Expert: James Gort - 5/5/2008

Question
how does the earths rotation stay relatively predictable/constant???

Answer
Hi Jenn,

Isaac Newton discovered the law of inertia - that an object at rest stays at rest, and an object in motion stays in motion unless acted upon by an external force.  The earth started rotating on its axis as it cooled from the gas and dust from which it was formed (conserving the rotation that the original dust cloud had).  Since it was rotating about its axis, it continues to rotate fairly constantly - unless there is an external force.  The earth is subject to some external forces - mainly, the gravity of the moon and the sun.  Since the earth isn't perfectly round, and the gravity of those two bodies cause the earth to bulge slightly (causing tides), the earth actually wobbles a little.  Every 26,000 years, the axis of the earth makes one full "wobble".  But other than that, it is pretty constant.  But all those tidal forces (from the sun and moon) do affect the rotation - it's slowing down!  The earth loses about 2 thousandths of a second per day, every 100 years.   

Hope that helps.

Prof. James Gort