Tuesday, June 22, 2010

It's summer!

Today is the first day of summer, which makes it a great day to address the most common misconception people have about the universe around us: what causes Earth's seasons?

Most people think they know the answer, and most people are wrong.  So, the chances are good that you think you know why we have seasons, and that you are wrong.  Now don't get angry about being wrong, and don't get defensive.  I'll lead you to the right answer,  I'll never tell a soul that you were wrong, and you can go forth and pretend that you knew all along.  It'll be our secret.

Let's dig in:

Wrong: Earth's distance from the sun causes the seasons.  Most people think that the Earth is closer to the sun in the summer and further away in the winter, and this changing distance is what causes the seasons.  It is true that the distance between the sun and the Earth changes, and that distance changes from to 91.3 million miles to 94.4 million miles.  But the Earth is closest to the sun in early January, and furthest from the sun in July, almost exactly backwards from what most people in the Northern Hemisphere think! Also, remember that the northern and southern hemispheres of the Earth have opposite seasons, which would not be true if the Earth-Sun distance were responsible for Earth's seasons.

In short, the Earth-Sun distance does not cause the seasons.

Correct answer: Earth's tilt causes the seasons.  The Earth is not straight up and down with respect to the sun. (Stated more scientifically, Earth's rotation axis is inclined, or tilted, with respect to its orbit).  Our north pole is pointed in a direction toward Polaris, the North Star.  And it always points in that direction.  As the Earth orbits the sun, sometimes that direction is toward the sun, and sometimes it is away from the sun.  On the summer solstice (today!), the Earth's North Pole is pointed as close to the sun as it can get.  On the winter solstice (around December 22), the North Pole is pointed as far away from the sun as it can get.

So, why does this tilt cause the seasons?  Imagine yourself outside on a chilly but sunny day, and your face is a little chilly.  What do you do?  If you are like me, you close your eyes and point your face right at the sun to get some warmth.  Or imagine you are outside on a cold night, your hands are cold, and the only source of heat is a small candle.  What do you do to get your hands warm?  You likely will cup them around the candle's flame.

In both of these cases, you are placing your cold skin so that it receives the most direct light from the sun or the flame.  If you are not facing directly into the sun, only the part of your face looking toward the sun gets maximum heat.  If your fingers are pointed away from the candle's flame, they don't get heat.

The same is true for the Earth.  When Earth's northern hemisphere is pointed more toward the sun, it gets more direct light and more direct heat.  Therefore, that hemisphere experiences higher temperatures and what we call the summer season.  When the northern hemisphere is pointed away from the sun, it gets less direct energy, and so it experiences cooler temperatures and winter.

The Earth's southern hemisphere, which points in the opposite direction as the northern hemisphere, therefore points toward the sun when the north pole points away, and vice-versa.  This is why, although it is summer here in the United States, the soccer teams at the World Cup in South Africa are struggling against cold weather.

So, it's that simple.  When your hemisphere points more toward the sun, you get warmer weather.  When it points away, you get colder weather.  The Earth's distance from the sun doesn't matter. Remember this, and you'll have mastered one of the most basic and most misunderstood facts of astronomy!

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