|Since the distance between the Earth and the Sun
does not tell us the seasons, how does it affect us?
|Question Date: 2007-01-10|
I like this question because it highlights a
common misconception about our seasons. It is true
that the distance between the Earth and the Sun
does not create the yearly seasons (the seasons
are controlled by the tilt of the Earth's axis of
rotation). However, this distance is responsible
for more extreme, longer-term, climate changes. It
is responsible for setting the pace of the Ice
Ages which covered a good portion of our planet
with ice hundreds of meters thick during the past
3 million years. The Earth's orbit around the Sun
(and thus the distance between the Earth and the
Sun) goes through 100,000-year cycles where it
flips between a circle and something more like an
oval (this wobbling is due to interactions with
the gravitational field of other planets). When it
is an oval, earth's climate goes through greater
extremes. This 100,000-year cycle is one of three
Milankovitch cycles that are the pacemakers of the
Ice Ages. There is also a 40,000-year cycle (dues
to periodic changes in the tilt of Earth's axis of
rotation) and a 20,000-year cycle (due to periodic
wobbling of the Earth's axis of rotation).
In January, when the Earth is closest to the Sun,
it does, in fact, receive more intense light from
the Sun than when it is farthest away in early
July. This excess energy serves to warm us
slightly. However, this effect is much less than
the effects of axial tilt, which is responsible
for the seasons. So the Earth-Sun distance does
raise our temperature when we are closer to the
Sun, but axial tilt still controls the seasons.
Because of this, summers in the Northern
Hemisphere are slightly cooler than those in the
Southern Hemisphere, and Northern Hemisphere
winters are slightly warmer.
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