Precession

Precession has an immediate impact on astronomers but a much
slower one on the seasons. Right now, the Earth’s north axis points
toward the Sun in June. But due to precession, 13,000 years from
now—half a precession cycle—the Earth’s north pole will be pointed
away from the Sun in June and toward it in December. Seasons
will be reversed relative to our current calendar.
Remember, too, that we are closest to the Sun on our elliptical
orbit in January. So half a cycle from now the northern hemisphere
of the Earth will experience summer when the Earth is closest to
56 FROM THE EARTH TO THE MOON
the Sun, amplifying the heat. It’ll also be winter when we’re farther
from the Sun, amplifying the cold. Seasons will be more severe. In
the southern hemisphere, the seasons will be even milder than they
are now, since they’ll have summer when we are farther from the
Sun and winter when we are closer.
This works the other way, too: 13,000 years in the past, the seasons
were reversed. Summers were hotter and winters were colder
in the northern hemisphere. Climatologists have used that fact to
show that things might have been profoundly different back then.
The slow change in the direction of the Earth’s axis might have
even been the cause of the Sahara becoming a desert! On a yearby-
year basis precession is barely noticeable, but over centuries
and millennia even small changes add up. Nature is usually brutal
and swift, but it can also display remarkable subtlety. It just depends
on your slant.