Even with climate change throwing things out of whack and speeding us toward our planet’s ultimate demise (or the demise of humanity, at least; Earth will live on long after we’re gone), the changing of the seasons is as regular and reliable as, well, the sun rising in the east and setting in the west. But what causes Earth’s seasons to change?
Many people think it has to do with our orbit around the sun. It does, to some extent, but that’s not actually the biggest factor—if it were all about how close to or far from the sun our planet is, the seasons would be the same all over the world. As it is, the seasons are direct opposites in opposite Hemispheres: when it’s summer in the Northern hemisphere, it’s winter in the Southern.
What really causes the cycle of seasons (with the attendant temperature changes and wildly varying differences in the amount of daylight we get) is the slowly shifting angles of the sun’s rays as they strike the earth. If you’re looking for an easy but effective way to visually demonstrate how sunlight angles change as the calendar does, look no further!
Comb Through the Evidence (or, The Evidence Through A Comb)
For this Demo Science science demo, you’ll need a comb, a long, rectangular piece of cardboard or tag board, and a light source (the sun works, obviously, but a flashlight will be easier to control and maneuver as needed). The cardboard should be either very light or very dark colored, so the contrast of light and shadow will show up as much as possible. You may also need an assistant for this one.
Lay your cardboard flat on a table. Hold the comb at one edge of the cardboard, with the teeth down. Align your flashlight so that it shines through the teeth and onto the cardboard, making a display of alternating stripes of light and shadow.
Keeping your comb and light source in place, tilt the cardboard upward slowly from the side opposite the comb. As its angle away from the table increases, the length of the light-and-dark stripes will change accordingly. The steeper the angle, the shorter the stripes will be. However, shorter stripes will be brighter, as there is less area for the light to cover.
Slowly tilt the cardboard back down and watch as the stripes grow longer, but dimmer.
But the Earth’s Not Flat OR Cardboard…
The earth is neither flat nor cardboard, it’s true, but the principle remains the same. Our planet tilts on its axis, and while this angle never really changes, the orientation of Earth on this angle does. When we’re tilted away from the sun, as in winter in the Northern Hemisphere, sunlight doesn’t strike our half of the world as directly; this causes the light that does hit us to be spread more “thin,” if you will. Less direct sunlight equals less direct heat and, of course, lower temperatures.
When we swing back around as winter becomes spring becomes summer, our Hemisphere is tilted toward the sun again. Light rays hit us more directly and are less spread out, heating us up again.