Why the Moon Causes the Tides
The Moon's gravity raises the ocean tides—but the explanation is subtler than it seems. Explore the real science of why the tides rise and fall.
Twice a day, the world's oceans rise and fall in the great rhythm of the tides. The cause is famously the Moon—its gravity pulling on the seas. But the popular one-line explanation hides something genuinely subtle. If the Moon simply pulled the water toward it, there would be one bulge of high tide, not two. Understanding why there are two is the key to truly understanding the tides.
Gravity That Varies With Distance
The essential idea is that the Moon's gravitational pull is not the same everywhere on Earth. Gravity weakens with distance, and different parts of the Earth are at different distances from the Moon.
- The side of the Earth facing the Moon is closest, and feels the strongest pull.
- The center of the Earth feels an intermediate pull.
- The side facing away from the Moon is furthest, and feels the weakest pull.
This difference in pull across the Earth—not the pull itself—is what creates the tides. Forces that arise from such differences are called tidal forces.
Why There Are Two Bulges
Now the two high tides make sense.
On the side facing the Moon, the water is pulled toward the Moon more strongly than the solid Earth as a whole. So the water is drawn ahead, away from the Earth, forming a bulge toward the Moon.
On the side facing away from the Moon, the situation is reversed. There, the water is pulled more weakly than the Earth as a whole. The Earth, in effect, is pulled away from that water, leaving the water behind in a bulge pointing away from the Moon.
The result is two bulges of high water, on opposite sides of the Earth—one facing the Moon, one facing away. The regions between the bulges experience low tide.
Why Two Tides a Day
As the Earth rotates on its axis, any given coastline is carried through this pattern of bulges. In roughly a day, a location passes through both high-water bulges and both low regions.
This is why most coasts experience roughly two high tides and two low tides each day. The bulges stay roughly aligned with the Moon; the Earth turns beneath them.
The Sun Plays a Role Too
The Moon is the main driver of the tides, but the Sun also exerts a tidal influence—weaker than the Moon's, despite the Sun's vastly greater mass, because the Sun is so much further away.
When the Sun and Moon are aligned, their tidal effects add together, producing especially large tides. When they are at right angles, their effects partly cancel, producing more modest tides. This interplay creates the familiar cycle of stronger and weaker tides through the month.
The Rhythm of a Difference
The tides are a magnificent, planet-scale demonstration of physics—and a lesson in careful thinking. The two daily high tides cannot be explained by a simple pull; they emerge only from the difference in the Moon's pull across the body of the Earth. Understanding the tides means understanding tidal forces, and it transforms a daily rhythm of the oceans into one of the most elegant pieces of science written across the natural world.