Ocean Tides: Daily, Spring and Neap Cycles

What causes tides?

Tides are the regular rise and fall of sea level. The main cause is gravity, the force of attraction between objects with mass.

The Moon pulls on Earth with gravity. The Moon is much smaller than the Sun, but it is the nearest large object to Earth. The closer an object is, the stronger its pull. Ocean water can flow, so it responds to the pull and piles up into a bulge.

The Sun is far larger than the Moon, and it also pulls on Earth's oceans. But the Sun is about 400 times farther away than the Moon. The Sun pulls harder overall, but tides depend on the difference in pull from one side of Earth to the other. Because the Sun is so far away, that difference is very small. So the Moon has the stronger effect on tides. The Sun's effect is real, but smaller.

Earth rotates once about every 24 hours. The bulges stay lined up with the Moon, so a place on the coast turns through them. When the place is under a bulge, it has high tide. When it is between bulges, it has low tide.

Why are there two bulges? Earth has one on the side facing the Moon, where the Moon's pull is strongest. There is another on the far side, where the Moon pulls less than on the rest of Earth, so the water is left behind. Either way, a coast passes through two high tides and two low tides.

The daily tide cycle

Most coasts have two high tides and two low tides each day. This is called a semidiurnal pattern. Some places, like parts of the Gulf of Mexico, can have only one of each a day, but two is the usual pattern.

The time between two high tides is about 12 hours and 25 minutes. The Moon moves a little along its orbit while Earth turns, so Earth needs about 50 extra minutes to catch up to the Moon each day. That is why tide times shift later each day.

Here are Aaliyah's water level readings at a dock in Maine. The first high tide is near 3:00 a.m.

The peaks of the curve are high tides and the dips are low tides. The curve repeats in a pattern, which is why tides can be predicted.

Spring tides and neap tides

The Moon's phase changes over about 29.5 days, so the Sun, Earth and Moon change their positions relative to one another. The Sun and Moon both pull on the ocean. Whether their pulls add together or work against each other changes how big the tides are.

Spring tides happen when the Sun, Earth and Moon are in a straight line. This is at new moon and full moon. The pulls add together. High tides are higher and low tides are lower, so the difference between high and low water is largest. (The name has nothing to do with the season.)

Neap tides happen when the Moon, Earth and Sun form a right angle. This is at first quarter and third quarter. The Sun's bulge falls where the Moon's low tide is, so they partly cancel. High tides are lower and low tides are higher, so the difference is smallest.

Using tides in real life

Because tides follow the Moon, we can predict them far ahead. NOAA publishes tide tables for many US coastal towns, showing the time and height of each high and low tide.

Spring tides come about twice each month. The Moon cycle is about 29.5 days, with a new moon and a full moon in each cycle, so spring tides come about $29.5 \div 2 \approx 14.75$, or roughly 15 days apart. Neap tides also come about twice a month, roughly 15 days apart and about a week from the spring tides.

Back to the start: Sofia's beach changed because Earth rotated through the Moon's tidal bulges, giving two highs and two lows each day. The water did not leave; the coast moved from a bulge to a low-tide zone and back.

Summary