The Earth: Spinning seasons

Space is pretty cool and all, but it doesn't really affect us, does it? Are you kidding? Do you even know how night turns to day? Or winter to summer? It's the sun into the earth, locked to each other in rotation like a clock work.

A clock work pretty much shapes our life on the earth. Let's go see. The earth is round like a ball - a ball that spins - with the sun shining on its side. Here, from where we are looking, we can see that a place on the earth moves into the sunlight and then out of it again. But from the earth, the rotation is seen as if it was the sun that was moving, going up and down.

One full rotation of the earth takes 24 hours. That leaves 12 hours in the dark and 12 in the sun - one night and one day for each rotation. Imagine a long line that passes from the North Pole to the South Pole. The earth rotates around this imaginary line. This is the rotational axis of the earth.

If you stand close to the rotational axis, far north or far south on the earth, then the sun will appear to move in a low arc across the sky. But if you're right between the poles, near the equator, you will see the sun rise straight up and pass right above your head, then down on the other side. The rotation of the earth isn't the only thing going on in space that matters. The earth is also revolving around the sun in a large, nearly perfect circle. Imagine this circle being a big disk, a flat plane in space with the sun in the center.

This plane is called the ecliptic. Now, look at the Earth's rotaional axis and compare it to the ecliptic. The Earth is not spinning very striaght, is it? It's tilted to one side. Right now, the South Pole is closer to the sun.

Measure carefully and you'll find that the rotational axis is tilted 23.5 degrees in relation to the ecliptic. That may not sound like a big deal, but for life on Earth, it makes all the difference. It's late in December and the Northern Hemisphere is tilted away from the sun. If you lived here, you'd be close enough to the North Pole to be left in complete darkness, even at noon. It's a polar night here.

What about living here, a bit south of the Polar Circle? Here you can get some sunlight in the winter, but nights are long and days are short. Since the sunlight hits this part of the earth at a low angle, every beam of it has to travel a long distance through the atmosphere. When it hits the ground, it's spread out over a large surface. That means less energy per square meter and a colder climate compared to a beam of light hitting the Earth closer to the equator.

Close to the equator, each beam takes a shorter path through the atmosphere and is not as spread out on the ground. These two things - shorter days and the sun at a lower angle - leaves the north with winter when the axis of the rotation is leaning away from the sun. When it's winter in the north, it's summer in the south. This place is in the Southern Hemisphere. Here, the earth's rotational axis is leaning towards the sun.

Days are longer and nights are shorter, and the sun shines at a steeper angle, heating the earth more here. It's summer in the Southern Hemisphere. But it isn't always like this. Let's fast forward six months. The Earth revolves around the sun and in one-half years, it has made one-half lap, and now it's the Norther Hemisphere's turn to be tilted towards the sun.

Now that's where the days are longer and the sun shines from higher up on the sky, heating the ground more. It's summer in the north. Close to the equator, the tilt of the Earth's rotational axis is observed as a barely noticeable difference in the position of the sun. At noon, the sun is always high up in the sky, nearly straight above the head, and day and night are always about equal in length. But seasonal changes still have an impact, as they are the cause for seasonal changes in weather, bringing droughts and rain periods.

Everyone on Earth is affected by the seasonal changes caused by the Earth's axial tilt.