Tides
Tides come from differences in the Moon's pull across Earth.
About this topic
Tides are caused by the difference in the Moon's gravitational pull across Earth: the near side is pulled more strongly than the centre, which is pulled more than the far side, stretching the oceans into two bulges. Earth rotating through them gives two high tides daily. The key idea is that tides come from the variation in gravity, not the pull itself.
The Sun adds its own, weaker tide. At new and full moon the Sun and Moon line up, giving large spring tides; at the quarter moons they pull at right angles, giving smaller neap tides. Since the Moon orbits as Earth spins, high tides come about 50 minutes later each day, roughly every 12 hours and 25 minutes.
Worked example: the tidal effect falls with the cube of distance, so although the Sun is vastly more massive, it is so far away that its tide is only about 46% as strong as the Moon's. Coastal funnels such as the Bay of Fundy can amplify the range to over 15 metres.
Tides are used for tidal power, for planning shipping and fishing, and they slowly drag on Earth's rotation, lengthening the day by about 2 milliseconds per century. A common misconception is that the Moon only pulls the near water; in fact there is a second bulge on the far side. Another is that tides are caused by the Moon pulling oceans away from the Earth, when really the Earth itself is also pulled.
Key ideas
- Two bulges: near and far side
- Spring tides at new and full moon
- Neap tides at quarter moons
Quick check
Use cases: where Tides helps
- Homework: review the key idea behind Tides before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Tides without notes.
- Visual learning: play with the interactive lab to see how Tides changes when you adjust its inputs.
- Teaching a friend: explain Tides in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Tides?
Tides come from differences in the Moon's pull across Earth.
What will I learn in Tides?
Two bulges: near and far side. Spring tides at new and full moon. Neap tides at quarter moons.
Is Tides part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Tides lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Tides?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.