80
Now if climatic precession considered the movement of the axis only, earth’s solstices would realign with
Earth’s semimajor axis every 26,000 years. However, this doesn’t happen because the orientation of the
semimajor axis is also moving, but in counterclockwise direction. Gravitational tugs from other planets, primarily Jupiter, change the degree of eccentricity and rotate the semimajor axis. Given time, these opposite
motions move aphelion from the southern hemisphere’s summer to the northern hemisphere’s summer
and back again at cycles that vary from 19,000 to 23,000 years.
Figure 9: Seasons moving clockwise with semimajor axis moving counter-clockwise induce a climatic precession cylee that varies between 19,000 and 23,000 years.
ICY SCIENCE | QTR 1 2014