# Probing Question: Why does the Earth rotate?

##### August 6, 2007

We spend our lives on a spinning globe -- it takes only 24 hours to notice that, as night follows day and the cycle repeats. But what causes Earth to rotate on its axis?

The answer starts with the forces that formed our solar system.

A fledgling star gathers a disk of dust and gas around itself, said Kevin Luhman, an assistant professor of astronomy at Penn State. As things coalesce, the star's gravitational orbit sets that dust and gas to spinning. "Any clump that forms within that disk is going to naturally have some sort of rotation," Luhman said.

As the clump collapses on itself it starts spinning faster and faster because of something called conservation of angular momentum. Figure skaters exploit this law when they bring their arms closer to their bodies to speed up their rate of spin, Luhman explained. Since gravity pulls inward from all directions equally, the amorphous clump, if massive enough, will eventually become a round planet. Inertia then keeps that planet spinning on its axis unless something occurs to disturb it. "The Earth keeps spinning because it was born spinning," Luhman said.

Different planets have different rates of rotation. Mercury, closest to the sun, is slowed by the sun's gravity, Luhman noted, making but a single rotation in the time it takes the Earth to rotate 58 times. Other factors affecting rotational speed include the rapidity of a planet's initial formation (faster collapse means more angular momentum conserved) and impacts from meteorites, which can slow down a planet or knock it off stride.

Earth's rotation, he added, is also affected by the tidal pull of the moon. Because of the moon, the spin of the Earth is slowing down at a rate of about 1 millisecond per year. The Earth spun around at a faster clip in the past, enough so that during the time of the dinosaurs a day was about 22 hours long.

In addition to slowing the Earth's rotation, the moon's tidal pull is causing the moon to slowly recede from the Earth, at a rate of about 1 millimeter per year. In the distant past, the moon was closer. "It would have appeared much larger in our sky than it does now," Luhman said.

Millions of years from now, he added, the cycle of a day on Earth will likely stretch to 25 or 26 hours. People will have to wait a little longer for the rising of the sun.

Source: By Mike Shelton, Research Penn State

Explore further: Probing Question: What are Shooting Stars?

## Related Stories

#### Probing Question: What are Shooting Stars?

April 9, 2008

In the early morning darkness on April 15, 1912, as the R.M.S. Titanic was sinking in the freezing Atlantic, survivors witnessed a large number of streaking lights in the sky, which many believed to be the souls of their ...

#### Length of exoplanet day measured for first time

April 30, 2014

Observations from ESO's Very Large Telescope have, for the first time, determined the rotation rate of an exoplanet. Beta Pictoris b has been found to have a day that lasts eight hours. This is quicker than any planet in ...

## Recommended for you

#### What lies beneath: Venus' surface revealed through the clouds

July 18, 2016

Using observations from ESA's Venus Express satellite, scientists have shown for the first time how weather patterns seen in Venus' thick cloud layers are directly linked to the topography of the surface below. Rather than ...

#### X marks the spot at the center of the Milky Way galaxy

July 19, 2016

Two astronomers—with the help of Twitter—have uncovered the strongest evidence yet that an enormous X-shaped structure made of stars lies within the central bulge of the Milky Way Galaxy.

#### Alien solar system boasts tightly spaced planets, unusual orbits

July 21, 2016

Tightly spaced planets inside an alien solar system known as Kepler-80 boast a rare orbital configuration.

#### Asteroid that formed moon's Imbrium Basin may have been protoplanet-sized

July 20, 2016

Around 3.8 billion years ago, an asteroid more than 150 miles across, roughly equal to the length of New Jersey, slammed into the Moon and created the Imbrium Basin—the right eye of the fabled Man in the Moon. This new ...

#### First atmospheric study of Earth-sized exoplanets reveals rocky worlds

July 20, 2016

On May 2, scientists from MIT, the University of Liège, and elsewhere announced they had discovered a planetary system, a mere 40 light years from Earth, that hosts three potentially habitable, Earth-sized worlds. Judging ...

#### Five years after shuttle, NASA awaits commercial crew capsules (Update)

July 20, 2016

Five years after Atlantis completed the space shuttle program's final voyage, NASA is still at least a year away from launching its astronauts from U.S. soil.