# Q & A: Spinning Earth

Q:
why does the earth spin and how fast does it spin
- jennifer (age 15)
eston park, middlesbrough england
A:
The answer to your question sounds a bit odd, but it is nevertheless true: The earth spins now because it was spinning as it was created. When something is spinning it has a property we call angular momentum. The angular momentum of an object can not change unless outside forces are exerting twists on it. For example, the way you get a top going is you give a quick twist to its axle. The top will spin for quite a long time before stopping. If there were no friction, the top would spin forever, just like the earth. The difference between the top and the earth is that to top was first made by someone, and then after it was made we gave it a twist to start it spinning. The earth was made as lost of space dust came together over a very long time, attracted by the force of gravity. Since this dust was swirling around a bit as it came together to make the earth, the earth ended up spinning, and the rest is history.

It's easy to figure out how fast the earth spins. We know it turns once every day (about 24 hours), and we know the diameter (size) of the earth is about 13 million meters. Combining these two facts lets us calculate that the surface of the earth (at the equator) moves at a speed of about 1000 miles per hour due to its spinning.

MS

(published on 10/22/2007)

## Follow-Up #1: Do all objects in space rotate?

Q:
I want to know whether all the objects up in the space like stars, planets, comets asteroids rotate/spin?
- Indrajit
Delhi, India
A:
Yes, to a greater or lesser extent.  Space objects are formed by accumulation of gasses, dust, small asteroids etc.  As such, there is almost always an imbalance of angular momentum and a finite net amount, although it could be very small.   Some objects have their spin correlated with respect to a neighboring object.  In these cases tidal forces tend to equalize spin and rotation rates.  For example the moon rotates once per lunar month, i.e. its spin rate has been synchronized with its rotation rate around the earth.

LeeH

(published on 06/15/2010)