What if Earth stopped rotating?

Earth spins, so we get day and night. Do not worry: Earth will keep on spinning.

But in a pretend lab, what if it stopped? Would day last forever?

Earth's spin carries you from day into night and back again every 24 hours. Earth is not going to stop spinning; it has been turning for billions of years and slows only a tiny bit each century.

But imagine it did stop, so that it never turned at all compared with the distant stars. Would your side of Earth face the Sun forever? Or would something else still bring night? In this lab you can stop Earth's spin in a model and see.

Day and night come from two motions: Earth's rotation and its orbit around the Sun. Normally rotation dominates, which is why the solar day is only about four minutes longer than one rotation.

This lab removes rotation entirely, a purely imaginary case, and asks what is left. The answer depends on the orbit, and it is not what most people first expect.

Earth spins once a day. It also travels around the Sun once a year.

The spin gives us day and night. The trip around the Sun gives us the year.

Earth does two things at once. It spins once every 23 hours and 56 minutes, and it travels once around the Sun every 365 and a quarter days.

Because Earth moves a little way along its orbit each day, it has to spin a tiny bit extra to face the Sun again. That is why sunrise to sunrise takes 24 hours, about four minutes longer than one spin. Usually that small difference is the only sign of the orbit in our day.

The solar day S, the sidereal rotation period P and the sidereal year Y are linked by 1/S = 1/P − 1/Y. With P = 23.9345 h and Y = 365.256 days, S comes out at 24.0 h.

The second term is small because Earth rotates about 366 times per orbit. The orbit's contribution to day length is therefore only about four minutes, which is why we rarely think about it.

In our pretend lab, Olly stops Earth's spin.

Guess what? The Sun still moves across the sky, very slowly. One day now lasts a whole year!

Switch on 'stop spinning'. Earth no longer turns compared with the stars, but it still travels around the Sun. As it does, the Sun appears to drift slowly across the sky. One sunrise to the next now takes a whole year: about six months of daylight, then six months of night.

In the model, the half-year of sunshine heats the equator to about 100 °C, and the half-year of darkness cools it to about −27 °C.

With no rotation, 1/S = −1/Y, so the solar day equals one sidereal year: 8,766 hours, with 4,383 hours of daylight at the equator. The model's patch nearly reaches balance with the noon Sun, about 103 °C, and cools to about −27 °C, held up by the fixed heat sharing; with no air it falls to about −183 °C.

This is a thought experiment carried out with a model. Earth's spin cannot stop, and the model changes only the timing of sunlight, nothing else about the planet.

Earth keeps going around the Sun. So the Sun seems to move across our sky.

It moves so slowly that one trip takes a year.

Imagine walking slowly around a lamp in the middle of a room while always facing the same wall. Even though you never turn, the lamp is in front of you for half the walk and behind you for the other half. That is a stopped Earth going around the Sun.

The long day is very hot because the Sun shines on the same ground for months. The long night is very cold because the ground has months to lose its heat.

A non-rotating body in orbit still sees its star sweep 360° across the sky each orbit, so its solar day equals its orbital period. With months of continuous sunshine, the surface reaches near-equilibrium with the noon Sun; with months of darkness it cools until radiation balances the model's heat sharing.

The model's limits: it is one equatorial patch, heat sharing is fixed (climate simulations of slowly turning Earth-like planets show winds moving much more heat to the night side), and it ignores what stopping would do to Earth's shape and oceans.

The Moon always shows us the same face. But it still has day and night!

Its days and nights last about two weeks each.

The Moon always keeps the same face toward Earth, so it can look as if it never turns. But it does turn, once each trip around Earth, and that gives it days and nights about two weeks long each.

A stopped Earth would change in ways this lab cannot show. Earth's spin makes it bulge a little at the equator, so without the spin its shape would slowly change, and its oceans would probably shift too. The model only follows sunlight and temperature at one spot.

The Moon is tidally locked: its rotation period equals its 27.3-day orbit, so its solar day is about 29.5 days. Mercury rotates once every 1,407.6 hours yet has a solar day of 4,222.6 hours, about 176 Earth days: three times as long as one spin.

Earth's equatorial radius (6,378.1 km) exceeds its polar radius (6,356.8 km), a flattening of 0.0034; NOAA explains that this equatorial bulge comes from Earth's rotation. A stopped Earth would therefore slowly lose its bulge, and its oceans would probably redistribute; how, and what else would change, are ideas outside this temperature model.