Happy Equinox! (or (various) new year(s))
22 March 2010
Well, even if the OED defines them as such, neither the September equinox nor the just-past March variety actually counts an exactly symmetrical twelve hours of daylight and darkness. Often, the fussy details of things in astronomy (like whether an equinox is labelled as spring, or just March) are related to an observers location on Earth. This time, though, it mainly matters that this was only the spring equinox if you live north of the equator, so identifying it by its month is less hemisphere-centric. Which is absolutely a word.
At any rate, what the OED doesn’t relate is that atmospheric effects of refraction cause the Sun to appear above the horizon when its actual position is below our geometric horizon, as defined by the curvature of the Earth. The practical effect of this is that the Sun ‘sets’ (goes below that geometric horizon) before we see it set below our apparent horizon. Thus, even on the equinox we get a few extra minutes of daylight, making it more like twelve hours and eight minutes of daylight and eleven hours and fifty-two minutes of darkness at mid latitudes. The exact amount, I think, does vary a little bit with the observer’s latitude, but the US Naval Observatory once again has a very precise definition.
Another interesting component about this particular day/event is that is marks the start of a new year for many disparate cultures. To wit, it did for the Romans prior to some of the many calendar adjustments performed by various rulers. Persian new year (sometimes called Noruz according to some I have talked to, but please feel free to correct me) is a festival of about ten days that also begins with the equinox. Passover begins within this lunar cycle, in fact at Sundown on Monday 29 March, though I am still not familiar with all the details of how to calculate months in the Jewish calendar. Easter is determined according to a complex formula that begins with the equinox but then explicitly avoids Passover. The sources I’m familiar with on Celtic culture are more vague, but Beltane is a good, old-fashioned bonfire building spring fertility and planting festival which would make sense to hold about now. Chinese new year happened with an earlier lunar cycle, but again, I’m not totally clear on the details of this calendar.
And last but not least, the First Point of Aries is the spot in the sky where the track of the position of the Sun against the background stars throughout the year (aka, the ecliptic) crosses the projection of the Earth’s equator onto the same background stars (aka, the celestial equator). Believe me when I say that if I could find a good diagram, I would, but the only ones I know are still under copyright. Anybody know how to use Illustrator or SolidWorks?
Now back to your regularly scheduled programming…