Expert: Jayendra Upadhye - 11/12/2004

Question
Is the speed of the earth's rotation around its axis and its revolution around the sun constant?  Does it flucuate?  Because if it does, that would mean that every day, year, etc. is different in length.

Answer
Hi Shane,
If you look at it too exactly, both are inconstant!
1 - The gravitational tidal coupling between the moon and the earth has two consequences.
(a) - The tidal "interaction" SLOWS the earth's rotation by a "small" fraction, continuously.(Currently the Earth runs slow at roughly 2 milliseconds per day. After 500 days, the difference between the Earth rotation time and the atomic time would be 1 second. Instead of allowing this to happen, a leap second is inserted to bring the two times closer together)
refer leap second:
http://tycho.usno.navy.mil/leapsec.html
(b) - The moon due to law of conservation of angular momentum, actuall speeds up as a result, and recedes from the earth approximately 4 cm per year!
This is called lunar RATE OF RECESSION. (3.8 cm/year)
ref:- http://home.cwru.edu/~sjr16/advanced/earth_moon.html
2 - Since the earth's orbit though almost circular is NOT a perfect circle, but an ellipse, (almost circle) it follows kepler's laws (T squared varies as R cubed)
Thus it picks up speed as it approaches perihelion and looses apeed as it approaches apahelion.
Mean orbital velocity (km/s) =  29.78
Max. orbital velocity (km/s) =  30.29
Min. orbital velocity (km/s) =  29.29
ref: earth fact sheet (excellent data on earth)
http://64.233.179.104/search?q=cache:VcPwDBzsGSQJ:www.missionspace.info/pfs/eart...

Shane, no cause for despair though!
WE could contine with the good old Gregorian stuff and LEAVE all this to the astronomers! (For these minute changes to become noticieable, it would TAKE several life times!)

So cheers!
Jayen