Q. I always understood that the first day of spring, around March 21, coincided with the vernal equinox, the time when daylight was exactly 12 hours long. The same was true of the autumnal equinox around Sept. 21. Yet in the paper last week, I noticed that the equinox appeared to occur here on March 17, when sunrise and sunset were both at 7:35. On March 21, sunrise was at 7:28 a.m., and sunset was at 7:39 p.m., giving 12 hours and 11 minutes of daylight on the day of the equinox. What gives?