|The Changing Seasons:
A Brief Tutorial
The actual reasons may not be quite as dramatic, but certainly are just as interesting.
Because the orbit is an ellipse, that means that at some point, the Earth travels slightly closer to the Sun during its 365 day journey. The Earth reaches the point in its orbit closest to the Sun in January, and it reaches the farthest point from the Sun some six months later, resulting in a slight change of distance from the Sun. If that were all that governed weather, we'd have summer in January, and Winter in July! This may be true for our Southern Hemisphere friends, but not up in the North. Something else must be going on.
It ends up that the largest contributor to the change in seasons is the 23 degree tilt of the Earth on its axis, the imaginary line that runs through the middle of the earth from the North Pole to the South Pole. The change in distance of the Earth to the Sun is a very minor player.
Because the direction of the Earth's tilt
changes in relation to the Sun, the northern and southern halves of our
planet get differing amounts of sunlight over the course of the year.
In the Northern Hemisphere of the Earth the axis points most toward the
Sun in June. Because the Earth is leaning toward the Sun, it receives
direct rays of sunlight and is warmer, as seen in the image below.
On the other hand, the Earth's axis points
most away from the Sun in December. Because the Earth is leaning
away the sun, it receives the least amount of direct rays of sunlight and
is cooler, as seen in the image below.
But wait, there's more. In the summer, the angle of the Sun is higher, and therefore the days are longer. This gives the Sun more time to heat the Earth, so it gets hotter. In the winter, the angle of the Sun is lower, and the days are short, giving the Sun less time to heat the Earth. This is a secondary effect.
Make sense yet? Try thinking about it this way. When the Sun is higher in the sky, the light is falling straight on you, and so more light (and more heat) hit each square centimeter of the ground. When the Sun is low, the light gets more spread out over the surface of the Earth, and less heat (per square centimeter) can be absorbed. Since the Earth's axis is tilted, the Sun is higher when you are on the part of the Earth where the axis points towards the Sun, and lower on the part of the Earth where the axis points away from the Sun.
Because the Earth's orbit around the Sun
does not change, we can predict the seasons;
The distance of the Earth to the Sun is
a smaller effect yet, but it does exist! So the Southern Hemisphere gets
slightly hotter summers and slightly colder winters than the North.
But only by a couple of degrees, and only on average. Your mileage may
And, if all of that wasn't enough, here
is one more thing to think about! The days are longer in summer and
shorter in winter the further you move from the equator. It's generally
dark on a summer night in Florida by 8:30 p.m., but in Vermont, there will
still be some light at 10:00 p.m. The situation is reversed in the
winter, where the Sun will go down in Vermont by 3:45 while it remains
light in Florida until 5:15. Northern Alaska is called the
"Land of the Midnight Sun" because it never gets dark during their summer
months. That part of the Earth is facing the Sun all day and all
night. Antarctica never sees daylight during those months.
Let's hope the penguins have nightlights! Of course, the situation
is reversed in December and January when northern Alaska never sees the
Sun and Antarctica sees the "Midnight Sun".
All of these conditions combine to create the different seasons we experience living on Earth.