The Vernal Equinox
The word equinox is derived from the Latin words meaning “equal night.” All over the world, days and nights are approximately equal. Today, the Sun rises exactly in the east and sets exactly in the west.
At the equinoxes, the tilt of Earth relative to the Sun is zero, which means that Earth’s axis neither points toward nor away from the Sun. (However, the tilt of Earth relative to its plane of orbit, called the ecliptic plane, is always about 23.5 degrees.)
Astronomically speaking, the March equinox occurs when the Sun crosses the celestial equator on its way north along the ecliptic. In the Northern Hemisphere, the March equinox is known as the vernal, or spring, equinox, and marks the start of the spring season.
In the Southern Hemisphere, this equinox is known as the autumnal, or fall, equinox and marks the start of the fall season; the vernal equinox for the Southern Hemisphere occurs in September.
The March equinox happens at the same moment across the world but is converted to local time. In 2015, it falls on March 20 at 6:45 P.M. EDT, 5:45 P.M. CDT, 4:45 P.M. MDT, and 3:45 P.M. PDT, for example.
Meteorologically speaking, however, in the Northern Hemisphere the official spring season always begins on March 1 and continues through May 31. Summer begins on June 1; autumn, September 1; and winter, December 1.
Weather scientists divide the year into quarters this way to make it easier to compare seasonal and monthly statistics from one year to the next. The meteorological seasons are based on annual temperature cycles rather than on the position of Earth in relation to the Sun, and they more closely follow the Gregorian calendar. Using the dates of the astronomical equinoxes and solstices for the seasons would present a statistical problem because these dates can vary slightly each year.