This year, the arrival of spring falls on March 20. The cherry trees in Japan have started to bloom, the Iranians have already celebrated Nowruz, the Persian New Year and the first day of spring. In the West, it is the equinox, which marks the change of weather season. It takes place this Monday at 9:24 p.m. and 24 seconds, according to the Institute of Celestial Mechanics and Ephemeris Calculation (IMCCE), based at the Paris Observatory.
The vernal equinox is one of the two times of the year when the duration of the day is equal to that of the night, that is to say exactly twelve hours. The sun is then perfectly aligned with the Equator. This happens a second time in the year, for the autumnal equinox: during the transition from summer to autumn.
Gregorian calendar and leap years
Since 2008, the arrival of spring has been celebrated on March 20. It will no longer be celebrated on March 21 until the next century. It will be necessary to wait until the year 2102 to celebrate the arrival of spring again on March 21. In the 21st century, therefore, spring will begin almost exclusively on March 20. The vernal equinox can sometimes fall on March 19, but this is much rarer. Next time will be in 2044.
This variation in the date is due to the fact that the Earth’s orbit is not perfectly circular and that the seasons therefore have an unequal duration. Also, the Earth goes around the sun not in 365 days but in 365.2422 days. That is 365 days, 5 hours and 48 minutes.
This small surplus is compensated every four years by leap years (of 366 days), which gives an average year of 365.25 days. In the Gregorian calendar, leap years are suppressed for vintages that are multiples of 100 but not 400. “We observe a slow shift towards the beginning of the month which is accentuated at each end of the Gregorian cycle (1600, 2000, 2400 and 2800)”, notes the Paris Observatory.
“After each end of the cycle, the number of equinoxes falling on March 19 increases to the detriment of the equinoxes on March 21, then the three following end of centuries there is a shift in dates towards March 20 and 21”explains the Institute of Celestial Mechanics and Ephemeris Calculation.
If autumn and spring begin at the equinox, this is not the case with winter and summer. They are defined by the solstices (the shortest day and the longest day of the year). These also vary: December 21 or 22 for winter, and June 20 or 21 for summer.