Welcome to Spring: March 20, 2023. But what is the equinox?

 Punctual as a Swiss clock, Spring will arrive today at 22:24 UTC+1 20 March 2023. The equinox (from the Latin night equals) is that moment when the Sun is at the zenith of the Earth’s equator. The Earth's axis of rotation has an average inclination of 23 degrees and 27 degrees. Thus the solar light never affects at the same angle but it constantly varies. During the equinoxes, however, the Earth’s axis is perpendicular to the direction of the sun’s rays and therefore the duration of the day is equal to that of the night.

Springtime

Obviously for the northern hemisphere, the March equinox marks the arrival of spring while in the southern one the beginning of autumn. The opposite happens with the September equinox.

What about the solstices?

Literally the solstice is the moment when the Sun reaches, in its apparent motion along the ecliptic, the point of maximum or minimum declination. This means that the summer and winter solstices are respectively the longest and shortest day of the year. Like the equinox, the solstice is due to the inclination of the Earth’s axis of rotation with respect to the ecliptic (apparent path that the Sun makes in a year with respect to the background of the celestial sphere). The term solstice comes from the Latin solstitium, that is, the 'Sun stops', because, depending on the solstice, the Sun ceases to rise (or go down) compared to the celestial equator and seems to stop in these days. The Sun reaches the maximum value of positive declination in June (marking the beginning of the boreal summer and the austral winter) and the maximum value of negative declination in December (beginning of the boreal winter and the austral summer).

The precession of the equinoxes

The precession of the equinoxes is one of the motions of the Earth which modifies the orientation of the axis of rotation in relation to the fixed stars. The Earth’s axis undergoes precession (similar to a spinning top) due to two factors: the shape of the Earth (oblate spheroid, projecting to the equator) and also the gravitational forces of both the Moon and the Sun. The precession motion makes a full turn every 25,772 years or so: in 12,000 years the pole star will be Vega.

Source: NASA