Wednesday, June 21, 2006

Happy summer solstice!

This morning, at 8:26am EDT, 5:26am PDT, or 12:26 Greenwich Mean Time, the sun reached its most northern part in the sky for the year, marking the summer solstice and the beginning of summer for the northern hemisphere. And our poor friends in the southern hemisphere are now looking at the start of winter. (They'll get their laughs in six months.)

This means it is also time for me to harp on one of the largest misconceptions in public astronomy knowledge -- the cause of the seasons. The cause of summer is not that the Earth is closer to the sun in the summer. In fact, the Earth is getting further away from the sun for the next few weeks yet, and reaches its closest point to the sun in January. The Earth's distance from the sun has nothing to do with the seasons!

The cause of summer is that, right now, the North Pole is tilted toward the sun, so the sunlight hits us more directly. The southern hemisphere is having winter because the South Pole is tilted away from the sun. Think about a nice cold day (a pleasant thought in this heat!). If it is sunny and you want to warm your face, you naturally tilt your head so the sun is shining directly on your face. Now that it is summer, you naturally tilt your face away from the sun and run for shade or air conditioning. It works the same for the Earth. When it is warm, we are tilted toward the sun, when it is cold, we are tilted away from the sun.

Another experiment -- Go out at lunchtime in the next few days and look at your shadow. If you are in the Northern Hemisphere, you notice your shadow is short, because the sun is nearly overhead. Now, this winter, look at your shadow again at noon, and you'll notice it is much larger. That's because the sun will no longer be overhead.

So, promise to remember -- the tilt of the Earth causes seasons. And I promise not to harp on this issue for another six months.

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