Joe F's post is entirely correct, save in one piffling detail. In addition to precessing, the angle of inclination of the earth's axis does indeed change regularly from a maximum of about 25 degrees to perpendicular to the orbital plane, to a minimum of about 21 degrees. (It is currently about 22.5 degrees and decreasing). The period of this oscillation is about 40,000 years.

This is one of the Milankovitch Cycles (the other two being precession and changes in the eccentricity of the earth's orbit), which have periodic effects on the earth's climate over a time scale of thousands of years.

A greater inclination of axis produces colder winters and hotter summers. (Precession determines which hemisphere gets the more extreme weather. Eccentricity of orbit determines the total amount of radiation the Earth receives from the Sun.)


Joe is quite correct; the presence of the moon is the major factor which stabilises the earth's rotation. The cause of instability in a planet's rotation is its unequal balance of masses. Neither Earth nor Mars is a perfect sphere. Mars has a bulged northern hemisphere and slightly flattened southern hemisphere. Earth isn't quite so skewed, but there is a slight preponderance of mass in the northern hemisphere.

Not strictly fact; but, science-fiction author, Kim Stanley Robinson has written a trilogy of novels (Red Mars, Green Mars, Blue Mars)which concern the colonisation of Mars in the coming decades, and these give good descriptions of how the eccentric orbit and rotation of Mars affect the seasons and weather.