our methodology page. Not for navigation.