Tuesday, September 22, 2015

Fall Equinox

The autumnal equinox is at 4:21am tomorrow.  What does that even mean?

The earth circles the sun once each year.  The earth's axis is tilted 23 1/2 degrees out of the vertical.  (Recall every globe you've seen).  Since the direction of that tilt does not change, but the earth's position around the sun does change, the sun shines most on the northern hemisphere from late June to late September, and on the southern hemisphere from late September to late June.  At a moment between these two periods the sun shines directly on the equator, as the earth transitions from one hemisphere to the other: this is the equinox.  So tomorrow fall officially begins in the northern hemisphere, while spring officially begins in the southern hemisphere.  (Here's a satellite's eye view.)

(NOT to scale: sun is roughly 100 earth's wide, and roughly 100 suns away!)
Image by Kevin Niewood

At the equinox day and night are equal in length everywhere in the world--hence the name.  Only at the equinox does the sun REALLY rise due east and set due west.  It also roughly marks the time when sunrise and sunset are at their shortest: in takes only a few minutes at our latitude for the sun to completely appear at dawn, or disappear beneath the horizon at sunset.  Most importantly, the equinoxes are equal in the amount of solar radiation at any given place.

That means the sun is giving us the same amount of daily energy as it did on March 21st!  Why the difference in temperature?  Think of the daily solar energy input as a day's pay.  The heat held by Earth's surface (mainly in the enormous storage capacity of ocean and lake water) is your bank account.  Our income is the same, but we have a big bank balance of heat thanks to all the bonuses we got during the summer!  That is why you are probably dressed differently now than you were last March 21!

Everything you need to know is here!

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