How Does the Moon Really Cause Waves?
By: Sam Westreich
Tags: Environment, Astronomy, Nature, Science, Oceans

Look, I know that the reason why the ocean has waves is because of the Moon. I learned that back in elementary school, and I’ll state it with full confidence.

But if you asked an innocent follow-up question, my confidence would unravel. I know that there’s some connection, that there’s some sort of link between the phases of the moon and the strength of the tides. Didn’t sailors used to know about the tides by looking at the moon?

How are tides caused? Does the Moon pull harder at some times to make stronger waves? Are tides in freshwater lakes, or just in the ocean? Why does the tide go in and out once per day in some places, but multiple times per day in other places? What about the Sun? Shouldn’t the Sun’s gravity also cause tides?

Let’s learn about tides! Here’s how they are caused, what the Moon (and maybe the Sun) have to do with shifting water, and what else can contribute to the splashy waves that we see on the beach.

1. How are tides caused?

The first thing to understand is that not all waves are equal.

There are two types of waves — surface waves, which is what we see crashing on the shores of the ocean every few seconds — and long-period waves, which take hours to slowly hit the land.

The waves that we see on the surface aren’t caused by the pull of gravity, but are driven by wind. These surface waves can still travel very long distances, even across an entire ocean, but they’re mostly on the surface. These waves don’t actually carry water over long distances, but instead transmit the energy of the wind through the rise and fall of the water.

Surface waves, caused by the wind, not by gravity. Photo by Jeremy Bishop on Unsplash

Long-period waves, on the other hand, are the waves caused by the pull of other celestial bodies on the Earth. These waves move the entire ocean, and reach deep below the surface.

When a long-period wave hits the shore, it takes hours for the full wave to exhaust itself against the land, and it pushes the entire ocean against the shore, making the water rise. We call the ebb and crash of these long-period waves against the shore tides.

2. Does the Moon pull harder at some times to make stronger waves?

Some tides are stronger than others — the strongest tides, with the highest rise and the lowest fall of the ocean, occur during a full moon or during a new moon.

However, this isn’t because of just the Moon itself, but instead is due to the interaction of the Moon and the Sun.

Long-form waves, which we know as tides when they hit the shore, are caused by the gravitational pull of other celestial bodies, most notably the Moon (because it’s closest) and the Sun (because it’s very large and heavy).

And when the Moon and Sun happen to be pulling in the same direction, we get the strongest tides. This happens during a new moon (when the Moon is directly between the Sun and Earth), and during a full moon (when the Earth is directly between the Moon and the Sun).

In this new Moon situation, where the Moon and Sun are aligned, their gravitational pulls are both in the same direction and we get stronger tides. Source.

The majority of the strength of the tides comes from the Moon, as it is much closer, but the Sun still contributes somewhat to influencing tidal strength.

3. Are tides in freshwater lakes, or just in the ocean?

Since tides are caused by the gravitational pull of celestial bodies, mainly the Moon, they aren’t just limited to oceans! Theoretically, a lake can have tidal effects as well.

However, because there’s a lot less water in a lake, pond, or other freshwater body, the gravitational waves that pull on the water are much smaller, and the tides are much less noticeable. Very large lakes, like the Great Lakes in the northeastern United States, do see a regular tidal effect, but it’s much smaller than ocean tides.

4. Why does the tide go in and out once per day in some places, but multiple times per day in other places?

Ah, now things start getting more complex. Someone may ask this question in Britain, for example, where vacationers at seaside resorts enjoy a regular two tides per day.

In some locations, the spinning of the Earth and Moon generates centrifugal force, which also pulls on the water on the Earth’s surface. This leads to additional tides.

The first tide occurs when Britain faces the Moon, and the pull of the Moon’s gravity is strongest. The second tide occurs when Britain faces away from the Moon, and centrifugal force pulls the water higher.

Essentially, as the Earth rotates, this means that sometimes multiple tides stack up in some regions, while other regions may have less frequent tides, or sometimes almost no tidal activity at all!

5. What about the Sun? Shouldn’t the Sun’s gravity also cause tides?

As mentioned above, in point #2, the Sun does have an effect on tides as well! But despite being much more massive, the Sun is much further away than the Moon, and so its effect is reduced.

The strength of the Sun’s gravity is about 177 times that of the Moon — but it’s also 390 times as far away. When these two numbers are combined, we end up with the Sun having about half the gravitational strength of the Moon, when it comes to tides.

As mentioned above, this is why tides are strongest during a full moon or a new moon! At those times, the Sun’s gravitational pull is along the same plane as the Moon’s pull, and thus we get strongest tides.

These tides, by the way, are called spring tides — not named because they occur in the spring, but because this is when the water most strongly “springs forth”.