Tides are caused mainly by the Moon’s gravity. As the Moon pulls on Earth, it stretches our oceans into two bulges of water, one on the side facing the Moon and, surprisingly, one on the opposite side too. As Earth spins beneath these bulges, most coastlines sweep through both of them each day, which is why most places get two high tides and two low tides in a little over 24 hours. The Sun plays a supporting role, adding to the effect or softening it depending on where it sits.
That’s the short version. But the details, especially that mysterious second bulge on the far side of the planet, are where tides get genuinely interesting. Let’s wade in.
The Moon Does the Heavy Lifting
Gravity gets weaker with distance, and that simple fact is the whole secret of tides. The Moon doesn’t pull on all of Earth equally. It tugs hardest on the side of Earth closest to it, less hard on the planet’s center, and least of all on the far side.
On the near side, where the Moon’s pull is strongest, ocean water gets drawn toward the Moon and piles up into a bulge. That bulge is a high tide. So far, so intuitive: the Moon is overhead, and the water rises to meet it. If you’d like to appreciate just how much influence our companion has over Earth, our piece on what makes our Moon so special is a lovely companion read.
The Mystery of the Second Bulge
Here’s the part that stumps almost everyone. There’s a second high-tide bulge on the far side of Earth, the side pointing away from the Moon. How can water bulge toward the Moon and away from it at the same time?
The answer is that difference in gravitational pull again. The Moon pulls the solid body of Earth more strongly than it pulls the water way out on the far side. In effect, the Moon tugs the whole planet slightly toward itself and away from that far-side water, leaving the water “behind” in a second bulge. NASA describes the tide as the Moon’s gravity squeezing and sliding Earth’s water around the globe until it piles up on both the near and far sides at once.
Those two bulges are the key to the daily rhythm. Because there are two of them on opposite sides of the planet, any given coastline rotates through a high tide, then a low, then another high, then another low, roughly every 12 hours and 25 minutes.
The Sun Gets a Vote Too
The Moon runs the show, but it isn’t alone. The Sun also raises tides on Earth, and here’s a fact that surprises people: even though the Sun is about 27 million times more massive than the Moon, its tidal pull is less than half as strong.
Why? Distance. The Sun sits roughly 390 times farther away than the Moon, and the tide-generating force falls off incredibly steeply with distance, dropping with the cube of it rather than the usual square. So the Sun’s enormous mass gets overwhelmed by its enormous distance, leaving it with only about 46% of the Moon’s tide-raising power. Still significant, just second fiddle.
Spring Tides and Neap Tides
When the Sun and Moon team up, things get dramatic. Twice a month, at the new moon and the full moon, the Sun, Earth, and Moon line up in a row. Their gravitational pulls combine, producing especially high high tides and especially low low tides. These are called spring tides, and despite the name, they have nothing to do with the season. The word comes from the sense of water “springing” up.
About a week later, at the first and last quarter moons, the Sun and Moon sit at right angles to each other. Now they’re working against one another, and the tides partly cancel out, giving milder highs and lows known as neap tides. That neat connection means you can guess the tides just by glancing at the Moon’s phase. Our Moon phase and supermoon calculator lets you check the phase for any date, and when a supermoon lines up with a spring tide, you get the biggest tides of all.
Why Your Local Tides Are Different
If tides were only about the Moon and Sun, every coast would behave identically. They don’t, because the real Earth is messy.
The shape of coastlines, the depth of the ocean, the width of bays, and even the wind all shape the tides you actually see. Some places get two roughly equal high tides a day, some get two unequal ones, and a few get just one. The narrow Bay of Fundy in Canada funnels water so effectively that its tidal range can top 50 feet, while some spots barely notice any tide at all. There’s also a small daily delay: because the Moon keeps moving in its orbit, each high tide arrives about 50 minutes later than the one the day before. For the full official rundown, NASA’s guide to tides is well worth a read.
So the next time you’re at the beach and the water creeps up over your towel, you’ll know exactly who to blame. It’s the Moon, quietly reaching across a quarter of a million miles of space to move the entire ocean, twice a day, every day, without fail.
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