Why this spring will not start on 21 March – Le Figaro

The astronomical spring begins the day of the March equinox, which this year falls on March 20, at 5:30 am in France. And meteorologists, it’s still early.

spring often taught in school that begins the day of the equinox March 21 of this month. Unfortunately, this is a simplification, sometimes belie the astronomical parameters involved in determining this very special day.

According to the tables of the Institute of Celestial Mechanics and Ephemeris calculation of the Observatory paris (IMCCE), the spring equinox this year falls on Sunday 20 March, precisely in 5h30m and 12s (French time). This is the exact moment when the sun crosses the earth’s equator. Daily illumination then becoming longer in the northern hemisphere than in the south, which marks the end of winter and the coming of spring in our hemisphere. Day length is so as long as that of the night, the Sun rises due east and sets due west.

Orbite not quite circular

But why the equinox does not always he falls on March 21 but sometimes 20 or even (very rare way) 19? It is precisely to avoid the drift of season dates that our Gregorian calendar was introduced in 1582, but the revolution of our planet around the Sun is actually quite complicated to fit into any schedule. Firstly because the Earth’s orbit is not perfectly circular (the Earth-Sun distance varies from 147 to 152 million kilometers), so that the seasons have unequal length. Moreover, the full rotation around the sun lasts exactly 365.2422 days, which is a little different from the statutory 365-day year.

The leap years, like 2016, used to add one day every 4 years making it to one year averaged 365.25 days. But this compensation is too high, and after 130 years on average, the equinox falls a day earlier. Which is the case now. The twenty-first century, only the years 2003 and 2007 had the spring equinox falling on March 21. Very rarely during some leap years (during which February has 29 days) the same spring begins March 19. Next time it will be in 2044.

offset meteorologists Des

To complicate things, the astronomical seasons, punctuated by the ballet of the Earth around the Sun, do not match not to such seasons defined by international meteorological agencies. For the astronomical seasons are not all the same length (winter lasts 89 days and 93 were in the northern hemisphere), and as we have seen they do not start at a fixed date.

“in meteorology, summer is the period of the warmest year, says the Meteo France website. The maximum duration of sunshine is around the summer solstice (20 or 21 June). But because of the inertia of the atmosphere, it’s only about three weeks later than the average temperature is usually at its maximum, that is to say in mid-July. “Considering the hottest time is the middle of summer, it begins in early June and ends in late August. Meteorologists believe that each season lasts three months. Spring begins for them well before the equinox, on March 1.