What Causes Ocean Tides?

If you spend a day at the beach, the water's edge can move a long way. Hours later it comes back. That slow rise and fall is the tide, and it happens on a schedule you can predict years in advance. The reason is gravity, mostly from the moon, with a smaller push and pull from the sun.

1. Gravity is the starting point

NOAA's National Ocean Service explains that ocean tides result from the gravitational attraction of the sun and moon on Earth's oceans. Isaac Newton first described this in 1687.

Newton's law of gravitation says the pull between two objects gets stronger with more mass and weaker with distance. Specifically, it weakens with the square of the distance: twice as far away means one quarter of the pull.

2. Why the moon beats the sun

The sun is enormously more massive than the moon. NOAA says the sun is 27 million times more massive than the moon, and its overall gravitational attraction on Earth is more than 177 times greater than the moon's.

So why does the moon control the tides?

Because tides do not depend on the total pull. They depend on how much that pull differs from one side of Earth to the other, and that difference fades with distance even faster than gravity does. NOAA explains that tide-generating forces vary with the cube of the distance, not the square.

The sun is about 390 times farther from Earth than the moon. Here is how that plays out:

NOAA's conclusion: the sun's tide-generating force is about half that of the moon, so the moon is the dominant force behind Earth's tides.

3. Why there are two high tides a day

Most coasts get two high tides and two low tides each day. NOAA describes Earth rotating through two tidal "bulges," so a given beach passes through high water twice as it turns.

4. Why high tide is about 50 minutes later each day

If tides followed the 24-hour solar day, high tide would come at the same time every day. It does not, because tides follow the moon instead.

NOAA defines a lunar day (also called a tidal day) as the time it takes for a spot on Earth to rotate from directly under the moon back to directly under the moon. That takes 24 hours and 50 minutes. The extra 50 minutes happens because the moon moves along its orbit in the same direction Earth spins, so Earth has to turn a little farther to catch up.

From NOAA's numbers:

• Two high tides and two low tides happen every 24 hours and 50 minutes.
• High tides are about 12 hours and 25 minutes apart.
• Water at the shore takes about 6 hours and 12.5 minutes to go from high to low, or low to high.

So if high tide is at 9:00 a.m. today, expect the next one around 9:25 p.m. and the morning one tomorrow around 9:50 a.m.

NOAA notes there are exceptions: not every coast follows this two-a-day pattern.

5. Spring tides and neap tides

The sun's smaller tidal force does not create a separate set of tides. NOAA explains that it shows up as a change in the size of the moon's tides.

Spring tides. When the sun, moon, and Earth line up, at new moon and full moon, the sun's effect adds to the moon's. High tides are extra high and low tides are extra low. The name has nothing to do with the season.

Neap tides. About a week later, when the sun and moon are at right angles to each other as seen from Earth, the sun's effect partly cancels the moon's. The result is moderate tides, with a smaller difference between high and low.

Each lunar month has two sets of spring tides and two sets of neap tides.

6. Distance changes tides too

The moon's orbit is not a perfect circle. NOAA says the distance between Earth and moon varies by about 31,000 miles over a month.

• Perigee (moon closest, once a month): tide-generating forces are stronger, and the tidal range is above average.
• Apogee (moon farthest, about two weeks later): forces are weaker, and the range is below average.

Earth's distance from the sun matters as well. Around January 2, when Earth is closest to the sun (perihelion), tidal ranges are enhanced. Around July 2, when Earth is farthest (aphelion), they are reduced.

7. Putting it together

On any given day, the tide at a beach depends on all of these at once:

1. Earth's rotation through the two tidal bulges, giving roughly two highs and two lows per lunar day
2. The moon's phase, which decides whether the sun is helping (spring tides) or working against the moon (neap tides)
3. The moon's distance, which strengthens or weakens its effect
4. Earth's distance from the sun, which adds a smaller yearly variation

8. Key takeaways

• Tides come from the gravitational pull of the moon and sun on Earth's oceans.
• Tide-generating force fades with the cube of distance, so the nearby moon has about twice the tidal effect of the far more massive sun.
• A lunar day is 24 hours and 50 minutes, so high tides are about 12 hours and 25 minutes apart and arrive later each day.
• Spring tides, with the biggest range, happen at new and full moon; neap tides happen in between.
• Tides are also larger when the moon is closest to Earth and in early January, when Earth is closest to the sun.