A surfer checking the tide chart before paddling out probably isn't thinking about the Moon, Earth's rotation, or the mechanics of the solar system.
They're thinking about whether the reef will have enough water over it, whether the sandbar will finally start working, or whether the waves are going to improve as the tide comes in.
Fair enough.
But there's something about that tide chart that most surfers probably don't know.
The tides are slowly changing the length of Earth's days.
NASA notes that tidal interactions have been slowing Earth's rotation for billions of years. And we have geological evidence showing just how different Earth once was.
For example, sedimentary records in South Australia indicate that around 620 million years ago, a day on Earth lasted only about 21.9 hours.
So, yes, your tide chart is connected to a planet that is very, very slowly changing.
How does that happen?
The ocean acts like a brake
You probably already know the basic story.
The Moon's gravity pulls on Earth and its oceans, creating the tidal bulges responsible for the rise and fall of sea level.
But there's a little more going on.
Earth is rotating once roughly every 24 hours, while the Moon takes about 27.3 days to orbit Earth.
The two movements don't line up perfectly, and the tidal bulges are dragged slightly ahead of the Moon as Earth spins.
Then comes the ocean itself.
Tidal water doesn't move around a smooth, frictionless planet. It encounters coastlines, shallow seas, continental shelves, underwater ridges, and the seafloor.
All that movement generates friction and dissipates some of the energy involved in Earth's rotation.
Think of it as a tiny brake on the planet. It isn't a brake you'll ever feel. Your surf session won't suddenly become 23 hours and 59 minutes long.
But over geological time, even a tiny effect can become significant. And that's where things get really interesting.
Earth's days are getting longer
Tides and Earth's rotation might seem like two completely separate things.
They aren't.
Some of the rotational energy that keeps Earth spinning is transferred into the tidal system and eventually dissipated as heat. As a result, Earth's rotation gradually slows.
And when Earth spins more slowly, the length of the day increases. The change is incredibly small on a human timescale.
Nobody is going to check the clock halfway through a surf and discover that Tuesday has mysteriously gained an extra minute.
But go back hundreds of millions of years and the difference becomes much easier to see.
Earth used to rotate faster.
That means a year contained more days than it does today - not because Earth needed longer to travel around the Sun, but because each day was shorter.
The ancient Earth was, quite literally, spinning faster. And that brings us back to the water washing over the reef in front of you.
Every tide is part of the same process.
And the Moon is moving away
There's another remarkable side effect.
As Earth's rotation loses energy through tidal interactions, some of that energy is transferred to the Moon's orbit.
The Moon responds by gradually moving farther away from Earth.
Today, the Moon is receding at roughly 3.8 centimeters per year. That's not exactly something you'll notice during your next dawn patrol.
At that rate, the Moon only moves about as far as the length of a fingernail in a year. But geological time has a way of making tiny numbers much more interesting.
Over hundreds of millions of years, centimeters become kilometers.
So two things are happening at the same time: Earth is gradually spinning more slowly, and the Moon is gradually moving farther away.
The tides are part of the connection between them.
Not bad for the water level you were checking before paddling out.
The tide that shapes your surf session
Of course, surfers experience tides in a much more immediate way.
A few centimeters or tens of centimeters of water can make a huge difference to a wave.
A rising tide can cover a shallow reef, change the depth over a sandbar or alter the way a wave breaks.
A falling tide can expose rocks, change currents and turn an average-looking lineup into something completely different.
But there's no universal "high-tide wave" or "low-tide wave."
The Moon may be responsible for the gravitational force behind the tides, but the coastline and seafloor decide how that tidal energy behaves locally.
Continental shelves, underwater ridges, reefs, bays, estuaries, and the shape of the coastline all influence how tidal waves travel through the ocean.
That's why two peaks relatively close to each other can have very different tidal ranges and timing.
It's also why local knowledge matters.
Locals and experienced surfers don't just know whether a spot works on a rising or falling tide. They know how that particular wave responds to changing water levels.
Some breaks wake up as the tide fills in; others start firing as the water drains away. Some have a very narrow window when everything comes together.
Every surf break has its own relationship with the tide.
Rocks that remember ancient tides
And here's where the story gets even more fascinating.
Scientists don't have to rely entirely on computer models or old observations to understand how tides behaved in the distant past.
Rocks can remember them.
Certain sedimentary deposits, known as tidal rhythmites, contain layers produced by repeating tidal cycles.
By studying those patterns, scientists can investigate how Earth's tides behaved hundreds of millions of years ago.
These geological records offer clues about a very different Earth - one where the planet was rotating faster, and the Moon was closer.
In a sense, they're geological tide charts that have been sitting around for hundreds of millions of years.
No surf forecast, smartphone app, or tide table pinned to the refrigerator: just layers of rock recording the rhythm of an ancient ocean.
And if you're a surfer, there's something pretty cool about that.
The same gravitational relationship that helps determine whether your favorite wave works at high tide has been shaping Earth's oceans - and leaving traces in its rocks - for hundreds of millions of years.
So the next time you check the tide before paddling out, remember that you're watching a small part of a planetary process that is slowing Earth down, pushing the Moon away and changing the length of our days.
All while you're waiting for the next set. Isn't that fun and exciting?
Words by Luís MP | Founder of SurferToday.com
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