Equal day and night.

Today (22 September) is the fall equinox – when we have equal amounts of the day with sun and no sun. The length of the day is changing fast – indeed, faster than any other time of the year except spring equinox – and it is really noticeable every day. Today was rather cloudy, not a great day for solar power:

But yesterday was a very good day:

Our solar tracker is produced by Solaflect Energy, who sent out this brief description:

As a solar company, we pay a good deal of attention to the sun! So, a few fun equinox facts for you on the occasion of the Automnal Equinox, which officially happens at 3:21 today.  

Equinox comes from equal nights. Almost everywhere on earth has 12 hours of daylight and 12 hours of night today. The exceptions are the north and south poles. At the north pole, today is sunset, and if you were standing there, the sun would circle around you on the horizon all day – a very long sunset. There will be weeks of twilight before it becomes totally dark until late winter. You would see the same thing at the south pole, except that it would be sunrise.

The sun is directly overhead at the equator today. If you were standing on the equator, it would rise due east, be directly overhead at noon (no shadows – that never happens here), and set directly west. It also rises due east and sets due west here, (although with today’s weather, you’re unlikely to see either) but will be 45 degrees above the horizon at noon (at least if you were standing on the border between Vermont and Quebec, which is 45 degrees latitude).

Author: dfkotz

David Kotz is an outdoor enthusiast, traveller, husband, and father of three. He is also a Professor of Computer Science at Dartmouth College.

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