Fall Arrives

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Fall Arrives
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People have been following the motions of the Sun for thousands of years. They’ve even built special observatories for the job.

One of the more impressive ones is at Dengfeng in China. Astronomers built a giant sundial there almost 750 years ago. The city was a center of science, religion, education, and politics for centuries. So it became known as “the center of heaven and earth.” That made it a perfect spot for following the Sun.

Astronomers built a smaller sundial there more than five centuries earlier — part of a network of sites across the country. From the different lengths and motions of the shadows cast by the Sun, astronomers measured the length of the year to within about a half-minute of its true value — the most accurate measurement by anyone up to that time. The findings were used to create a calendar, and to measure the extent of different territories.

The observatory built in the 13th century consisted of a tall platform shaped a bit like a pyramid. A pole atop the platform cast a shadow along a hundred-foot scale on the ground.

The observatory is still there — the centerpiece of a World Heritage site — a site for following the Sun.

And this is a good time to follow the Sun because tonight is the equinox — the beginning of autumn in the northern hemisphere. The Sun rises due east and sets due west across the whole planet — a key point for skywatchers across the globe — and across the centuries.
 

Script by Damond Benningfield

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