How do tides work?

The principle

Tides. Think of the sea as a loose blanket of water wrapped around Earth. The Moon tugs at it, and as Earth turns underneath, every beach slides in and out of the deep parts of the blanket.

Step 1: The Moon pulls the sea. The Moon’s gravity pulls on everything on Earth, but it pulls harder on things that are closer. The water on the side facing the Moon is pulled the most, so it heaps up into a gentle bulge. It’s like a crowd leaning towards a stage: the people at the front lean the most.

Step 2: Two bulges, not one. The water on the far side of Earth is pulled the least, less than the solid Earth itself. So the Earth is pulled away from it a little, and that water gets left behind as a second bulge. Imagine three runners tied loosely together and the front one sprints: the line stretches out at both ends. In between the two bulges the sea is lower: that’s low tide.

Step 3: Earth spins through them. Earth turns once a day, so your beach passes through both bulges: about 2 high tides and 2 low tides a day. But the Moon also moves along its orbit in the same direction, so Earth needs about 50 minutes extra to catch up with it. That’s why high tides come about every 12 h 25 min, and about 50 minutes later each day.

Step 4: The Sun helps, or not. The Sun is about 27 million times heavier than the Moon, but about 390 times farther away, so its tide effect is only about half the Moon’s. Twice a month, at new and full Moon, Sun and Moon line up and their pulls add up: high tides are higher and low tides lower (spring tides, nothing to do with the season). About a week later they are at right angles and partly cancel out: smaller neap tides.

Step 5: At the beach. You don’t see bulges, you see the water creep up the sand for about 6 hours and then back down for about 6 hours. How big the difference is depends hugely on the coastline: a few centimetres in some places, many metres in narrow bays.

Spring tide or neap tide?

• Spring tide: Sun and Moon in a line (new and full Moon). Upside: the lowest low tides, great for walking the shore. Downside: the highest high tides, with more risk of flooding in storms.
• Neap tide: Sun and Moon at right angles (half Moon). Upside: calmer, smaller changes in the water level. Downside: less of the beach is uncovered at low tide.

Try it yourself: look up your nearest harbour’s tide table for a week. The high tide moves about 50 minutes later each day, and the biggest tides land near full and new Moon.