(c) Anthony K. Grafton 2003

The Length of Days

By now you’ve probably noticed that the days are getting longer. In December, it was dark by 6 pm, and now its daylight past 6:30 pm. And soon, as we enter June and July, it will still be daylight an hour or two later. What causes this to happen?

The change of seasons and the length of days on the Earth are directly caused by the tilt of the earth axis as it spins. Imagine someone spinning a basketball on his finger as he walks in a circle around you. If the ball is spinning on an axis that runs straight up and down, then it looks the same to you whether it is on your right side or your left side. But if the ball were spinning with its axis tilted, then on one side of you the top of the spinning ball would be pointed towards you, and as the person walked around to your other side the top of the spinning ball would be pointed away from you.

This is the same as what happens to the Earth as is orbits around the sun. Sometimes the top of the Earth (what we commonly think of as the Northern hemisphere) is pointed more toward the sun, and at other times the top is pointed away from the sun. When the Northern hemisphere is pointed slightly towards the sun, we in the United States experience longer, summer days. When our hemisphere is pointed away from the sun, we experience shorter, winter days.

Here are a couple of questions to consider. When we in the United States are having a hot July summer, what season is it in South America? And is the Earth closer to the sun during our summer or winter?