Topic 2.7: Tides

If you spend a day at a beach, you will see the water slowly come in and go out. These tides are caused mainly by the Moon's gravity, with some help from the Sun's. They are one of the clearest ways to see that the Moon's pull on Earth is real.

Two panels of Earth with its ocean bulges toward and away from the Moon, showing spring tides when the Sun and Moon line up and neap tides when they are at right angles — Spring tides and neap tides.

How the Moon Makes Tides

Gravity gets weaker with distance. The Moon pulls hardest on the side of Earth facing it, less on Earth's center, and least on the far side. The difference in pull stretches Earth slightly in the direction of the Moon. The ocean, which can flow, shows this the most. The result is two bulges of water:

One on the side facing the Moon.
One on the side opposite the Moon.

Earth turns once a day through these two bulges, so most places have two high tides and two low tides each day. Because the Moon also moves along its orbit, the time between high tides is about 12 hours 25 minutes, and the tide times shift about 50 minutes later each day.

Spring Tides and Neap Tides

The Sun also pulls the oceans, about 46 percent as much as the Moon.

Spring tides (nothing to do with the season): at new and full Moon, the Sun, Earth and Moon are in a line. The pulls add up, giving the highest highs and lowest lows.
Neap tides: at first and third quarter Moon, the Sun and Moon pull at right angles and partly cancel. The tidal range is smallest.

Not All Tides Are Equal

The Bay of Fundy in eastern Canada has the biggest tides in the world, up to about 16 meters (53 feet) between low and high, because its funnel shape and size match the tide's natural rhythm.
The Gulf of Mexico has very small tides. Along Alabama's coast the tide is typically only about 0.3 to 0.6 meters (1 to 2 feet), and many days have just one high tide and one low tide (a diurnal tide), instead of two. Wind and storms often change the water level more than the Moon does, which is why hurricane storm surge is so dangerous.

A sea stack at Hopewell Rocks in New Brunswick surrounded by water at high tide — Bay of Fundy, high tide: the water has risen around the rock stacks. Photo: Mhsheikholeslami (CC BY-SA 4.0).

The same kind of sea stack at Hopewell Rocks standing on dry sea floor at low tide — Six hours later, at low tide, you can walk on the sea floor. The Bay of Fundy has the biggest tides in the world. Photo: Mhsheikholeslami (CC BY-SA 4.0).

Tides Do More Than Move Water

Tides rub against the sea floor, which slowly slows Earth's spin (days get longer by about 2 milliseconds per century).
That same interaction pushes the Moon farther away, by about 3.8 centimeters per year, about the speed your fingernails grow.
Tidal pools, salt marshes and estuaries, like Mobile Bay, are full of life that depends on the rhythm of the tide.

What Causes Tides? — NOAA SciJinks

Tides: How Gravity Creates Tidal Patterns — Miacademy

Worth a Pause

The sea has always been a picture of a power we can't control. Job heard it: "Hitherto shalt thou come, but no further: and here shall thy proud waves be stayed" (Job 38:11). The tide's rhythm is another piece of that promise: the sea comes in, and the sea goes out, and the clock for it is the Moon. A fisherman on Mobile Bay and an astronomer with a calculator are both reading the same sky.

Try It: Moon Phase Match. Match each spot in the Moon's orbit to the phase you would see from Earth, then call the tides. Play Moon Phase Match »

Practice Problems

What causes tides?
Why are there two tidal bulges, one on each side of Earth?
About how much later does high tide come each day?
What are spring tides and when do they occur?
What are neap tides and when do they occur?
How is the tide on Alabama's coast different from the tide at the Bay of Fundy?
How are tides slowly changing the Earth-Moon system?

View Practice Problem Solutions →

Next: Topic 2.8: Telling Time by the Sky →