The March equinox or Spring equinox is the equinox on the Earth when the subsolar point appears to leave the Southern Hemisphere and cross the celestial equator, heading northward as seen from Earth. On the Gregorian calendar, the Northward equinox can occur as early as 19 March or as late as 21 March at Greenwich. For a common year the computed time slippage is about 5 hours 49 minutes later than the previous year, and for a leap year about 18 hours 11 minutes earlier than the previous year. Balancing the increases of the common years against the losses of the leap years keeps the calendar date of the March equinox from drifting more than one day from 20 March each year.The March equinox may be taken to mark the beginning of astronomical spring and the end of astronomical winter in the Northern Hemisphere but marks the beginning of astronomical autumn and the end of astronomical summer in the Southern Hemisphere. In astronomy, the March equinox is the zero point of sidereal time and, consequently, right ascension. It also serves as a reference for calendars and celebrations in many human cultures and religions.
A question revolving around the vernal equinox concerns the length of day versus night. We have been taught that on the first days of spring and autumn, the day and night are equal to exactly 12 hours all over the world.
Yet, if you check the calendar pages in our Almanac, you will find that this is not so. In fact, our tables tell you that on the days of the spring and fall equinox, the length of daylight is actually longer than darkness by several minutes.
The reason this happens can be attributed to our atmosphere. If Earth was a planet that did not have an atmosphere, then yes, on the equinox days the length of the day and night would be exactly even.
However, our atmosphere acts like a lens and refracts (bends) its light above the edge of the horizon. Put in another way, when you watch the Sun either coming up above the horizon at sunrise, or going down below the horizon at sunset, you are looking at an illusion, the Sun is not really there, but already below the horizon.