Autumn begins today

| September 22, 2008 @ 7:30 am | 10 Replies

Autumn (or as we all say it for real, Fall) offically starts today at 10:43 am CDT (1543 GMT). This is the time when the earth’s tilt and revolution around the sun allow the entire world to have 12 hours (+/-) of daylight and 12 hours (+/-) of dark. The view below is a satellite view taken near the fall equinox in 2002 (from Naval Research Lab). Click here for a very cool movie of satellite images through one whole year, showing the sun’s changing angle.

These Geochron-style charts (from the US Naval Observatory) show the pattern of daylight on a world map at 10 am CDT today, compared to how it was back on July 1. Notice how it’s even north-to-south now, but the light was concentrated in the northern hemisphere on July 1. (These are similar to the “Geochron” charts you see in Tom Clancy movies.)


(July 1)

Our days are getting shorter, too. In Birmingham, we had 14 hours, 24 minutes of daylight on June 21, we’ll have 12 hours, 7 minutes today (the extra 7 minutes are due to refraction, etc.), and we’ll only have 9 hours, 56 minutes on December 21. Here is a chart of the length of day at Birmingham through the year.

For people who like math, it’s a sine wave. The derivative (rate of change) of a sine wave is a cosine wave, which means we are losing daylight rapidly right now. About 2 minutes a day.

The sun is also at a lower angle than it was in the Summer. This is the chart of measured solar energy by hour at UAH yesterday and on June 27. The up and down changes are due to clouds, but notice that the peak right now is only about 800 Watts per square meter (W/m2), and in early Summer it was around 1100 W/m2.

With the sun’s energy weakening over the Northern Hemisphere, it should really cool off in the next 2 months. Check out temperatures in the Arctic from last evening:

These are in Celsius…but it’s already way below freezing at high latitudes, with air in the 30s and 40s (Farenheit) all the way into Europe, Alaska, and Canada. And, it’s already -40 on the ice cap on Greenland!

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