Which way is the Earth spinning? (Natural Sciences, Cambridge)

It all depends on how you look at it.¹ It spins eastwards, of course, which is why the sun comes up in the east and sinks in the west as the Earth whirls us away out of sight of it. And if you travel far out into space above the North Pole, you might also say it spins anticlockwise.

Astronomers might then go a little further and describe Earth’s rotation as ‘prograde’. Prograde simply means turning in the same direction (retrograde means turning in the opposite direction). So when they say Earth’s rotation is prograde, they are saying that it is spinning on its axis in the same direction as it is orbiting around the sun. It’s as if the Earth is rolling forever forward as it journeys through space. In fact, most of the planets in the solar system spin the same way. The only exceptions are Venus and Uranus, which spin slowly backwards.

Strikingly, not just the rotation but also the ‘revolution’ or orbit of all the planets, including Venus and Uranus, is prograde. That means they all orbit the sun in the same way the sun is turning, too. In fact, pretty much everything in the solar system is spinning the same way. Even moons orbit their planets in the same way. And now as astronomers discover other planetary systems circling other stars, they are finding that these planets mostly turn the same way as their stars as well, although there are exceptions. So prograde motion seems to rule, despite the odd rebel.²

Astronomers have been aware of this widespread tendency to prograde motion in the solar system for several centuries. It was in trying to explain it that thinkers such as Kant and Laplace developed the nebular hypothesis of how the solar system came into being in the later 1700s. There are numerous theories about the origins of the solar system, but the nebular hypothesis is still the most widely accepted.

In the nebular hypothesis, the solar system began as a huge gas cloud or nebula. Perhaps triggered by a collision with another cloud, or by a star exploding nearby, it began to collapse inwards under the force of its own gravity. As it contracted, the material gained what is called angular momentum.

Angular momentum is a hugely important quality in the motion of space. In fact, it’s why everything in space seems to whirl around like some infinitely vast clockwork toy. Everything in motion has momentum, the natural tendency to carry on moving in the same direction. Angular momentum is momentum in a circle, and occurs whenever the direction of momentum is pulled continuously off course by some additional force. In space, that force is mostly gravity. Wherever there is gravity and motion, which is essentially everywhere, gravity turns motion into angular momentum. So circular motion is universal. Angular momentum is what makes the Milky Way and other galaxies turn, the solar system rotate, and planets and moons orbit. It’s also what makes the Earth spin.

The key thing to remember about angular momentum is that, like linear momentum, it can’t just get lost; it is always conserved. In the nebular hypothesis, any slight rotation the original cloud had was magnified as it collapsed. Its angular momentum was packed into a smaller and smaller space so that it began to spin ever faster.³ There is a famous analogy about the conservation of angular momentum with a spinning skater. As she tucks her arms in, the concentration of momentum means she spins faster. So it was with the infant solar system.

With the nebular hypothesis, a cloud perhaps a light-year across was reduced to the size of the solar system. So there’s a lot of momentum to pack into a tiny space, and the collapse of the nebula was like winding up a giant clockwork toy. As the original ball of the cloud collapsed, so its matter was concentrated and flung out in a flat, spinning pancake disc. Then, as gravity pulled material in the disc together to form planets, all the momentum of the vast original cloud was concentrated into spinning them like tops.

That original momentum was enough to keep the Earth spinning relentlessly at the same speed far into the future. There are small frictional forces, known as tidal forces, caused by the gravitational interaction between the Earth, moon and sun that act as brakes. But they slow it down by barely 2.3 milliseconds per day every hundred years. Weather systems in the atmosphere can be a drag on the planet and affect the speed of the spin, too. Earthquakes can actually speed the Earth up or slow it down by shifting its mass. The quake that struck Japan in 2011 apparently accelerated the Earth’s rotation and shortened the day by 1.8 microseconds by shifting mass towards the equator.

I’ve talked about the speed of Earth’s spin as if it’s the same everywhere. This is not quite true. If you stand right on the poles, for instance, it takes you a whole day to turn right round, but you’re not moving any distance. Yet if you stand on the equator, you’re whirled round at 1,667km/h – faster than the speed of sound. That’s why spacecraft are often launched from tropical locations, to give them extra launch speed.

If you go down into the Earth’s interior, too, the speed varies. This is because the centre of the Earth is fluid and magnetic. The spinning of the magnetic material in the Earth’s core creates a magnetic field, and this in turn affects the metal in the core. It pushes the inner core eastwards, making it spin faster than the rest of the Earth, and it pushes the fluid outer core in the opposite direction so that it actually turns westwards relative to the rest of the Earth.

In talking about the way the Earth spins, I’ve talked so far about the direction it spins. But I could also talk about how it spins. From the geometry of our view of the sun and planets, we know that the Earth doesn’t spin perpendicular to its orbit around the sun but at a slight angle. Its axis, which runs in a line between the North Pole and the South Pole, is tilted over at an average of 23.4° to the plane of the orbit. This tilt is why we get seasons, since it ensures that as Earth moves through its orbit, the direct line of sun strikes at different places.

In fact, the way the Earth spins varies continually. Over a 42,000-year cycle, for instance, its spin axis wobbles a little between 22.1° and 24.5°. Over 26,000 years, too, the axis moves slowly round in a circle, tracing out a cone, in a movement called precession. And over just eighteen to nineteen years, a tiny wobble called nutation is superimposed on the precession because Earth’s equator is not perfectly aligned with the moon, making the rotation slightly unbalanced by the pull of their mutual gravity. Serbian mathematician Milutin Milankovitch showed how these variations might alter the warming power of the sun and create variations in climate, now called Milankovitch cycles.

In the 1978 movie, Superman used his super-strength to reverse the spin of the Earth to turn back time and save Lois Lane. Maybe some other force will one day intervene and change the rotation of the Earth. But until then, we can be certain the sun will rise in the east and set in the west, and that’s how it should be.

Footnotes

1 You could just say it’s spinning my way – because any description of motion depends entirely on the frame of reference. The only easily visible signs that the Earth is spinning are the daily movement of the sun and stars through the sky, and the sun’s shifting shadows on the ground. Yet we have so little sensation of movement it’s no wonder that for a long time most people thought just the sun and the stars moved while the Earth was forever fixed. Even now, it’s not always easy to remember as you watch the sun sink below the horizon in the west that it’s really the Earth and you moving, not the sun.

Some clever Ancient Greek astronomers such as Aristarchus guessed that the Earth spins over 2,000 years ago. Yet the clues that it is spinning are so subtle that they were not really picked up until the 16th century when Polish astronomer Copernicus introduced to Europe the idea that the Earth orbits the sun. Even then, a century on, Galileo was drawn into ferocious battles that ended in his house arrest when he tried to make the case for a moving Earth in the face of official opposition from the Catholic church. It’s easy for us, four centuries on, to ridicule the church elders who abused Galileo – but his evidence that the Earth moves amounted to little more than the observation of moonlike phases on Venus through a telescope added to Copernicus’s ingenious (and correct) interpretation of the loops in the tracks of the planets through the sky. After Galileo, Newton and others tried with little success to detect the Earth’s motion in the deflection from the vertical of falling objects. But it wasn’t until 1851 that French physicist Léon Foucault was able to provide clear evidence of Earth’s motion with his pendulum, which slowly shifts the direction it swings during the day as the Earth rotates while its momentum carries in the same direction. Nowadays, of course, we are finally able to move beyond the Earth and witness its rotation from the outside, from space.

2 Actually it’s not just planets, stars and galaxies that are on the whirl. A fascinating study by a team led by Michael Longo at the University of Michigan recently looked at the direction of rotation of over 15,000 spiral galaxies in the part of the sky towards the north pole of the Milky Way. What they found was that significantly more are rotating anticlockwise than clockwise. If more galaxies do spin in this direction, Longo argues, then the universe must have a net anticlockwise angular momentum. Since that momentum must have come from somewhere, it implies the universe was born spinning.

3 With neutron stars the gravitational collapse is so dramatic and the concentration of angular momentum so huge that these tiny stars can spin up to 642 times a second!