What Causes Tides: Gravity, the Moon, and Moving Oceans

Most people learn that the Moon’s gravity pulls the ocean toward it, creating a bulge of water on the side of Earth facing the Moon. That is half right, which is why the next question — “then why is there also a high tide on the opposite side of Earth?” — stumps most people. The full answer requires understanding not just gravity but the gradient of gravity.

The tidal force: it is a difference, not a pull

Gravity weakens with distance according to an inverse square law. The side of Earth closest to the Moon is about 12,700 kilometres closer to the Moon than the centre of Earth, and the far side is 12,700 kilometres further away. These distances matter because they mean the Moon’s gravitational pull on the near side is slightly stronger than on Earth’s centre, and pull on the far side is slightly weaker.

The tidal force is what you get when you subtract out the average pull (the pull at Earth’s centre, which governs Earth’s overall orbit around the Moon). What remains is a residual force that points toward the Moon on the near side — because the near side is pulled more strongly than average — and points away from the Moon on the far side, because the far side is pulled less strongly than average.

These residual forces stretch Earth and its oceans along the Earth-Moon axis. The result is two tidal bulges: one pointing toward the Moon, one pointing away. This is why two high tides occur in a day. As Earth rotates under these bulges over roughly 24 hours, any given coastline experiences two high and two low tides.

Why the cycle is 24 hours 50 minutes, not 24 hours

If Earth simply rotated under two stationary bulges, the tidal cycle would match the 24-hour day exactly. But the Moon is also orbiting Earth, moving about 13 degrees further along in its orbit each day. For a point on Earth to catch up to where the Moon was — and thus complete a tidal cycle — Earth needs to rotate a little extra. This adds about 50 minutes, giving a tidal day of roughly 24 hours 50 minutes, which is why high tides arrive about 50 minutes later each day.

The Sun’s role

The Sun is vastly more massive than the Moon, but it is also vastly further away. Because tidal forces depend on the gradient of gravity (how much gravity changes across a distance), and the Sun is so far away that the gradient across Earth’s diameter is proportionally smaller, the Sun’s tidal effect is only about 46 percent as strong as the Moon’s despite its enormous mass.

The two effects combine in predictable ways. When the Sun and Moon are aligned — at new moon and full moon — their tidal forces add together, producing the higher-than-average spring tides (the name comes from the Germanic word for “to jump,” not the season). When the Moon is at a right angle to the Sun as seen from Earth — at the quarter moon phases — the forces partially cancel, producing the more moderate neap tides.

Why tides vary so much by location

If tides were purely a matter of the tidal force acting on a global ocean, the water rise everywhere would be modest and roughly equal. In practice, tidal ranges vary dramatically: the Bay of Fundy in Canada regularly sees tides of over 16 metres, while some locations in the Mediterranean barely notice tides at all.

This is because the actual behaviour of tides depends heavily on ocean basin geometry, depth, and resonance. Ocean basins have natural oscillation frequencies. When the tidal forcing frequency is close to a basin’s natural frequency, resonance amplifies the tide enormously. The Bay of Fundy is the classic example: its length and depth create a natural sloshing period of about 12.4 hours — almost exactly half the tidal day — which drives the tidal range to extremes.

In semi-enclosed seas like the Mediterranean, the connection to the open ocean is too restricted to allow much tidal exchange, so the tide barely registers. Coastline shape, continental shelf depth, and even the Coriolis effect all influence how the tidal energy distributed by the Moon and Sun translates into the local rise and fall any particular beach actually experiences.

Tides and Earth’s rotation

The friction of tidal movement — ocean water sloshing over the sea floor — is gradually slowing Earth’s rotation. The day is lengthening by about 1.7 milliseconds per century. The angular momentum lost by Earth is transferred to the Moon, which is very slowly spiralling outward at about 3.8 centimetres per year. Over geological time, this has made the day significantly longer; ancient coral fossils preserve daily growth rings that suggest days were around 22 hours long 370 million years ago.