The regular rise and fall of sea level, caused mainly by the gravitational pull of the Moon and the Sun on the ocean waters, combined with the centrifugal effect of the Earth-Moon system.
- The Moon's pull is the dominant cause; though far smaller than the Sun, the Moon is much closer, so its tidal effect is about twice the Sun's.
- High tides occur on the side facing the Moon and on the opposite side; most coasts experience two high and two low tides in roughly a day (semidiurnal).
- Spring tides (the highest high and lowest low) occur at new moon and full moon, when the Sun, Earth, and Moon are in a line (syzygy) and their pulls add.
- Neap tides (the lowest high and highest low, a smaller range) occur at the first and third quarters of the Moon, when the Sun and Moon are at right angles and partly cancel.
- The Bay of Fundy in Canada has the world's highest tidal range; in India, the Gulf of Khambhat and Gulf of Kachchh have large tidal ranges suited to tidal power.
The Moon-over-Sun dominance, the spring tide (new and full moon) versus neap tide (quarter moon) distinction, and the Bay of Fundy and Gulf of Khambhat examples are recurring oceanography items.
Spring tide (new and full moon, largest range) versus neap tide (quarter moons, smallest range); "spring" here means to surge, not the season; the Moon, though smaller, has a larger tidal effect than the Sun because it is closer.
The Moon's pull mainly raises tides; spring tides (largest) at new and full moon, neap tides (smallest) at the quarter moons.