All Things Being Equal…

Happy Autumnal Equinox to our northern readers!  And to our friends  in the southern hemisphere, sorry about the northern bias in the naming of this day; I'll just say, "Happy September Equinox to you!"

The word equinox comes from the Latin for "equal night" and it is the point at which each part of the world is receiving 12 hours of day and night.  Here's why:

Image by Przemyslaw "Blueshade" Idzkiewicz

The Earth is tilted on it's axis (about 23.5°).  As we revolve around the Sun, that tilt stays the same and so for half of our trip around the Sun, the northern part of the world is tiled toward the Sun and for the other half of the journey, the southern part of the world is tilted toward it.  An equinox occurs at the point where neither half of the Earth is tilted toward the Sun.

This does not mean that the Earth isn't tilted on its axis during an equinox; with respect to the Sun, the Earth is just tilted to the side.  Since neither hemisphere is tilted more to the Sun than the other one during an equinox, both halves of the Earth receive the same amount of light (12 hours).  

Everybody got that?

After the September Equinox, the southern hemisphere will just barely begin to lean toward the Sun and the northern part, away, giving the Southern Hemisphere just a tad bit more daylight than it's northern counterpart as the two hemispheres head into their spring and autumn respectively.  That's why using the term Autumnal Equinox is not really accurate…there is a whole half of the world that is going into spring, not autumn.

What happens at the poles during an equinox?

Excellent question dear reader!

Today, the Sun is halfway above the horizon at both the North and South Poles.  It will set over the next couple of days at the North Pole and plunge Santa and the elves into six months of night, while it will simultaneously rise at the South Pole like a celestial Kilroy on the horizon and in a couple of more days it will be fully risen, giving the South Pole six months of daylight.  In March, the situations will be reversed.

It' really kinda cool when you think about it…at the North and South Poles (at opposite times of the year, of course), the Sun slowly spirals up in the sky for three months, gets to it's maximum elevation, 23.5° above the horizon (in June for the North Pole and in December for the South Pole) and then corkscrews back down to the horizon where it will then disappear for six months.

Neat.

Want to learn something that will make you sound really pretentious?  

I thought you would!

Technically, an equinox is not the day that receives 12 hours of day and twelve hours of night.  An equinox is the point at which the center of the Sun is on the same plane as the Earth's equator, it only lasts for a brief moment in time.  The day on which an equinox occurs is called an equilux.  Today, September 23, 2011, the equinox occurred at 9:04 AM, but the equilux lasts all day long. 

Now go forth and irritate your friends with that annoyingly accurate information.

Good day, and have fun looking up.