Why are there two high tides a day?

The Moon stretches Earth's oceans into two bulges — one on the near side, one on the far side. Earth rotates through both each day, so most coasts see two high tides and two low tides.

Tidal forces

The Moon stretches Earth into two bulges.

Gravity pulls the near ocean toward the Moon and leaves the far ocean behind. Earth spins through both — two high tides a day. Add the Sun to swing between spring and neap.

Lunar phase

New Moon

Tide type

Spring

Bulge (illustrative)

1.46×

Playback

Moon position

Orbital angle0°

Include the Sun

Solar tide ≈ 46% of the Moon's

Jump to phase

Why two bulges?

Gravity is stronger closer to the Moon. The Moon pulls hardest on the near-side ocean (bulge 1), less hard on Earth's centre, and least hard on the far-side ocean.

From Earth's frame of reference, this means the near-side water gets pulled toward the Moon, and the far-side water is left behind (it's pulled less than Earth itself). Result: two bulges, one on each side.

Earth rotates through both bulges every ~25 hours (the extra hour because the Moon orbits too). That's why coastal areas see two high tides and two low tides per day.

Spring vs neap tides

The Sun also creates tidal bulges, ~46% the size of the Moon's. When Sun + Moon are aligned (New or Full Moon), their bulges combine → very high spring tides.

When Sun and Moon are 90° apart (First/Last Quarter), they partially cancel → smaller neap tides.

This is why coastal flooding during typhoons is often worse during full/new moon — surge stacks on already-elevated spring tides.

Inverse-cube law

Tidal acceleration falls off as 1/r³, not 1/r² like gravity itself — it's the difference in pull across Earth's diameter that stretches the oceans.

That steep falloff is why the Moon wins despite the Sun's enormous mass: the Sun is ~390× farther away. Run the numbers and the Moon's tidal force comes out about 2.2× the Sun's.

What about the Earth-side?

The solid Earth itself flexes about 30cm from tidal forces (called solid earth tide). The ocean's response is bigger because water flows. Continental coastlines amplify or dampen the effect based on bathymetry — the Bay of Fundy in Canada gets 16-metre tides; the Mediterranean barely 50cm.

The Moon is also slowly stealing Earth's rotational energy through tidal friction — Earth's day is lengthening by about 1.7 milliseconds per century, and the Moon is drifting away at 3.8 cm/year.

Citations

Pugh & Woodworth (2014) — Sea-Level Science, 2nd ed., Cambridge.
Doodson (1921) — harmonic analysis of tidal constituents, Proc. R. Soc. A 100, 305.
Cartwright (1999) — Tides: A Scientific History, Cambridge.
NOAA Tides & Currents — current global tidal predictions.
Williams & Boggs (2016) — Lunar laser ranging on Earth-Moon recession (3.83 cm/yr), Cel. Mech. Dyn. Astron.

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