How the Vernal Eqinox Works

Happy Vernal Equinox to everyone. Well let's say that at 7:44 AM Eastern Daylight Time it will be the Vernal Equinox or the First Day of Spring in the Northern Hemisphere. That doesn't necessarily mean that today you will have 12 hours of sunlight and 12 hours of night, that would depend on where you are in the world. What it does signal is that the sun is directly overhead of equator and will rise due east and set due west. But to explain why it is on March 20th some years and March 21st some years and could someday be on the 19th, lets see what National Geographic has to say to make it more clear...

"Using the Julian calendar, the spring and fall equinoxes and the seasons were arriving 11 minutes earlier each year. By 1500 the vernal equinox had fallen back to March 11.

To fix the problem, the pope decreed that most century years (such as 1700, 1800, and 1900) would not be leap years. But century years divisible by 400, like 2000, would be leap years.
Under the Gregorian calendar, the year is 365.2425 days long [note: the astronomical tropical calendar year is 365.24219 days long. OoS]. "That gets close enough to the true fraction that the seasons don't drift," MacRobert said.

With an average duration of 365.2425 days, Gregorian years are now only 27 seconds longer than the length of the tropical year—an error which will allow the gain of one day over a period of about 3,200 years.

Nowadays, according to the U.S. Naval Observatory's Chester, equinoxes migrate through a period that occurs about six hours later from calendar year to calendar year, due to the leap year cycle.

The system resets every leap year, slipping a little bit backward until a non-leap century year leap nudges the equinoxes forward in time once again."

I couldn't have said it any better. Have a nice day.