Every 24 hours, the Earth makes a full rotation, spinning west to east, which is why the sun rises in the east and sets in the west and the stars at night appear to move across the sky.
Earth spins along an imaginary line between the North Pole and the South Pole called the rotation axis. The rotation axis points toward a bright star, Polaris, which is visible on clear nights in the Northern Hemisphere.
But why does Earth spin in the first place? And what would happen if it suddenly stopped?
To understand why, let's see what we can learn from other bodies in space.
The sun spins. In fact, it spins in the same direction Earth does, counterclockwise.
Not only that, Earth orbits the sun in the same direction, as do most other planets and more than a million asteroids and dwarf planets.
Jupiter and Saturn spin quite a bit faster than Earth, taking only about 10 hours to rotate. Saturn's spin is a little bit tilted, so we get to see changing views of its rings over time.
There are two exceptions: Uranus and Venus. Uranus is tilted so far over, it's virtually spinning on its side. Nobody knows exactly how or why it got this way. Maybe it collided with another planet. Venus is also odd — it spins clockwise.
We don't know for sure whether it formed that way or got knocked over. Most scientists now think its spin has been reversed over time by tidal forces involving the sun and Venus' thick atmosphere. Both Venus and Uranus rotate in what's known as retrograde manner, meaning opposite the rotation of the sun.
All that leads astronomers like me to wonder: Is there something about how the solar system formed that kind of "baked in" that direction of spin?
Stars Are Born and Slowly Rotate
For more clues, we can look at a young star, one that is just forming its system of planets.
A famous one is called Beta Pictoris. It is surrounded by a thin disk of dust, gas and little bits called planetesimals that range in size from grains of sand and large rocks to objects the size of mountains. Astronomers are pretty sure the disk formed from material left over when the star was born 20 million years ago.
Every star is born from a cloud of gas and dust that moves through space surrounded by other similar clouds. The force of gravity causes these clouds to tug on one another as they pass, which makes them slowly rotate.
Even when one of these clouds collapses to form a star, it continues to rotate. The star forms, spinning at the center of a flat pancake of rotating gas and dust called a protoplanetary disk. All of it — the star, the gas, the dust — spins in the same direction.
Astronomers think that our solar system looked a lot like Beta Pictoris in its early years.
We think that inside the disk, the gas and dust can stick together in a process called accretion. As a baby planet starts to grow, it gets heavier, and its gravity attracts more and more little pieces.
When the baby planet gets massive enough, the force of gravity begins crushing it, making it denser. Because of the force of gravity, the planet spins faster, like an ice skater drawing in her arms to spin.
Rising pressure in the core then causes the core to melt. Denser materials sink toward the core, and lighter materials float to the planet's surface. We end up with a planet with an iron core surrounded by rock, and maybe water and ice on the outer edges.
Earth's spin is important for life, mainly because it causes day and night. On Mercury, which spins very slowly, temperatures vary drastically. During the day, temperatures can reach as high as 800 degrees Fahrenheit (430 degrees Celsius) and then drop as low as minus 279 degrees Fahrenheit (minus 173 Celsius) at night.
It's also important for ocean tides. Without the daily ebb and flow of water, it's possible life on Earth would never have emerged from the sea onto land.
So, astronomers believe Earth spins because the entire solar system was already rotating when Earth formed. But there are still a lot of questions about how planets' spins change over time, and how spin affects the evolution of life.
With more than 5,000 planets now known beyond the solar system, future scientists are going to be busy exploring.
Silas Laycock, Ph.D., is a professor of astronomy. He researches pulsars, black holes, binary stars, X-rays, high energy radiation, optical spectroscopy and adaptive optics.