Tides: why the sea rises and falls

Gravity gets weaker with distance. The side of the Earth facing the moon is therefore pulled slightly more strongly than the Earth's centre, and the far side slightly more weakly. What matters for the tides is not the pull itself but this difference. It draws the water on the near side towards the moon and, in a sense, leaves it “behind” on the far side – so two tidal bulges form on opposite sides.

The Earth turns beneath these bulges once a day. But because the moon moves on along its orbit at the same time, it takes about 24 hours and 50 minutes for a place to return to the same position relative to the moon. On coasts with two high waters a day there are therefore a little over twelve hours between high waters, and every day the times shift by almost an hour.

The sun produces its own tides in the same way. Although it is far more massive, its tidal effect is only about half as strong as the moon's because of its enormous distance. When the sun, Earth and moon are in line – at new and full moon – both effects add up: there are spring tides with particularly high high water and particularly low low water. At the quarter moons they partly work against each other: neap tides with a small difference.

The simple picture of two tidal bulges would produce similar tides everywhere. In reality, however, continents stand in the way of the water masses, and ocean basins have their own shapes and depths. The water sloshes back and forth in them like water in a bathtub. That is why some coasts have two high waters a day, others only one, and others a mixture.

The tidal range, that is the difference between high and low water, also varies greatly from place to place. In funnel-shaped bays the water can pile up high – the Bay of Fundy in Canada is known for one of the largest tidal ranges in the world. In almost enclosed seas such as the Mediterranean, by contrast, the tides are barely noticeable in many places.

On top of this, at many places spring tides do not occur exactly on the day of full or new moon but often a day or two later. All this shows: general rules explain the principle, but for a specific place only the official tide tables count.

Get the high and low water times for the nearest reference port from official tide tables, such as those published by national hydrographic services. Make sure the date, place and time zone are right – summer time can make an hour's difference too.

The water does not rise evenly. A well-known rule of thumb from seafaring, the rule of twelfths, describes this roughly: in the six hours between low and high water, the water rises in the first hour by about one twelfth of the tidal range, in the second by two, in the third and fourth by three each, in the fifth by two and in the sixth by one twelfth. So it rises fastest in the middle. The rule is an approximation and does not apply everywhere.

Plan your return so that you are back long before the tide comes in, not just before. At spring tides the water falls further, uncovering large areas of mudflat – but it also comes back faster and higher. Wind and air pressure can also bring the water earlier than the table says.

Incoming water can cut off coves, headlands and sandbanks faster than expected. On mudflats, channels and gullies fill first – often behind you, between you and the land. If you see water running behind you, turn back at once. Fog on the mudflats can make orientation impossible within a short time. So walk on mudflats only with a local guide.

If you are cut off, do not wade back through deep or flowing water. Tidal currents can be very strong, especially in narrows and around headlands. Find a safe, high spot, make yourself noticed and call the local emergency number (112 in the EU) or the coastguard.

Wind and air pressure change the water level compared with the table. Low air pressure lets the sea rise a little, and onshore wind pushes water against the coast. When a storm coincides with a spring tide, a storm surge can occur in which the water rises far above the table values. Then the authorities' warnings and instructions apply.

Many people wonder why there is also a tidal bulge on the side facing away from the moon. The answer lies in the difference in pull: the Earth's centre is pulled more strongly towards the moon than the water on the far side, which is thus, in a sense, left behind. So tides arise from differences in gravity across the Earth's diameter.

Because the orbits of the Earth and moon are known precisely, tides can be calculated years in advance. Only the weather brings uncertainty into it. And anyone who sees the full moon rising in the evening knows: in these days the tides on many coasts are particularly strong – a good reason to read the table twice as carefully.