There’s lots to celebrate this week — the vernal equinox, also referred to as the first day of Spring, Pi Day, and St. Patrick’s Day. The vernal equinox is the moment when the sun crosses the celestial equator — an imaginary line in the sky above Earth’s equator — from south to north. According to the National Oceanic and Atmospheric Administration (NOAA), it will occur at 11:06 p.m. Eastern Daylight Time. This is the earliest date that it can be, and is largely due to the Leap Year system that our calendar uses. The vernal equinox was on March 19 in 2020, but that was the first time since 1896. This is because a Leap Day is not added in years divisible by 100, unless it is divisible by 400, as the year 2000 was. So, the addition of a Leap Day in 2000 reset the calendar so that the vernal equinox comes earlier.
We will see spring arrive on March 19 a number of times during this century, not just in Leap Years, but when there is no Leap Day in 2100, it will go back to the equinox occurring on March 20 or 21. That is, until 2400.
I will save you from going down the rabbit hole of checking the date historically. Before 1753, in the United States, the vernal equinox occurred much earlier – according to the Julian calendar, which was the calendar in use here at that time. The Gregorian calendar was originally introduced in some European countries in 1582, but its adoption took over 300 years. To get the calendar back in sync with astronomical events like the vernal equinox, a number of days had to be dropped.
