Vernal Equinox: The length of the time between sunrise and sunset changes daily because of the 23.5 degree tilt of the earth's rotational axis from its orbital plane. There are minor variations caused by the movement of other celestial bodies and the variable speed of the Earth in orbit, and, theoretically at least, teensy changes caused by the movement of people and things on the surface of our planet. Twice a year, the tilted Earth and the sun line up just right, and the number hours of daylight and of darkness are equal: "equinox" is derived from Latin and means "equal night". In the Northern Hemisphere the equinox in March is called "vernal" from the Latin word for Spring, and the one in September is called "autumnal" from the Latin word for Fall. There are minor variations in time and date from year to year, but scientists have figured out precise times for us. Their calculations should hold true unless there is a major perturbation in our orbit or a major precession in the tilt of our axis of rotation or unless something really, really big starts to move around on the Earth's surface.

This year the Earth will hit the Northern Hemisphere Spring sweet spot at precisely 8.35 AM Rome time on March 20. If you measure the amount of daylight in the 24 hour periods before and after that time and average the two measurements, you should get precisely 12 hours (with a little bit extra because of atmospheric light bending -- nothing is ever perfect).

The ancient Romans restarted their calendar every year at the vernal equinox and called that day the first (or kalend) of March. There has been some slippage, and the equinox is now within a day or so of March 21 every year. The original Roman calendar only had 10 months and lasted 295 days. They were initially perfectly happy to let the winter pass uncounted, but, according to legend, King Numa Pompilius split the uncounted days into January (for two-faced Janus who looked forward and back) and February (a time of preparations -- februa in Latin -- for the new year). Numerous reforms resulted in the calendar we use today.

Romans counted their days in nundinae, which means "nine-days" but actually lasted only eight. It would be like saying a week lasts from Sunday through Sunday -- it sounds silly, but everyone knew the convention. The Roman solar/lunar/nundinae calendar became such a nightmare -- there were eventually 244 holidays -- that nobody bothered to remember it. It stayed the same every year, barring feriae imperativae (literally, holidays on demand), so people just painted a copy on the wall inside the front door and checked it on the way in and out. Calendars were also painted on the walls of public buildings. They were pretty much the same throughout the Roman world, although some days bore additional labels for local feasts and celebrations.

For information on the vernal equinox, go to http://www.celestia.com/SRP/MA96/Html/VernalEquinox.html, or to http://www.nws.noaa.gov/er/box/equinox.html

For Roman Calendar Information, go to http://www.geocities.com/SiliconValley/Lab/6024/roman-cal.html, or to http://www.ancientsites.com/er/romankalendar.html.

For an interactive Roman calendar like the one painted on Roman walls, see http://www.clubs.psu.edu/aegsa/rome/romecal.html.