Physics
The moon doesn't just pull the ocean toward it, it stretches the entire planet's water into two bulges at once, and the second one is the part most people never learn about.
How the moon's gravity actually moves water
The moon's gravity pulls hardest on whichever side of Earth is currently closest to it, creating a bulge of water on that near side. That part matches most people's intuition. What's less obvious is that Earth's solid body is also being pulled toward the moon, while the ocean on the far side gets left behind slightly, creating a second bulge on the opposite side of the planet at the same time. That's why most coastlines experience two high tides and two low tides roughly every 24 hours and 50 minutes, not one.
The sun contributes to this too, though its effect is smaller, about 46% of the moon's, because while the sun is vastly more massive, it's also far enough away that its gravitational pull on Earth's water is weaker than the much closer moon's. When the sun and moon align during new and full moons, their pulls combine into stronger spring tides. When they're at right angles during quarter moons, they partially cancel into weaker neap tides.
Why the far-side bulge matters
The far-side bulge is the detail that trips people up, because it seems to defy the simple idea that gravity should only pull water toward the moon. It happens because tidal force depends on the difference in gravitational pull across an object, not just the pull itself. Earth's near side feels a stronger pull than Earth's center, and Earth's center feels a stronger pull than the far side, and that difference in pull, not the pull alone, is what stretches the ocean into two opposing bulges.
- The extra 50 minutes in the tidal cycle exists because the moon is also orbiting Earth, so Earth has to rotate slightly further to catch up to the same lunar position.
- Tidal range varies enormously by coastline shape; the Bay of Fundy sees over 50 feet of difference between high and low tide.
- Lakes and other small bodies of water technically experience tidal forces too, just at a scale too small to notice.
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Shop All T-Shirts →Frequently Asked Questions
Why are there two high tides a day instead of one?
Because the moon's gravity creates a bulge on the side of Earth closest to it and a second bulge on the opposite side simultaneously, due to the difference in pull across the planet.
Does the sun affect tides too?
Yes, at roughly 46% the strength of the moon's effect. Its alignment with the moon is what creates stronger spring tides and weaker neap tides.
Why is the tidal cycle 24 hours and 50 minutes instead of exactly 24?
Because the moon is orbiting Earth in the same direction Earth rotates, so Earth needs a little extra time each day to rotate back under the moon's position.
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