Although in the collective imagination the beginning of spring is associated with March 21, this date has not been valid for some time. Since 2007, in fact, the spring equinox always falls on March 20 and will continue to do so at least until 2102, with rare exceptions in which it could even anticipate on March 19th.
But what did this change have determined? There are two main causes: the inclination of the earth’s axis and the structure of the Gregorian calendar.
The role of the inclination of the Earth axis
The rotation axis of the earth is not perfectly vertical, but inclined with respect to the orbit plane. This means that during the year, our planet approaches and moves away from the sun in a cyclical motion. The spring equinox marks the moment when the day and night have exactly the same duration, due to the position of the earth compared to the sun. However, since the terrestrial orbit is not perfectly regular, the date of the equinox undergoes light variations over the centuries.
The effect of the Gregorian calendar
The Gregorian calendar, adopted in the 16th century to correct the inaccuracies of the previous Giuliano calendar, also contributes to the change in the date. A calendar year lasts about 365.2422 days, but our calendar considers only 365 days, with the addition of an extra day on the bissestils to compensate for the blur. However, this correction is not perfect and leads to a progressive movement of the date of the equinox.
What will happen in the future?
Until 2102, the spring equinox will continue to fall on March 20, with sporadic exceptions in which it could anticipate on March 19th. Only in a distant future, due to the variations in the motion of the earth and of any future corrections of the calendar, could return to being March 21st.
Therefore, it’s time to update school books: March 21 no longer marks the beginning of the summer! And if someone asks you when spring begins, now you know that the exact answer is March 20.
