Folks who have lived along the coast for a while know that no two tides are quite the same. That’s because the tides are the result of a gravitational tug-of-war between Earth and two other astronomical bodies.
In the absence of complications due to bathymetry, spring tides are exactly at the full and new moons and neap tides are exactly at the one-quarter and three-quarter moon. Credit: Wikipedia The best-known tides are those caused by the Moon. Its gravity pulls a little more strongly on the side of Earth that faces it, producing “bulges” in the oceans -- one on the side of Earth that faces the Moon, the other on the opposite side. As Earth turns on its axis, these bulges of water travel around the globe to stay aligned with the Moon. When the bulges hit land, the water level rises, causing high tides. And half-way between high tides, the water is at its lowest level in the cycle, causing low tides. Because of the Moon’s orbital motion around Earth, the tides peak about 50 minutes later each day.
The height of the tides is also influenced by the Sun. The Sun’s gravity is stronger than the Moon’s, but the Sun is also 400 times farther than the Moon. So the lunar tides are more than twice as strong as the solar tides.
When Earth, Moon, and Sun align -- at full Moon and new Moon -- the gravity of the Moon and Sun combine to create higher high tides and lower low tides, known as “spring” tides. But when the Moon is at first or last quarter, the Moon and Sun are pulling at right angles to each other, so high tides are lower than average, while low tides are higher -- a phase known as “neap” tide -- making every tide a little different from all the others.
