Seasons and Solstices
Why are there seasons? For many years, people pondered this same
question, and it took humankind until the 1500s to finally figure
it all out. The solution, it turns out, is as simple as the Earth
orbiting the Sun. Almost every person born since the Civil War has known
this since he (or she) was a child, but still, just because the Earth goes
'round the sun, this does not imply that seasons should necessarily occur;
there's something a little more complex than just the orbit of the Earth
involved. This section of the tutorial shall examine how it all works,
and why, exactly, there are seasons.
A few terms you ought to know
- A time at which the days and nights are the same length around the world.
- Occurs around March 21 and September 21 (but not necessarily on
- Occurs when the Sun is directly over the equator.
- Is either vernal (in the spring) or autumnal (in the fall).
- A time at which either day or night is the longest it will be during
- Occurs around June 21 and December 20 (but not necessarily on
- Goes simply by winter or summer solstice.
- Occurs when the sun is directly above 23.5 N latitude (Summer
Solstice) or 23.5 S latitude (Winter Solstice).
- Will allow one pole to have 24 hours of daylight, while the other pole
has a 24 hour night.
The Axis of the Earth
The Earth spins on its axis, an invisible line through the center
of the Earth. The northernmost point of this axis is the North Pole. The
southernmost point, therefore, is the South Pole. The Equator is an
invisible line that encircles the widest point of the Earth, and is
equidistant from either pole at every point; that is to say, the
Equator is the same distance away from each pole at every point along it.
The Earth rotates along the plane of the equator, meaning that the Earth
spins in a circle represented by the equator, or any latitude line, for
that matter. (It wobbles a little, actually, but it's such a small wobble
that it really doesn't matter much unless you're thinking in terms of
26,000 years at a time.) If you stood out in space so that you could look
down over the North Pole, you would notice that the Earth spins
counterclockwise, which makes sense if you consider that the sun rises in
the East and sets in the West.
North Polar and Equatorial views of Earth and Solar Beams
The interesting thing about all this is that, even though the Earth
rotates on the plane of the equator, the Sun doesn't always follow the
equatorial path. In fact, the plane that the Sun appears to follow as
Earth rotates is dependent upon the time of the year. It's a little tough
to grasp at first, but it's this tilt that gives us the seasons.
Earth's axis tilt and equatorial plane compared with its plane of
Even atmospheric scientists have trouble with this concept at first, so
don't be discouraged. If it still isn't that clear, click on the picture,
and you'll go to a place with instant, Java-friendly animations, and a
more extensive explanation.