Your turn
Each lab is one step from the video. It's all pretend: Earth isn't going to stop.
Each lab isolates one step of the thought experiment. Shapes are exaggerated and the models are simplified.
Tap, drag and slide. It's just pretend!
Drag the pin north or south. Where is the ride fastest?
Move the pin in latitude. Speed ≈ 1,670 km/h × cos(latitude).
Move the pin. Where is Earth's ride fastest?
Stop the merry-go-round fast, then gently. What does the toy car do?
Compare an instant stop with a gradual one. Where does the car's momentum go?
Stop it fast. Then stop it slowly. What does the car do?
Drag the non-spinning Earth around the Sun. When does your home get daylight?
With no rotation, how many sunrises does your home see in one orbit?
Move Earth around the Sun. When is it sunny at home?
Slide the spin down. Where does the water end up?
Reduce the rotation rate and watch the equatorial water bulge relax toward the poles.
Slide the spin down. Watch the water move!
Shapes exaggerated about 150×. The map is a simplified sketch of Esri's model.
Turn the dial. How long is a day at half speed?
Simple model: day = 24 h ÷ spin. How long is a day at 10%?
Drag back in time. How long was a day 1.4 billion years ago?
Scrub back 1.4 billion years. (Three measured points joined by straight lines: a rough guide.)
Slide back to long ago. Were days shorter?
Don't worry: this can't really happen! It's a pretend "what if" game that helps us learn how our Earth works.
Our Earth spins around once every day, and you ride along with it. Near the middle of Earth you go fastest. Near the top you go slower.
Put a toy car on a merry-go-round. If the merry-go-round stops all at once, the car keeps going and slides off! Moving things like to keep moving. That's called inertia. The air and the sea would be like that car.
If Earth slowed down very gently, your home would stay sunny for a long, long time, then dark for a long, long time. One day and one night would take a whole year! Find out what makes the seasons.
Spinning makes Earth a bit fat in the middle, and the oceans pile up there too. Without the spin, the water would flow to the top and the bottom. There would be one giant land in the middle!
Earth will keep spinning, day after day, because nothing out in space is pushing hard enough to stop it. You can keep on playing!
First, the good news: it can't actually happen, so this is a thought experiment. There is no known way to suddenly stop a whole planet, and scientists say it is very unlikely Earth will ever stop spinning.3 But imagining it is a great way to see how much the spin does for us.
Earth turns once on its axis every 23.9 hours.12 Its equator is about 40,000 km around (the equatorial radius is 6,378 km).2 Covering that distance in one turn means a spot on the equator moves at about 1,670 km/h. Esri's scientists use 1,667 km/h,1 and Space.com rounds it to about 1,000 mph.3
Closer to the poles you ride around a smaller circle each day, so you move more slowly. At the poles themselves you just turn on the spot.3 Our figures are simple geometry: the speed is the equator speed times the cosine of your latitude.
| Where | Latitude | Speed from Earth's spin |
|---|---|---|
| Equator (e.g. Quito, Singapore) | 0° | ~1,670 km/h (1,040 mph) |
| New York, Madrid, Beijing | ~40° | ~1,280 km/h (800 mph) |
| Oslo, Anchorage | ~60° | ~840 km/h (520 mph) |
| North or South Pole | 90° | 0 |
So why don't we feel it? Because the ground, the air, the oceans and you all move together at the same steady speed. You only feel a change in motion, not the motion itself.4
This is where inertia comes in. Newton's first law says that an object in motion stays in motion, at the same speed and in the same direction, unless a force acts on it.4 Picture a toy car on a spinning record player. It rides around happily. Stop the turntable in an instant, and the car keeps going and shoots off the edge.
In our thought experiment, the air and the oceans are the toy car. If the solid Earth stopped suddenly, they would keep moving east at the speed they had before. Physicist Andrew Layden told Space.com that the water, the air and anything loose would "keep going".3 The result would be winds and waves far stronger than any storm. The poles, which barely move, would be the least affected.3
That's why a sudden stop is the boring version: everything is just a mess. The slow version is where the surprises are.
Imagine instead that the spin fades gently over many years. Esri's model imagines it happening over a few decades.1 Earth would still orbit the Sun, and its axis would keep the same tilt. Here is what would change.
Without spin, a place on Earth only turns to face the Sun because Earth travels around it. So one side would get about six months of daylight while the other side had six months of night, and then they would swap.3 One day plus one night would last a whole year.1 Physicist Andrew Layden told Space.com that this would change our weather a lot, because air and ocean currents depend on Earth's spin.3
Today, winds and ocean currents curve because Earth turns under them: to the right in the Northern Hemisphere and to the left in the Southern. This is the Coriolis effect. NOAA explains that on a non-rotating Earth, air would simply flow straight back and forth between high and low pressure.5 Weather everywhere would change.3
This is the biggest surprise. Earth is not a perfect ball. Spinning makes it bulge at the equator, a bit like pizza dough tossed in the air. The distance from the centre to the equator is about 21 km longer than the distance to the poles.12 Sea level is a balance between gravity, which pulls water toward Earth's centre, and the outward push of the spin.1 Because of that, the oceans bulge too, about 8 km at the equator.1
Take away the spin and nothing holds that water bulge up. Witold Fraczek of the mapping company Esri used its ArcGIS software to model the result.1 The oceans would slowly flow toward the poles. Land would rise out of the sea around the equator and join into one giant equatorial continent, with two separate polar oceans. The northern ocean would spread over the lowlands of Siberia and northern Canada. The southern ocean would sit about 1,400 m lower than the northern one, and the old peak of Chimborazo in Ecuador would stand about 13,600 m above the new sea level.1 Esri's model moves only the water and keeps today's rock shape.1
About the toys: the cross-section in chapters 4 and 5 exaggerates the bulge about 150 times so you can see it. The map is a simplified sketch of Esri's result, not a real map, and the ocean shapes in the dial toy come from a simple model, not measurements. The dial's day length uses a rule of thumb (24 hours divided by the share of today's spin), capped at one year when the spin stops.
Earth's magnetic field comes from its fluid outer core. Molten iron and nickel churn as heat escapes and swirl in whirlpools driven by Earth's rotation. The moving metal makes electric currents, which make the field.6 The field shields us from harmful particles from space.6 Because the spin helps stir the core, some scientists think a non-spinning Earth might have a weaker field,3 but no one knows for sure. Venus is an interesting clue: it spins very slowly, one turn every 243 Earth days,7 and unlike Earth it has no big magnetic bubble around it.6
Yes, but very, very slightly. The Moon's gravity pulls the oceans into tidal bulges.13 As Earth spins under them, the tides rub against it like a gentle brake. This tidal friction is the main reason the day is getting longer.18 The same tides slowly push the Moon away from us, by about 3.8 cm (roughly 4 cm) a year, as laser reflectors left by the Apollo astronauts show.10
Astronomers F. R. Stephenson, L. V. Morrison and C. Y. Hohenkerk studied ancient and medieval records of eclipses, plus later timings of the Moon passing in front of stars, from 720 BC to AD 2015. They found the length of the day grows by about 1.8 milliseconds per century. Tidal friction on its own would add about 2.3 milliseconds; other slow changes inside Earth, such as the crust rebounding since the last ice age and the core tugging on the mantle, are thought to explain the difference.8 Earth's spin also wobbles a little from year to year, which is why timekeepers have added leap seconds to atomic clocks.11
At 1.8 milliseconds per century, adding a single hour to the day would take about 2 million centuries, or roughly 200 million years. That's our own simple arithmetic, not a forecast.
If the day is getting longer, it must have been shorter long ago, and rocks confirm it. Stephen Meyers and Alberto Malinverno studied 1.4-billion-year-old layered rocks whose layers record ancient climate cycles. They worked out that a day then lasted about 18.7 hours, and the Moon was about 340,900 km away (today it averages 384,400 km).910 About 400 million years ago, a year had roughly 40 more days than today.1
| When | Length of a day | Source |
|---|---|---|
| 1.4 billion years ago | ~18.7 hours | Meyers & Malinverno 20189 |
| ~400 million years ago | ~21.6 hours (about 405 days a year) | Esri, from tidal slowdown1 |
| Today | 24 hours | NASA2 |
The time-slider toy in chapter 6 joins these three points with straight lines. It is a rough guide, not a model of how the slowdown really went.
The thought experiment shows how much our planet's spin quietly does for us. It gives us short days and nights instead of six-month ones. It curves the winds and currents that shape our weather,5 keeps the oceans spread around the globe,1 and helps stir the core that makes our magnetic shield.6 If you want to go further, see how gravity works, what causes tides, what causes the seasons, what causes wind and how the Moon formed, when Earth spun much faster.
A sudden stop would be extremely dangerous, because the air, the oceans and anything not attached would keep moving east at up to about 1,670 km/h. A very slow stop over many years would be easier to live through, but day and night would each last about six months, and most of the oceans would move toward the poles. Luckily, scientists say neither is expected to happen: it would take something like a collision with a planet-sized object.
Not in any way that matters to us. Earth's spin is slowing very slightly because of the Moon's tides. The day gets about 1.8 thousandths of a second longer every hundred years. At that rate it would take roughly 200 million years just to add one hour to the day.
Gravity would not change, so you would not float away. But in a sudden stop your body would keep moving east at the speed the ground used to move, up to about 1,670 km/h at the equator. That is inertia, the same reason a toy car slides off a record player when it stops suddenly.
Even a one-second stop would be sudden, so the air and oceans would keep moving east at the old speed, causing huge winds and waves. Starting the spin again would not undo that. It is still only a thought experiment: nothing realistic could stop a planet even for a second.
Earth would still go around the Sun once a year. So most places would get about six months of daylight followed by about six months of night, as Earth travelled around the Sun. One day plus one night would last a whole year.
Earth's spin makes it bulge at the equator, and the oceans bulge too, about 8 km higher at the equator than on a still Earth. Without the spin nothing holds that bulge up, so the water would slowly flow toward the poles. A model by the mapping company Esri shows two polar oceans and one giant continent around the equator.
Shorter than today. A 2018 study of 1.4-billion-year-old rock layers found that a day then lasted about 18.7 hours. About 400 million years ago a year had roughly 40 more days than now, so each day was around 21 to 22 hours.