Falling into the Equinox
Posted September 22, 2008
Our common means of marking the beginning of Fall takes place today as the earth reaches one of two points in its orbit around the sun at which our rotational axis is pointed neither toward nor away from the sun. This brief moment (the Autumnal Equinox) occurs at about 11:44 AM EDT, and of course most of us won't really notice any detectable effects apart from the fact that we continue to head toward shorter periods of daylight and longer nights, eventually leading us through Fall and into Winter.
There are some old myths about the equinox, principally that there is something special about the alignment of the Earth and Sun that makes it possible to balance an egg on the pointy end or something to that effect, but there isn't any such effect associated with the equinox. Of course, it's also worth noting that while the term derives from the idea that the length of day and night will be equal on this date, there are some physical reasons that doesn't quite work out either, and in fact we'll have to wait a few days yet for the time between sunrise and sunset to actually reduce to 12 hours. This is due to the fact that refraction by our atmosphere causes the sun to appear higher that it really is at sunrise and sunset, and gives us a few extra minutes of direct sunlight compared to what we'd experience on a planet with no air. I posted a little more on these effects in a blog about the "Equilux" last year, which you can find at http://www.wral.com/weather/blogpost/1240380/.
One point about the equinox that is true is that it is the time of year when the time separating sunrise and sunset changes most rapidly from day to day, that is the length of daylight is shrinking fastest from day to day, a process that has been speeding up since we passed the summer solstice and will slow down again as we approach the winter one in December.
Weatherwise, we just finished off a weekend that felt nice and "early Autumn-like" with highs in the 70s and fairly low humidity. We should warm up a bit today but will see high pressure over the northeastern U.S. push cooler air in our direction for the next few days, and this effect should be exacerbated by a low pressure area forming along an old stalled frontal boundary off the coast. Various computer models give differing projections as to the eventual intensity and movement of this low, so the forecast for later this week is only of moderate confidence, but it has at least a reasonable chance to provide us with a few periods of significant rain, stiff north to northeast winds, cloud cover and potentially notably cooler temperatures, and may well end up leading to a some ocean overwash, heavy surf and beach erosion along the coast.