Monday, September 21, 2009
Today the day and the night are 12 hours each, equal, everywhere in the temperate regions (everywhere that's not the tropics). Since the last, Spring, Equinox, the days have been longer the farther North you are, although they have been longer than the nights everywhere North of the Tropics. But after today farther North places will have shorter and shorter days, while the farther South you go the closer you get to a perfect 12 hour day and 12 hour night. The Equinox also marks the time of the year when the length of days is changing fastest (the second derivative is greatest). This is the time of year when people look up and remark "Fall is coming!".