Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Rate Earth turns 
Name: owl
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: 1993 - 1999
Question:
I have heard that the moon and earth are slowing in their respective rotations due to the gravitational attraction between them. Does this mean that an earth day was shorter in the past?

Replies:
Yes, in fact the angular momentum of the earth-moon system remains constant, but tidal forces cause a kind of friction that slowly slows the earth's rotation. In order to conserve angular momentum, the moon's orbit is slowing increasing its diameter. It's interesting that we (humans) exist right now because some millions of years ago the moon would have been closer and looked larger and total solar eclipses would not have been as spectacular as they are now. Similarly, some millions of years in the future, the moon will be further away, will look smalle r, and there will never again be a total solar eclipse, only annular eclipses. We just happen to be living during the relatively small period of time when the moon and sun have the same apparent size as seen from the earth!

John Hawley


Click here to return to the Astronomy Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory