Why are there two high tides per day?

The Moon's gravity pulls the ocean on the side of Earth nearest to it into a bulge — that's one high tide. But on the opposite side, the Moon's gravity is weaker there, so the ocean is "left behind" as the Moon pulls Earth's centre more strongly — creating a second bulge and a second high tide. As Earth rotates, most coastal locations pass through both bulges in about 24 hours 50 minutes (slightly more than a day because the Moon moves in its orbit). This is why high tides occur approximately every 12 hours 25 minutes. NOAA's tides explainer covers the full mechanism.

What are spring and neap tides?

Spring tides (highest high tides, lowest low tides) occur at new Moon and full Moon, when the Sun, Earth, and Moon are roughly aligned. The gravitational pulls of the Sun and Moon add together. Neap tides (smallest tidal range) occur at first and third quarter Moon, when the Sun and Moon are at 90° to each other as seen from Earth, and their pulls partially cancel. The Sun's tidal effect is about 46% as strong as the Moon's because, although the Sun is far more massive, it is much farther away and tidal force scales with the inverse cube of distance.

How does the Moon slow Earth's rotation?

Earth's tidal bulges are slightly dragged ahead of the Moon's orbital position by Earth's rotation. The Moon's gravity pulls back on these bulges, creating a torque that gradually slows Earth's rotation — adding about 1.4 milliseconds per century to the length of a day. As Earth loses rotational energy, the Moon slowly spirals outward (it recedes by about 3.8 cm per year). This is the same mechanism that has already tidally locked the Moon to Earth — the Moon rotates once per orbit, always showing us the same face. NASA's delta-T data tracks the accumulated effect on Earth's rotation.

Do tides occur on other planets and moons?

Yes — tidal forces are universal wherever gravitational gradients exist. The most dramatic example is Io, Jupiter's innermost large moon: Jupiter's immense gravity, amplified by orbital resonances with Europa and Ganymede, stretches and flexes Io's entire body, generating enough heat to make Io the most volcanically active world in the Solar System. Europa's subsurface ocean is also kept liquid in part by tidal heating — making it one of the best candidates for extraterrestrial life. See our exoplanet guide for how tidal locking affects habitability of worlds around other stars.

Tides Explained — Moon, Sun & Gravity

Tidal forces — gravity with a gradient

Gravity weakens with distance (following the inverse-square law). The Moon's gravitational pull on the side of Earth nearest to it is slightly stronger than on Earth's centre, which is slightly stronger than on the far side. This gradient — the difference in pull across Earth's diameter — is the tidal force. It acts to stretch Earth (and its oceans) along the Earth–Moon line.

The solid Earth also bulges tidally by about 30 cm, but it's the ocean that moves visibly because liquid can flow. The tidal range varies enormously by location: in the open ocean it is only about 30–60 cm, but in funnel-shaped bays like the Bay of Fundy (Canada) the range reaches 16 metres — the highest in the world — because the bay's natural resonance amplifies the tidal signal.

The Sun's contribution

The Sun also exerts a tidal force on Earth — about 46% as strong as the Moon's. Despite the Sun being 27 million times more massive, it is 390 times farther away, and tidal force scales with the inverse cube of distance (not inverse square like gravity itself), so distance penalises the Sun's tidal effect heavily.

At new Moon and full Moon, the Sun, Moon and Earth align and their tidal pulls add, producing the strongest tides of the month — spring tides. At first and third quarter Moon they pull at right angles, partially cancelling — neap tides. This connection to the Moon's phases means you can predict tidal strength from the lunar calendar.

Tides explained

Tidal forces — gravity with a gradient

The Sun's contribution

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