Learn / Gravity and orbits
Tides
Why there are two high tides a day, why the Moon keeps one face to us, and what keeps Io erupting.
First look
The idea, in plain words
The Moon’s gravity pulls on the whole Earth, but not evenly. The side of the Earth facing the Moon is nearer to it and is pulled a little harder. The far side is further off and is pulled a little less.
That uneven pull stretches the Earth slightly along the line towards the Moon. The oceans, which are free to move, pile up into two bulges: one facing the Moon and one on the opposite side.
The Earth spins underneath the two bulges once a day. Each stretch of coast passes through both, which is why most places get two high tides a day and not one.
The same stretching, over a very long time, slowed the Moon’s own spin until it kept one face towards us. And at Jupiter, tides squeeze the moon Io so hard that its inside melts, making it the most volcanic place we know.
How it works
Cause and effect, with real numbers
It is the difference that counts. A tide comes from the difference in pull between the near and far sides of a body, and that difference falls away with the cube of the distance, much faster than gravity itself. This is why the Moon beats the Sun. The Sun pulls on the Earth about 180 times harder than the Moon does, but it is 390 times further away, and its tide is a little under half as strong.
Spring and neap. At new and full Moon the Sun and Moon pull along the same line and their tides add up. These are spring tides, with the highest highs and lowest lows. At half Moon they pull at right angles and partly cancel, giving the weaker neap tides. The pattern repeats twice in every 29.5 days.
Timing. The Moon moves on round its orbit while the Earth turns, so a place on the Earth takes 24 hours 50 minutes to come back under it. High tides are therefore about 12 hours 25 minutes apart and arrive roughly 50 minutes later each day.
Size. In mid-ocean the bulge is only about half a metre high. Coasts change everything. Where the sea is funnelled into a narrowing bay the water heaps up: the range reaches about 16 metres in the Bay of Fundy in Canada and about 14 metres in the Severn Estuary.
The price. The bulges rub against the sea floor as the Earth turns, and that friction slows the spin. The day gets longer by about 2 thousandths of a second each century. In exchange the Moon is pushed outward. Lasers bounced off mirrors left on its surface by the Apollo crews show it moving away by 3.8 cm a year.
Locked. The Earth raises a tide in the solid Moon as well, and long ago that brought the Moon’s spin to match its orbit, so the same face always points at us. Almost every large moon in the Solar System has ended up the same way.
Io. Io is locked too, but its orbit is kept slightly oval by the regular tugs of its neighbours Europa and Ganymede. So its distance from Jupiter keeps changing, and its tidal bulge keeps rising and falling, by as much as 100 metres of solid rock. The rubbing heats the interior, and Io has hundreds of active volcanoes to show for it.
In the 3D view
The Earth's two bulges with the planet turning beneath them, then Io flexing as it rounds Jupiter.
Not built yet. This lesson will be able to fly you there.
Words used here
- Tide
- The stretching of one body by another's gravity, which pulls harder on the near side than on the far side.
- Tidal locking
- When a moon turns exactly once for each orbit, so the same face always points at its planet.
- Tidal heating
- Heat made inside a moon as changing tides squeeze and stretch it.
- Resonance
- When the periods of two orbiting bodies are in a simple ratio, so they tug each other at the same places again and again.
- Roche limit
- The distance inside which a planet's tides would pull apart a moon held together only by its own gravity.
Worth knowing
The Moon is moving away from the Earth by nearly 4 cm a year, about as fast as fingernails grow.
Io is the most volcanic world known, with hundreds of volcanoes. Some throw lava fountains dozens of kilometres high.
Io goes round Jupiter in 1.77 days, Europa in 3.55 and Ganymede in 7.16: each takes almost exactly twice as long as the one before. The regular tugs that result are what keep Io's inside molten.
Phobos goes round Mars in 7 hours 39 minutes, faster than Mars turns, so it rises in the west. It is creeping inward by about 1.8 metres a century and will be torn apart or hit the ground in about 50 million years.
Charon is half as wide as Pluto. The two keep the same faces towards each other all the time, and the point they both circle is out in the space between them.
Where this comes from
The words are written for this site. Sums in the text are worked from the figures in these sources.