Friday, September 23, 2005

It's the time of the season...

Ah, fall is in the air, as yesterday afternoon the sun crossed the celestial equator to head south for the winter. Now it is officially autumn. Here in Tucson it feels like fall, as the temperature is only 99 degrees today.

By far the number one mistake the general population makes about astronomy is the cause of the seasons. A lot of people believe that the cause of the seasons is that the Earth is farther from the sun in the winter than in the summer, but that is wrong. In fact, the Earth is a million miles closer to the sun in the winter than in the summer. So what causes the Earth's seasons? It is the Earth's tilt.

Let's say it is a sunny but cold winter day, and you are outside and want to warm your face up a bit. What do you naturally do? You close your eyes and tilt your face so you are looking directly at the sun. When your face is pointed toward the sun, you are warmest. Likewise, the Earth is warmest when it is pointed toward the sun. The Earth is coldest when it is pointed away from the sun.

During winter here north of the equator, the North Pole is pointed as far away from the sun as it ever gets. At the same time, the South Pole is pointed toward the sun, so those lucky Aussies and Kiwis get summer. But by June, the North Pole is pointed as much toward the sun as it gets, and we northerners get to enjoy summer, while the southern hemisphere is plunged into the cold of winter.

So, keep this fact straight: The Earth's seasons are caused by the Earth's tilt, nothing else.

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