On the equinox, day and night are not actually equal
Equinox means equal night, yet on that date daylight wins. The Sun is a disc, so its top edge clears the horizon before its centre does, and the atmosphere bends light over the horizon too. Together these add about 14 minutes of daytime at the equator. The truly balanced day, sometimes called an equilux, falls a few days away.
Astronomically, an equinox is the instant when Earth's equatorial plane slices through the centre of the Sun and the planet's axis leans neither toward it nor away. It happens twice a year, around 20 March and 23 September. On that day the Sun rises almost due east and sets almost due west, it stands directly overhead somewhere on the equator, and the line between day and night runs straight from pole to pole, lighting both hemispheres equally. For about four days around each equinox, both poles are in daylight at once.
The halves of the year are uneven. Because Earth moves faster when nearer the Sun, the stretch from the March equinox to the September one lasts about 186 days, while the return trip takes about 179. That gives the North Pole's summer-long day roughly a week more than the South Pole's. The names get muddled too: vernal means spring, yet in the southern hemisphere the March equinox arrives in autumn, which is why some prefer the neutral terms northward and southward equinox.
Calendars have long chased these dates. Julius Caesar fixed the spring equinox on 25 March in 45 BC, but his year ran about 11 minutes too long, so by the 1580s the equinox had slipped to 11 March. Pope Gregory XIII restored it to around 21 March, the date used for calculating Easter, by allowing only 97 leap years per four centuries instead of 100. Even so, the equinox wanders; this century's earliest March equinox will fall on 19 March 2096. Precession has also nudged the first point of Aries, where the Sun crosses northward, into Pisces.
The March equinox begins the new year in the Persian, Hindu and Assyrian calendars, and the ancient Antikythera mechanism was built to predict equinoxes and solstices. Around these dates communications satellites can suffer brief disruptions.
Source: Equinox
