Probing Question: Why does the Earth rotate?

Aug 06, 2007
Earth

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: NASA asteroid defense program falls short: audit

add to favorites email to friend print save as pdf

Related Stories

Better non-functional security tests for software

11 minutes ago

The integration of digital expert knowledge and automation of risk analyses can greatly improve software test procedures and make cloud computing more secure. This is shown by the latest results of a project ...

'Jaws' lived in Doncaster according to fossil record

12 minutes ago

(Phys.org) —Sharks, swamps and a tropical rainforest teeming with life – it's not what comes to mind when you think of Yorkshire. But for the first time evidence of Doncaster's 310-million-year-old past, including a ...

Invisibility cloaks closer thanks to 'digital metamaterials'

14 minutes ago

The concept of "digital metamaterials" – a simple way of designing metamaterials with bizarre optical properties that could hasten the development of devices such as invisibility cloaks and superlenses – is reported in a paper published today in Nature ...

Recommended for you

India's spacecraft 'on target' to reach Mars

17 hours ago

An Indian spacecraft is on course to reach Mars, an official said Monday, following a 666-million-kilometre voyage that could see New Delhi's low-cost space programme win Asia's race to the Red Planet.

Rosetta's lander Philae will target Site J

19 hours ago

(Phys.org) —Rosetta's lander Philae will target Site J, an intriguing region on Comet 67P/Churyumov–Gerasimenko that offers unique scientific potential, with hints of activity nearby, and minimum risk ...

User comments : 0