SIC-Reason 4 Seasons

Science in the City

Reason 4 Seasons

DEAR SCIENCE IN THE CITY,

There I was at 5 pm, shivering in the dark on my bike ride home, and seriously contemplating a move to Australia. I’d happily trade my December turkey feast for a beach barbeque. But then I wondered, why does Australia have summer when we have winter? Is it a Down Under oddity, like Vegemite?

Frostbite

Dear Frostbite,

It’s not just an Ozzie thing—the entire southern hemisphere, including South America, Southeast Asia, India and most of Africa, has summer in December and winter in July.

Lots of people think that it gets warmer when the Earth gets closer to the sun during its yearly orbit, but that wouldn’t explain Australians’ December beach-going. They’re on the same planet as us after all, so how could they be close to the sun if we’re far away? In truth, the Earth’s orbit around the sun is nearly circular. The Earth is a tiny bit closer to the sun during Vancouver’s winter.

To explain our dark, cold December days, you need to consider the angle of the sunlight. You can demonstrate this with a flashlight. I’ll wait while you go get one!

Hold the flashlight at the level of your head and aim it straight down at the newspaper. Notice that it lights up a small circle. Now, don’t change the flashlight’s height, but tilt it so the light hits the paper at a glancing angle. Notice that it lights up a bigger oval shape—but the light on the paper is not as bright. When you take the same light and spread it over a bigger area, it isn’t as bright—or as warm.

What does this have to do with the sun? The Earth’s axis (an imaginary stick connecting the north and south poles) points in the same direction no matter where the Earth is in its orbit. Sometimes the north pole points slightly towards the sun, sometimes slightly away from the sun and sometimes sideways-on to the sun. Check out the picture (ABOVE).

When the north pole is pointing towards the sun, the sun’s light hits northern parts of the Earth straight on. The light is bright and hot, and we have summer. Meanwhile, in the southern hemisphere, the sun’s rays hit at a glancing angle, like when you tilted the flashlight. The light and heat are weaker when they’re spread out more.

Australia gets its summer when the south pole is pointing towards the sun, so they get the sunlight straight on. That’s when we get sunlight at a glancing angle, in December, making Vancouver cold and gloomy.

When the Earth’s axis tilts sideways-on to the sun, the sun shines directly onto the equator. Both Canada and Australia get sunlight at an angle. This happens twice a year—in the spring and in the fall—and it’s called the equinox. So there you have it, Frostbite, you’re at the mercy of the tilt of the Earth’s axis.

~Seasonal Sage, Sandy Eix, Ph.D.