Calculating the start of spring gets messy

The meteorological calendar splits our Gregorian calendar into 4 segments for seasonal and statistical purposes. Meteorological spring started March 1. The astronomical calendar isn’t as neat and tidy.

At 12:30 a.m. EDT the sun passed directly over the equator, over the Indonesian portion of the island of Borneo in the South Pacific marking the March Equinox and beginning of spring in the northern hemisphere (and autumn in the southern hemisphere).This is also the earliest the March equinox since 1896.

The March equinox varies from year to year arriving between March 19 and March 21. The equinox most frequently falls on March 20 during the 21st century. March 21 was the most frequent throughout the 20th century.

Looking at the list of dates and times for the March equinox over the years, an interesting pattern emerges. It occurs 6 minutes later each year for 3 years, falls back by about 18 minutes each leap year, then the cycle continues.

This variance doesn’t come from any change in the Earth’s journey around the sun. It is driven by the slight difference between the mean tropical year and tropical year. The Gregorian calendar is based on the 365 day mean while it takes 365 days, 5 hours, 48 minutes, 45 second, a tropical year, for the sun to return to the same position as seen from Earth.

In 2020 the March equinox will be even earlier March 20, at 3:06 UTC (March 19 at 11:06 p.m. EDT). The earliest of the 21st century occurs March 19 2026 at 14:03 UTC, 10:03 a.m. EDT