The relationship between the Moon and the tides
Anyone who spends a few hours on a shoreline notices the sea rising and falling. The main author of this steady breathing is the Moon, some 384,000 kilometres away.
Short answer
The main cause of the tides is that the Moon's gravity acts with different strength on different parts of the Earth. The oceans bulge both on the side facing the Moon and on the side directly opposite, which is why most coasts experience roughly two high and two low tides a day. The Sun's contribution is about half that of the Moon.
What creates the tide: the difference in pull
Gravity weakens with distance. The Moon pulls on the face of the Earth turned towards it, on its centre and on its far face with different strengths. What raises the tide is not the pull itself but this difference. Ocean water responds to the difference readily; the solid crust also flexes a little, though we do not feel it in daily life. The tide, in other words, is not a phenomenon peculiar to water; it is the response of anything that can flex to a difference in gravitational pull.
Why are there two bulges at once?
The first bulge is intuitive: the water on the side facing the Moon is pulled more strongly and rises towards it. The second is surprising. The water on the far side experiences the weakest pull and in a sense "lags behind"; the result is a second bulge directly opposite the Moon. Because the Earth turns on its axis beneath these two bulges, a given coast passes through high tide about twice a day.
The Sun's share: spring and neap tides
The Sun is far more massive but also far more distant; its tidal effect is about half that of the Moon. At new moon and full moon the Sun, Earth and Moon line up along the same direction; the two effects reinforce one another and the tidal range grows. This is called a spring tide. At first and last quarter the two effects work at right angles to each other and the range shrinks; that is a neap tide. If you are curious about the lunar phases, see our guide to the phases of the Moon.
How many tides a day, and how far apart?
Because the Moon moves along its orbit while the Earth turns, the same point comes back beneath the Moon not in 24 hours but in about 24 hours and 50 minutes. This is why high tides fall roughly 50 minutes later each day than the day before, and why successive high tides are about 12.5 hours apart. The size of the tide varies a great deal with the shape of the coast, the sea floor and the size of the basin: differences of several metres are seen on ocean coasts, while in relatively enclosed seas such as the Mediterranean and the Black Sea the range in most places does not exceed a few tens of centimetres.
Knowing the tide in advance: tables and practical importance
Because the tide is the product of regular celestial mechanics, it can be calculated in advance; the tide tables published in coastal countries give the times of high and low water for each port days ahead. This information is considered vital by mariners: shallow passages can be crossed only at high water, anchoring depth is worked out against the tide, and walkers on the shore must avoid being caught by the rising sea. Tidal strength varies dramatically with geography: in funnel-shaped bays such as the Bay of Fundy in Canada the difference between the two water levels can reach 15 metres, while on open-ocean islands it may stay under a metre. Some countries also generate electricity from this regular motion; tidal power stations run the flow of rising and falling water through turbines. The theoretical framework behind all of this was built by Newton: once the way gravity changes with distance was understood, the reason for a rhythm observed for thousands of years was understood too.
The human body and the full moon myth
The thought that "most of our body is water, so the Moon must affect us too" sounds plausible at first but does not hold up physically. Tidal force scales with the size of the body affected: the oceans are thousands of kilometres across, so the difference in pull between their ends is significant, whereas the difference between the two ends of a human body is far too small to measure. There is no tide in a glass of water. We take up the cognitive background of the belief that the full moon influences behaviour in our guide to correlation and causation. When you next find yourself by the sea, you can watch the tide for yourself: marking the line the water reaches at the same spot a few hours apart is the most concrete way of seeing celestial mechanics with your own eyes.
Frequently asked questions
Why do some coasts have one high tide a day rather than two?
When the shape of the coast, the depth of the basin and the tilt of the Moon's orbit come together, one of the two high tides is greatly weakened in some regions. On such coasts a single pronounced daily tide is seen.
Do tides occur only in the seas?
No. The Earth's crust also flexes by a few decimetres a day, and tide-like pressure waves form in the atmosphere; but neither is noticeable in daily life.
Why are tides hard to notice on the coasts of Türkiye?
The Mediterranean and the Black Sea are relatively enclosed basins connected to the oceans by narrow straits. In basins where water cannot pile up freely, the tidal range on most coasts stays within a few tens of centimetres.
Is the Moon moving away, and will the tides change?
Yes; because of tidal interaction the Moon recedes by about 3.8 centimetres a year. This makes no difference at all on the scale of a human lifetime, but over millions of years it slowly lengthens the day.
This guide draws on the educational pages of official oceanographic and space agencies (NASA, NOAA) and on standard physics sources.