On Saturday evening (20 March 1999) we marked the passage of the vernal equinox (at 0146 Z on Sunday or 8:46 PM EST, 7:46 PM CST, and so forth), an event that is popularly known as the beginning of the astronomical spring season in the Northern Hemisphere. Residents of the Northern Hemisphere will note that the length of daylight rapidly increases toward a maximum on the summer solstice during midday on 21 June 1999.
At the instant of vernal equinox passage, the sun will appear to be directly over the earth's equator. If you were on the equator, Friday would be one of two days during the year when the sun would be directly over your head at local noon. At essentially every other place on earth (except for the poles) the sun would rise directly to the east of you, and set directly to the west - provided that clouds do not interfere.
As the name suggests, (equinox is derived from the Latin word "aequinoctium" meaning equality between day and night) this event will mark one of the two times during the year when essentially all places on earth experience equal intervals of daylight and equal night during a 24 hour period. In reality, the date when an even 12 hours of daylight has passed. The exact date depends upon your latitude. If you check the local sunrise and sunset times for Friday in your local newspaper or an almanac, you will note that the length of daylight exceeds 12 hours by several minutes. As described in a previous Supplement, this extended time is the result of the effect of the atmosphere bending the sun's rays combined with the fact that the sun is not a point, but a disc whose diameter is sufficiently large to make a noticeable difference. In the tropics, the day when the elapsed time between local sunrise and sunset is precisely 12 hours falls in late February, while at 40 degrees, the date is 17 March.
The precise prediction of the calendar date when the spring equinox occurs has much historical and religious significance, especially in Christianity, because the determination of when Easter is to be observed depends upon the date of the equinox. The U.S. Naval Observatory has prepared tables of the times of the start of the seasons (equinoxes, solstices) and other astronomical events through the year 2000. Based upon these tabulations, the equinox will occur almost 6 hours later this year than it did last year. The reason for this annual shift results because the earth makes one circuit of the sun in 365.2422 days. The insertion of an extra "leap year" day at the end of February of every fourth year (such as in 1996 and 2000) essentially corrects for this drift, causing the time of vernal equinox to be 18 hours earlier than in 1995. As discussed in an earlier Supplemental Information file, the Gregorian Calendar reform to adjust the problem with the early occurrence of the equinox, provides a better correction scheme than did the Julian Calendar, by requiring that only those century years as 1600 and 2000 be considered leap years, while the others (1700, 1800 and 1900) are "normal" years.