Spring is back

While the temperatures of the last days of winter and the first shoots and buds on the trees had already announced it, on 20 March spring officially began again at 4:33 p.m. (Italian time). At that exact moment, the Sun crossed one of the two points, in the celestial sphere, where the ecliptic and the celestial equator intersect: the so-called vernal point or spring equinox (the other corresponds to the autumn equinox). To be precise, therefore, the term equinox should not be used to refer so much to a day as to an instant.
This is not a mistake, in fact, contrary to what we learned as children, the beginning of spring does not necessarily fall on 21 March, but can be on 19, 20 or 21 March. How is it possible that the day changes? Let’s find out below.

Solstices and equinoxes are due to the trajectory with which our planet orbits the Sun. In fact, the Earth’s axis of rotation and orbital plane are not perpendicular, but meet at an angle of about 23.27 degrees. This gives rise to the apparent motion of the Sun in the sky during the course of the year, causes differing lengths of day and night throughout the year and causes the seasons to alternate.

Spring begins with the spring equinox (which falls between 19 and 21 March) and ends with the summer solstice, which falls between 20 and 21 June and marks the beginning of summer. On this day there are the highest number of hours of light, because the Sun is higher on the horizon (this happens in the northern hemisphere, while in the southern hemisphere winter begins). The autumn equinox occurs midway (22-23 September) between summer and winter, which begins on the darkest day of the year, falling on 20 or 21 December (in the southern hemisphere, summer begins). The seasons do not always have the same number of days because they begin and end in precise astronomical moments, which vary from year to year. Each year, in fact, solstices and equinoxes drift by about 6 hours: every year the Earth takes just over a year to rotate around the Sun, exactly 365 days and 6 hours. So to return to the same point of the orbit (in this case that of the solstice or equinox) every year takes about 6 hours more, but this drift is compensated every 4 years with a leap day. And this explains why the solstices and equinoxes do not always fall on the same day of the year.

The word equinox comes from the Latin aequinoctium, or ‘aequa-nox‘ i.e. night equal (to the day: using this definition the length of the day would be 12 hours. This is the point in the Earth’s orbit (the rotation of the Earth around the Sun) where our star is at the zenith of the equator. Actually, day and night are not exactly the same length during the equinox. In fact, the Sun’s rays begin to illuminate a region of the Earth before the solar disc is fully visible (at dawn) and disappear after it has dropped out of sight: the minutes of light that precede and follow the complete appearance or complete setting of the sun reduce the length of the night.

by Benedetta Palazzo