Autumnal Equinox - photo from wundergoround.com

Today marks the official start of fall or autumn (how did we get those two names we wonder?). It seems to us that as people have become more and more interested in "wellness" and healthy mind and body crap, they are now calling it the equinox. What ever happened to just the first day of fall? That conjures up memorable images of pumpkins, class trips to the apple orchard, and falling amber leaves more than "equinox". But the equinox has that implication of equality. In Latin, aequus, equal + nox, night. So today we have equal time of night and day, 12 hours of each right? Wrong. It looks like we have 12h 09m of daylight today.

What the...? Well it's not entirely a scam. The equal times of day and night actually occur slightly before or slightly after the equinox depending on where you are located. Okay this makes sense given that we live on a big blue marble. So lets get a little technical.

On the day of an equinox, the geometric center of the Sun's disk crosses the equator, and this point is above the horizon for 12 hours everywhere on the Earth. However, the Sun is not simply a geometric point. Sunrise is defined as the instant when the leading edge of the Sun's disk becomes visible on the horizon, whereas sunset is the instant when the trailing edge of the disk disappears below the horizon. These are the moments of first and last direct sunlight. At these times the center of the disk is below the horizon.

Another reason for the discrepancy is that atmospheric refraction makes the sun appear higher than it really is. This creates a longer day than if we had no atmosphere (not recommended).

So is today the day you try to impress your friends by standing eggs on their end? If you want, sure. But today is no different than any other day for the balance of an egg. The egg spinning and balancing thing is a myth. Sigh... we remember doing that as a kid. It was pretty exciting.

Incidentally, with the declared onset of El Niño, our days become slightly longer by thousandths of a second.

So the official time of the event is 16:30UT which is 12:30pm locally. And while our day and night isn't equal, maybe for now we call it the paene equinox.