The Short Answer
Ocean tides are the regular rise and fall of sea level caused mainly by the gravitational pull of the Moon, with a smaller contribution from the Sun. As Earth rotates, different coasts pass through regions where the water is pulled into a higher level, then out of them again, and the sea level at the shore goes up and down.
Gravity Gets Weaker With Distance
Every object with mass pulls on every other object, and this pull weakens as distance grows. This is the basic idea behind the theory of gravity. The Moon pulls on all parts of Earth, but it pulls harder on the side that faces it than on the center, and harder on the center than on the far side, because the distances are different.
That difference in pull across Earth’s width is what matters for tides. It is called the tidal force. It is a small effect compared with the overall pull, but it is enough to move water, which flows easily.
Why There Are Two Bulges
A common mistake is to think the Moon simply pulls water toward it, creating one bulge. In fact there are two bulges, one on the side facing the Moon and one on the opposite side.
- On the near side, the Moon pulls the water more strongly than it pulls Earth’s center, so water is drawn toward the Moon.
- On the far side, the Moon pulls the water less strongly than it pulls Earth’s center, so the solid Earth is drawn away from the water, and the water is left piled up on that side.
Another way to say this is that Earth and the Moon orbit around a common balance point, and the stretching effect of gravity creates the two bulges. The stretching explanation is the more precise one, and it explains why the far-side bulge is not an oddity.
How the Bulges Become Tides
The bulges stay roughly aligned with the Moon while Earth rotates underneath them. A given coastline is carried through one bulge, then through the low-water region between, then through the opposite bulge, then through another low region. That produces two high tides and two low tides in a bit more than a day, which is the pattern on many coasts.
The cycle takes slightly longer than 24 hours because the Moon moves along its orbit while Earth rotates. A point on Earth needs about 24 hours and 50 minutes to line up with the Moon again. For this reason, the time of high tide moves later by roughly 50 minutes each day.
The water does not actually form two neat hills that the land rotates through. Oceans are shallow compared to their width, the continents block the flow, and the water has friction and momentum. Real tides move as long waves that travel around ocean basins, and their timing at each coast depends on that complex motion.
The Sun’s Role
The Sun is much more massive than the Moon but much farther away, so its tidal effect is smaller, generally somewhat less than half of the Moon’s. It still matters, because the Sun’s and the Moon’s effects add together or partly cancel depending on their alignment.
Spring tides. At new moon and full moon, the Sun, Earth and Moon are roughly in a line. The two effects combine and produce the largest tidal ranges, with higher highs and lower lows. The name has nothing to do with the season; it refers to the water “springing” up.
Neap tides. At the first and third quarter moons, the Sun and Moon pull at right angles to each other. The effects partly cancel, and the tidal range is smaller.
This is why tides follow a roughly two-week rhythm between larger and smaller ranges. The relationship to the monthly changes in the Moon’s appearance is explained further in the topic of Moon phases, and the alignment at new and full moon is also what makes solar eclipses and lunar eclipses possible, though tides do not require an eclipse.
Why Tides Differ From Place to Place
Not every coast has the same tides. Several factors matter:
- Shape of the coast and sea floor. Funnel-shaped bays can concentrate the incoming tide and raise its height. Some places, such as the Bay of Fundy in Canada, are famous for very large tidal ranges, often described as among the largest in the world.
- Size of the ocean basin. Water in a basin can slosh back and forth at a natural rhythm, which can strengthen some tidal patterns.
- Local pattern. Some coasts have two high tides of nearly equal size each day, some have two of clearly different size, and some have only one a day.
- Small enclosed seas. The Mediterranean Sea, for example, is almost enclosed, so its tidal range is small.
Weather also affects sea level. Strong winds and low air pressure can raise water above the predicted tide, which is one reason storm surges can be dangerous. The damaging surges from tropical storms are related to hurricane formation and intensity, though a surge is separate from the astronomical tide.
Do Tides Affect Anything Besides Water?
The same tidal force deforms the solid Earth slightly, by a small amount that is measurable with precise instruments. Tides also create friction as water moves, which very gradually removes energy from Earth’s rotation. As a result, the Moon is slowly moving away from Earth, by a few centimeters a year, and days are lengthening very slightly over long time spans. These are tiny changes in human terms, but they have been measured and are well understood.
Tides matter for life as well. The intertidal zone, the strip of shore covered and uncovered each day, is a home to specialized plants and animals. Fishing, shipping and coastal engineering rely on tide tables, which are calculated from long records of local sea level and the known motions of the Moon and Sun. Ocean movements caused by tides can also affect ocean salinity near river mouths, where fresh water meets the sea.
Lakes and Tides
Large lakes also experience tidal forces, but the effect is very small because a lake is so small compared with an ocean, so it is generally negligible. Tides are noticeable mainly in the oceans and large seas.