Spring in the Northern Hemisphere officially begins with the Vernal Equinox, the day (actually a moment in time) when the position of the sun in our sky crosses to the northern side of the celestial equator, having spent the previous six months shining more directly on the southern half of our globe. For those who dwell in the Northern Hemisphere, this means a period of transition: around the time of the Vernal Equinox, day and night are of more or less equal length, following the longer nights and shorter days of winter and preceding the longer days and shorter nights of the summer season. (The word equinox comes from a Latin compound meaning “equal night,” although, due to the complex nature of astromechanics, the actual date on which the length of daytime and nighttime are closest to being equal tends to run a few days ahead of the spring equinox and lags a few days behind the fall equinnox, and also varies considerably according to the latitude of a given location.)
In our modern Gregorian Calendar (which is a 16th century slight modification of the Julian Calendar, established way back in the time of Julius Caesar), the Vernal Equinox falls, in any given year, on a date as early as March 19 and as late as March 21. The shifting of the sun’s apparent position throughout the year is caused not by any actual motion of the sun itself, but rather is the result of an effect created by the fact that earth’s axis is not perpendicular to the plane of its orbit around the sun, but is tilted at an angle of a little more than 23 degrees. This means that, for one half (six months) of the earth’s annual journey around the sun, one hemisphere is pointed more directly at the sun than the other, and then they trade places at the Equinoxes. (The corresponding Equinox in the month of September is called the Autumnal Equinox by those who inhabit the Northern Hemisphere.) The hemisphere receiving more direct sunlight (and for a longer portion of each 24-hour day) experiences summer, and the hemisphere receiving less direct sunlight (and for a shorter portion of each 24-hour day) experiences winter.
The Equinoxes also mark the only 2 days in each year when, as seen from every position on the globe (except the poles), the sun rises exactly due east and sets exactly due west. Also, in the weeks immediately before and after the equinoxes, there is a more noticeable and rapid change in the length of daylight hours than at other times of the year. At its peak, the difference is as much as 3 minutes from one day to the next. That means that, around the time of the Vernal Equinox, the length of daylight time on a given day is about 20 minutes longer than it was just a week before – a noticeable difference that prompts the oft-heard “My, the days are certainly getting longer!” and other similar comments during this season.
