The Tilt of the Earth and the Seasons
Astronomy is not just about the night sky, but about the motions of all planets, including our own. These motions
produce the noticeable changes in the length of day and night throughout the seasons. In this section, I will try to
explain how the tilt in the Earth's axis produces the effect of seasons.
Why are the poles cooler than the equator?
The polar regions of earth are not cooler because they are farther away from the equator, and they are not cooler
because they are farther from the sun. The poles are only about 3000 km farther from the sun than is the equator at
noon. This is about 0.002% of the earth-sun distance -- hardly significant.
The polar regions are cooler for the same reason as the morning and evening are cooler. From the polar regions, the
sun is low in the sky, so that the sunlight hits the earth at a low angle, as it does for us in the morning and evening.
This low angle means that the sunlight is more spread out, and thus warms the surface less.
Why are there seasons?
The Earth's orbit is eliptical - so it is sometimes closer to the sun than at other times. There is a misconception that
this causes the seasons. If that were the case, then the whole planet would experience summer at the same time, and
winter at the same time. But when it is summer in the northern hemisphere, it is winter in the southern hemisphere.
The Earth is actually closest to the sun (perihelion) during the northern hemisphere winter.
In fact, the reason for seasons is the tilt of the earth.
The earth's axis is tilted by about 23.5° from vertical. This tilt remains constant as the earth orbits around the sun. As
a result, in part of the earth's orbit the northern hemisphere is tipped more towards the sun, and in part of the orbit it
is tipped away from the sun. For the same reason as in the previous section, when the northern hemisphere is tilted
away from the sun, the angle of sunlight is lower, and there is less surface heating. This makes it winter.