The Changing Seasons

Four special days mark the astronomical calendar: the day the Sun is highest in the sky, the day it is lowest, and the days midway between these. One of them is about to happen: September 23rd, the fall equinox.

Unlike the stars, which always form the same patterns, called constellations, the Sun "wanders." Sometimes it is high in the sky, sometimes it is low.

We know that this motion is caused by the revolution of the Earth around the Sun, and the Sun appears high or low in the sky because our hemisphere is tilted either towards or away from the Sun. But people didn't always know this, nor is this modern understanding needed to understand the cause of the changing seasons.

Some stars are so close to the North Star that they never rise or set, but are always in the sky. The stars of the Big Dipper are like that. The closer a star is to the North star, the longer it stays in the sky. On the other hand, stars far in the southern sky are only in the sky briefly each night, if at all.

Now, how about that wandering star, the Sun? In summer, it is high in the sky. It is not close enough to the North Star to remain in the sky twenty-four hours; here in Vermont it is in the sky for fifteen hours. So not only can we bask in the direct rays of the Sunšs light and heat, the days themselves are longer. The Earth has lots of time to warm up. In winter, on the other hand, the days are very short. Only nine hours here in Vermont. The Earth cools off.

Sunday, September 23rd, the Sun enters the southern sky. From then on the nights will be longer than the days. But that day, they are equal. That is the equinox.

(09/19/07)