Tides & Earth
How the Moon moves the oceans
Twice a day, the sea rises and falls. Drag the Moon around Earth below and watch the two tidal bulges follow it, with the tide at a coastline plotted live.
AI-generated illustration. AI-generated illustration for BotBuhdy.
The tide machine
Drag the Moon (mouse or finger). Press play to spin Earth. The gold pin is a coastline; its water height is plotted below.
Rising tide
+0.00 m
- Next high
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- Next low
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- High to high (2 tides)
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- Today's range
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"Now" places the Moon and Sun where they really are at this moment (Astronomy Engine). Heights use the idealised open-ocean "equilibrium tide", about 0.24 m from the Moon and 0.11 m from the Sun. Real coasts can be very different (see below).
Why two high tides a day?
The Moon's gravity gets weaker with distance. It pulls the ocean on the side of Earth facing it a little harder than it pulls Earth's centre, and it pulls Earth's centre a little harder than the ocean on the far side. The difference stretches the oceans into two bulges: one under the Moon and one directly opposite.
Earth spins through both bulges each day. Meanwhile, the Moon moves along its orbit, so a spot on Earth needs about 24 hours 50 minutes to catch up with it again. That is why most coasts get two high tides and two low tides about every 24 h 50 min, and why high tide comes roughly 50 minutes later each day.
In the open ocean, the real bulge is only about half a metre high. The model above exaggerates it enormously so you can see it.
Spring tides and neap tides
The Sun raises tides too, a bit less than half as strong as the Moon's because it is so much farther away. Try the two buttons in the model:
- Spring tides (new moon and full moon): the Sun, Earth and Moon line up and their bulges add together. High tides are higher and low tides lower. The name has nothing to do with the season; it comes from the water "springing up".
- Neap tides (first and last quarter): the Sun pulls at right angles to the Moon, partly cancelling it, so the range is smaller.
When a full or new moon happens near perigee (a "supermoon"), spring tides are a little larger still, usually by only a few centimetres. That can matter during storms on low coasts.
Real tides are messier
Continents get in the way of the bulges, so the real ocean sloshes around in huge rotating systems, each circling a point with almost no tide. The shape of each bay can amplify the slosh:
In the Bay of Fundy, the natural back-and-forth time of the water in the bay is close to the 12 h 25 min tidal rhythm, so each tide pushes the water higher, like pushing a swing in time.
The tides are slowing Earth down
Tick Earth drags the bulge in the model. Earth spins faster than the Moon orbits, and friction carries the bulges slightly ahead of the line to the Moon. Two things follow:
- Earth's spin brakes. Tidal friction alone would lengthen the day by about 2.3 milliseconds per century. Other effects, such as the ground still rebounding after the last ice age, partly offset it, and the measured average over the last 2,700 years is about 1.8 ms per century.
- The Moon moves away. The leading bulge tugs the Moon forward, boosting it into a higher orbit. Lasers bounced off Apollo reflectors show it recedes about 3.8 cm a year.
Layered rocks laid down by ancient tides record how fast Earth used to spin. About 620 million years ago, a day lasted roughly 21.9 hours and a year had about 400 days (Williams, 2000).
The Moon also helps steady Earth's tilt. Today the tilt wobbles only between about 22.1° and 24.5° over 41,000 years. Calculations by Jacques Laskar and colleagues (1993) suggested that without the Moon it could swing much more widely; later work (Lissauer and colleagues, 2012) found such swings would be slower and more limited than first thought. The Moon helps, but it is not the only reason Earth's climate is stable.
Quick answers
What causes tides?
Mostly the Moon. Its gravity pulls harder on the near side of Earth than on the far side, stretching the oceans into two bulges. The Sun adds a smaller effect. Earth rotates through the bulges, giving most coasts two high tides a day.
Why are there two high tides a day and not one?
There are two bulges, one facing the Moon and one on the opposite side, and Earth turns through both in about 24 hours 50 minutes.
Why is high tide about 50 minutes later each day?
While Earth spins once, the Moon moves about 13° along its orbit, so Earth needs about 50 extra minutes to face it again.
What are spring tides and neap tides?
Spring tides happen around new and full moon, when the Sun and Moon line up and the tidal range is largest. Neap tides happen around the quarter moons, when their pulls partly cancel.
Where are the biggest tides in the world?
In the Bay of Fundy in eastern North America, where the range can reach about 16 m because the bay resonates with the tide.
Is the Moon making the day longer?
Yes, slowly. Tidal friction slows Earth's spin, and the measured day length has grown by about 1.8 milliseconds per century on average. About 620 million years ago, a day lasted about 21.9 hours.
Sources and credits
- NOAA National Ocean Service, Tides and water levels tutorial and What is a spring tide?
- NOAA, Where is the highest tide? (Bay of Fundy).
- NOAA, Perigean spring tides.
- Stephenson, F. R., Morrison, L. V. and Hohenkerk, C. Y. (2016), Measurement of the Earth's rotation: 720 BC to AD 2015, Proceedings of the Royal Society A 472.
- Williams, G. E. (2000), Geological constraints on the Precambrian history of Earth's rotation and the Moon's orbit, Reviews of Geophysics 38.
- Laskar, J., Joutel, F. and Robutel, P. (1993), Stabilization of the Earth's obliquity by the Moon, Nature 361; Lissauer, J. J., Barnes, J. W. and Chambers, J. E. (2012), Obliquity variations of a moonless Earth, Icarus 217.
- NASA Science, Tides; Williams, J. G. and Boggs, D. H. (2016), Secular tidal changes in lunar orbit and Earth rotation, Celestial Mechanics and Dynamical Astronomy.
- Positions for the "Now" button: Astronomy Engine by Don Cross (MIT licence).
Photos credited to NASA, NOAA and other public-domain sources are used with thanks; BotBuhdy adds a small botbuhdy.com mark. Pictures labelled AI-generated illustration are artist-style illustrations made for this page, not photographs.