How the Moon makes tides

Most coasts get two high tides a day, about 12 hours 25 minutes apart.

Two bulges, not one

Stand on a beach for a day and the sea comes in and goes out twice. NASA explains the cause like this. The Moon's gravity pulls on every part of Earth, but not equally. It pulls hardest on the side nearest the Moon, less on Earth's centre, and least on the far side. The near-side water is pulled towards the Moon more than the solid Earth is, and the solid Earth is pulled more than the far-side water. The result is a stretch along the Earth to Moon line, with water piled into two bulges: one facing the Moon and one on the opposite side.

The white arrows in the model show this difference in pull, called the tidal force. Under the Moon and on the opposite side the arrows point outward, away from Earth's centre. Halfway between, they point inward. NASA adds a helpful image: like water poured on a table, the ocean is much easier to slide sideways than to lift, so the tidal force mostly pushes water along the surface until it gathers in the two bulges. As Earth turns, each coast passes through both bulges and both low points, which gives two high tides and two low tides a day.

Why the tide is 50 minutes later each day

High tides are not exactly 12 hours apart. While Earth spins once, the Moon moves on along its orbit in the same direction, so a coast needs about 50 more minutes to come back under the Moon. One full cycle under the Moon takes about 24 hours 50 minutes, and the two highs within it are about 12 hours 25 minutes apart. NASA also points out that the bulges are not lined up exactly with the Moon: Earth's spin drags them a little ahead. Real coasts add their own twists, from the shape of bays to the depth of the sea, so you still need a local tide table.

The Sun joins in: spring and neap tides

The Sun raises tides too. NASA says the Sun has about 27 million times the Moon's mass but is about 390 times farther away, which leaves it a little less than half of the Moon's tide-raising force. The reason distance matters so much is that tides come from the difference in pull across Earth, and that difference shrinks with the cube of the distance: 27,000,000 ÷ 390³ ≈ 0.46. At new and full Moon, the Sun and Moon line up and their bulges add, giving extra-high and extra-low tides called spring tides (nothing to do with the season). At first and last quarter they pull at right angles and partly cancel, giving the smaller neap tides.

The model's readout gives the tide size with the Moon alone counted as 1. Adding a Sun tide of 0.46 in line gives 1.46 at spring tide; at right angles the two partly cancel to 0.54 at neap tide, so in this simple model spring tides are about 2.7 times as large as neap tides. Real ratios depend on the coast. Tides also happen in rock. NASA notes that the solid Earth stretches slightly too, and that Earth's pull raises and lowers the Moon's surface by about 10 to 15 cm, enough that laser ranging to the Apollo reflectors has to allow for it.