Beginner 9 minutes
Tides: how the Moon and Sun move the ocean twice a day
Two bulges, a planet spinning underneath, and a table that decides your dive window
Tide is the only ocean phenomenon that can be predicted years in advance with minute-level precision. Everything else (wind, waves, visibility) is forecast. Tide is calculation, which is why it's the most valuable information there is for planning certain sites.
In this article you'll see why two tides happen each day, what the difference is between spring and neap tides, why the same phenomenon lifts the water sixteen meters in one place and thirty centimeters in another, and how all of that turns into a concrete dive window.
In channels, reef passes, estuaries, and shore entries, tide isn't a detail of the day: it's what decides whether the site can be dived at all, and at exactly what time.
Why there are two, not one
The part almost nobody explains well.
The Moon pulls on the ocean's water, and that pull is somewhat stronger on the side of the planet facing it and somewhat weaker on the opposite side. That difference (not the pull itself, but its variation from one side to the other) is what generates the phenomenon.
The result is two simultaneous bulges of water: one on the side facing the Moon and another on the opposite side, where the water is, so to speak, left slightly behind relative to the rest of the planet. The Earth spins beneath those two bulges.
From that comes what's observed: every point on the coast passes through two zones of high water and two of low water in a bit more than a day. And since the Moon also moves along its orbit, that cycle doesn't last exactly twenty-four hours but about fifty minutes more, which is why tides get later each day.
The Sun does the same thing but with less effect: it's much farther away, and although its mass is enormous, what matters here is the difference in pull between one side of the planet and the other. Its contribution is on the order of half the Moon's.
What's interesting is what happens when the two combine, because that combination is where spring and neap tides come from.
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