Bad Astronomy: Misconceptions and Misuses Revealed, from ...

A DASH OF SEASONS 53
hot in the summer and cold in the winter, and that’s why we
have seasons. It’s not our distance from the Sun, but the direction
to the Sun and therefore the angle of the sunlight that makes the
difference.
Take a look at the diagram showing the Earth’s axis relative to
the Sun. Note that when the northern-hemisphere axis points
toward the Sun, the southern-hemisphere axis points away, and
vice versa. That’s why people in the southern hemisphere celebrate
Halloween in the spring and Christmas in the summer. I wonder
if the song “I’m Dreaming of a Green Christmas” is popular in
Australia . . .
There’s an added tweak, too: because of our axial tilt the Sun
gets higher in the sky in the summer, as we’ve seen. That means
the path the Sun appears to travel in the sky is longer, so the Sun
is up longer during the day. This, in turn, gives the Sun more time
to heat up the Earth. Not only do we get more direct sunlight, the
sunlight also lasts longer. Double whammy! In the winter the Sun
doesn’t get up as high, and so the days are shorter. The sun also has
less time to heat up the ground, and it gets even colder. If the Earth
were not tilted, days and nights would be 12 hours each, no matter
where you were on the Earth, and we’d have no seasons at all.
Take another look at the figure on page 51. It shows that the
Earth is actually closest to the Sun in January. This is the final nail
in the coffin of the misconception that distance to the Sun is the
main reason we have seasons. If that were true, we should have
summer in January in the northern hemisphere and winter six
months later in June. Since the opposite is true, distance must actually
be a bit player in the seasons game.
However, it is not completely negligible. Distance does play a
role in the seasons, although a minor one. For the folks in the
northern hemisphere it means that winters should be a couple of
degrees warmer on average than they would be if we orbited the
Sun in a circle because we are closer to the Sun in the winter. Conversely,
the summers are a couple of degrees cooler because we are
farther away. It also means that people in the southern hemisphere
should have hotter summers and colder winters than do those living
in the northern hemisphere.