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Seasons are caused by the Earth's distance from the Sun

On a cold winter's evening, you can huddle near a fire for warmth. If you get too close it can burn you, and if you are too far away it can hardly warm you at all. Clearly, the amount of warmth you get from something hot depends on its distance.

And hey, the Earth's orbit is an ellipse! So sometimes it's closer to the Sun, sometimes farther away. This must be why we have seasons, right?

Wrong. If you do the math, you'll find that the Earth should only be a few degrees warmer when it is at perihelion (closest to the Sun) than when it's at aphelion (farthest from the Sun). Yet the difference between summer and winter in most locations is a lot more than just a few degrees.

Even worse, when it's summer in the Northern Hemisphere, it's winter in the south, and vice-versa. So clearly it can't be the distance to the Sun that makes the difference.

The real reason for the seasons is the tilt of the Earth. Ever notice that a globe of the Earth is always tilted? That's because the Earth's spin axis (the line connecting the north and south poles) is tilted to the plane of the Earth's orbit around the Sun. The amount of the tilt is about 23.5 degrees.

In the summer, the Earth's axis is pointed toward the Sun (well, not exactly at the Sun, but in that direction). When that happens, the Sun gets higher in the sky. Its light is more concentrated, and it heats the ground more efficiently. Also, days are longer, giving it more time to heat things up. Summers are hot.

In the winter, when the Earth's axis is directed away from the Sun, the Sun is lower in the sky. The light hits the ground slanted, spreading it out. That makes it heat things a lot less efficiently. Days are also shorter, giving it less time to heat things up. Winters are cold.

That's why the opposite hemispheres have opposite seasons, too. When the Northern Hemisphere of the Earth is tipped toward the Sun, the southern one is tipped away, and vice-versa.

Sometimes, good science just depends on your slant on things.

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