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 Lunar Spin Down and Orbit
Name: Mark
Status: other
Grade: other
Location: NV
Country: N/A
Date: N/A

The earth has a natural moon. That moon's "year" is the time it takes it to make one trip around the earth. Our moon also has a "day," the time it takes to rotate once about its own axis. Our moon's "year" is exactly equal to its day. Not 99% the same. Not 99.999% the same but 100.000000000000000....% the same. This explains why the same face of the moon always is exposed to the earth.

My question is why is this so? In theory, would it be possible to place a huge rocket on the moon oriented at 90 degrees to a radial line from the rocket to the moon's center of gravity and change the duration of the moon's day, and thus forcing other portions of the moon's surface to face the earth?

Dear Mark,

The answer is a term called rotational lock. The Earth's gravity slows the Moon down until it is in rotational lock with the Earth and the rotation is as it is. Actually it is not 100%; there is a little libration.

I guess a big rocket could give us a different face, but it would have to be awfully big!

David Levy

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 (, or at Argonne's Educational Programs

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

Argonne National Laboratory