Tuesday, September 22, 2009

Autumnal equinox

Something especially for today:

Does the sun rise due east and set due west at the equinox?

Yes, it does. And that’s true no matter where you live on Earth, because we all see the same sky. No matter where you are on Earth, you have a due east and due west point on your horizon. That point marks the intersection of your horizon with the celestial equator – the imaginary line above the true equator of the Earth.

At the equinoxes, the sun appears overhead at noon as seen from Earth’s equator, as the illustration at the top of this post shows. That illustration shows the sun’s location on the celestial equator, every hour, on the day of the equinox.

That’s why the sun rises due east and sets due west for all of us. The sun is on the celestial equator, and the celestial equator intersects all of our horizons at points due east and due west. This fact makes the day of an equinox a good day for finding due east and due west from your yard or other favorite site for watching the sky. Just go outside around sunset or sunrise and notice the location of the sun on the horizon with respect to familiar landmarks.

If you do this, you’ll be able to use those landmarks to find those
cardinal directions in the weeks and months ahead, long after Earth has moved on in its orbit around the sun, carrying the sunrise and sunset points southward.
From a site called Earth Sky: A Clear Voice for Science.

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