It’s not quite true that the equinox is 12 hours of day/12 hours of night. For instance here in Oakland the length of day today will be 12 hours 8 minutes. Further north in Eureka, Montana (hi Aunt Kris!) the length of day will be 12 hours 10 minutes, and further south in Oceanside, California (hi Grandma!) it will be 12 hours 7 minutes.
For a more in-depth explanation check out timeanddate.com.
(Photo: Poison oak in it’s fall disguise at Mount Diablo State Park. © R. Bond 2009.)