Stand on a beach for a few hours and you will watch the sea slowly climb up the sand and then retreat again. Tides are one of the most reliable rhythms in nature, predictable years in advance, yet the explanation behind them is subtler than it first appears. The short answer is gravity, mostly from the Moon. The interesting part is why coastlines usually get two high tides a day rather than one.
The Moon pulls the ocean
Gravity from the Moon reaches across space and tugs on everything on Earth, including the water in the oceans. Because water flows freely, it responds to that pull by bulging slightly toward the Moon. The side of Earth facing the Moon experiences a stronger pull than the planet as a whole, so the ocean there is drawn upward into a bulge. A coastline passing through that bulge experiences a high tide.
The Sun contributes too. It is far more massive than the Moon, but it is also vastly farther away, and what matters for tides is the difference in gravity across the width of Earth. Because of that, the Sun's tidal effect is a little under half as strong as the Moon's. It does not create separate tides but reinforces or partly cancels the Moon's, which is why some tides are larger than others.
Why there is a second bulge on the far side
Here is the part that trips people up. If the Moon pulls the ocean toward it, you might expect a single bulge and just one high tide per day. Instead most places get two. The reason is that a second bulge forms on the side of Earth facing away from the Moon.
The cause is the difference in the Moon's pull across the planet. The Moon tugs hardest on the near-side water, pulling it away from the solid Earth. It tugs least on the far-side water, so the solid Earth is pulled away from that water, leaving it bulging outward on the opposite side. The result is two bulges on opposite sides of the planet. As Earth rotates once a day, a given coastline passes through both bulges, producing:
- Two high tides, roughly twelve hours apart, as the coast rotates through each bulge.
- Two low tides in between, when the coast is at the sides of the planet away from the bulges.
The timing does not land exactly on the clock. Because the Moon is also moving along its orbit, it takes a bit more than 24 hours for a spot on Earth to line up with it again, so tides arrive about 50 minutes later each day.
Spring tides and neap tides
Not all tides are equal. Twice a month the Sun and Moon line up, either on the same side of Earth or on opposite sides, and their tidal effects add together to produce especially large tides. Despite the name, these spring tides have nothing to do with the season; they simply spring up higher. About a week later the Sun and Moon are at right angles as seen from Earth, and their effects partly cancel, giving smaller neap tides.
- Spring tides: Sun and Moon aligned, tidal effects reinforce, the highest highs and lowest lows.
- Neap tides: Sun and Moon at right angles, tidal effects partly cancel, more modest range.
Why real coastlines vary so much
The simple two-bulge picture explains the basic rhythm, but real tides are also shaped by geography. The size of an ocean basin, the shape of a bay, and the depth of the water all influence how high the tide rises and exactly when it peaks. Some places, such as narrow funnel-shaped bays, amplify tides to enormous heights, while some seas barely notice them. A few unusual locations even experience only one high tide a day because of how local water sloshes back and forth.
Because the movements of the Sun and Moon are so regular, tides can be predicted far into the future, which is why tide tables are reliable enough to plan fishing trips, shipping, and coastal navigation around. Behind that everyday convenience lies a beautiful piece of physics: two bulges of water, chasing the Moon around a spinning planet.