Topics Earth, weather and climate

How do tides work?

Tides are the slow rise and fall of the sea, caused mainly by the Moon's gravity, with the Sun adding a smaller share. The key is that gravity pulls harder on whatever is closer. The Moon tugs on the near side of Earth more than on its center, and on the center more than on the far side. Those differences stretch the ocean relative to the solid planet.

That is why the simple picture shows two bulges of water, one facing the Moon and one on the opposite side. The far-side bulge is not water thrown away from Earth. It is water left behind, because Earth's center is pulled toward the Moon a little more strongly. This is the simplified equilibrium picture, and it is only a starting point. Real ocean tides also depend on the shape and depth of each basin, on coastlines, and on Earth's rotation, which is why a harbor a few hundred miles away can have a very different tide.

An episode would start with the bulges, then test them against what coasts actually do. It would cover spring and neap tides, why many places get two highs a day and some get one, and where the simple model breaks down. bre's hosts are AI, so they can be wrong, and you can press Talk to ask a question mid-episode.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. Gravity pulls harder on what is closerStart with the Moon's pull and why the difference in pull across Earth matters more than the pull itself.
  2. Why there are two bulges, not oneThe near-side bulge is easy to picture. The far-side one is water left behind relative to Earth's center.
  3. Spring tides and neap tidesWhen the Sun and Moon line up, their effects add and tides run larger. At right angles they partly cancel, and tides are smaller.
  4. Why the real ocean disagrees with the modelContinents, basin shapes, depth and Earth's rotation shape the actual tides, so the two-bulge picture is only an approximation.
  5. Two highs a day, or just oneMany coasts see two highs and two lows daily, others one, and some a mix. Local geography decides which.
  6. Funnels, harbors and big tidesNarrow bays can amplify the water's rise and fall. The Bay of Fundy is the famous example.
  7. What tides mean for peopleSailors, fishers and coastal planners all rely on tide tables, and the practical takeaway is to check them before walking out on tidal flats.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Tess. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Okay, a question I think most of us carry around: why does the sea go up and down twice a day? And the answer starts with the Moon, which is the easy part.

  2. TessAI host

    Easy part, sure. But the Moon is far away. How does it move an entire ocean?

  3. breAI host

    Gravity gets weaker with distance. So the side of Earth facing the Moon is pulled a bit harder than Earth's center, and the center a bit harder than the far side.

  4. TessAI host

    So the pull is uneven. Does that stretch the planet?

  5. breAI host

    Mostly the water, because it can flow. Picture water stretched relative to the solid Earth, so it piles up toward the Moon on one side.

  6. TessAI host

    Okay, but real talk. Why is there a bulge on the far side too? The Moon is pulling the other way there.

  7. breAI host

    Here's the part nobody tells you. The solid Earth is pulled toward the Moon a little more than the far-side water is. So that water gets left behind, and it looks like a bulge.

  8. TessAI host

    And this is a simplified picture, right? Real coasts don't behave that neatly.

  9. breAI host

    Right. Basin shapes and Earth's spin complicate things a lot, and that's where it gets interesting.

Questions people also ask

Why are there two high tides a day?
In the simple picture, Earth rotates through two water bulges, one facing the Moon and one opposite. That gives roughly two highs and two lows each lunar day, a bit over 24 hours. Local geography changes this, and some coasts get only one high a day.
What is the difference between spring and neap tides?
Spring tides, named for leaping and not the season, happen near new and full Moon, when the Sun and Moon line up and their effects add. Neap tides happen near the quarter Moons, when they pull at right angles and the tidal range is smaller.
Why are some tides much bigger than others?
Tidal range depends on the Moon's phase and distance, but mostly on local geography. Basin size, coastline shape and seafloor depth can amplify or dampen the water's movement. Narrow, funnel-shaped bays can produce some of the largest tides on Earth.
Do lakes have tides?
Technically yes, since the same gravitational pull acts on any body of water, but the effect in lakes is tiny. It is too small to notice without sensitive instruments. Wind and air pressure changes move lake water far more than the Moon does.

Related topics

More: all 300 topics, earth, weather and climate, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.