Spring equinox 2024: how to explain the phenomenon?


Spring equinox 2024: how to explain the phenomenon?

SPRING. The spring equinox will occur tonight, Wednesday, March 20, 2024, marking the end of winter and the beginning of the warm season. Date, time, meaning, we tell you everything!

[Updated March 19, 2024 at 11:46 a.m.] The spring equinox will occur this Wednesday, March 20, 2024. It is the transition from winter to spring, more precisely the time of year when the Sun crosses the equatorial plane of the earth. At this moment, the star is at the zenith of the Equator. Result ? Notably, nights and days have the same approximate length everywhere on Earth. The season of the first buds, the first rays of sun and the first sunburns, will start shortly... But be careful, the spring weather can sometimes be more capricious!

Why does spring, whose first day generally corresponds to March 21 in the collective imagination, arrive a day earlier some years? What is the difference between astronomical spring and meteorological spring? But also what are the traditions and cults associated with the spring equinox? Definition, date, time, pagan festival... Here are answers to the secrets of the spring equinox!

The spring equinox 2024 will take place on Wednesday March 20 at 4:06:21 (French time) exactly, i.e. 3:06:21 universal time, as reported by the Institute of Celestial Mechanics and Ephemeris Calculation. found at the Paris Observatory, in the 14th arrondissement of the capital.

Astronomically speaking, spring begins at the time of the vernal equinox, which can occur between March 19 and 21. It lasts until the summer solstice. Which means that spring lasts about three months. The date of the equinox is calculated for each year by astronomers and mathematicians. This involves predicting the exact moment when the plane of the equator and that of the Earth's trajectory coincide. A calculation made necessary by the discrepancy between our calendar, our time system and the movements of the stars. First, the Earth's orbit is not perfectly circular, which means that depending on its position in this orbit, the Earth can be more or less close to the sun (between 147 million kilometers minimum and 152 million kilometers maximum). This inevitably makes the duration of each season very irregular and therefore the date of spring variable.

Another explanation: It doesn't take exactly 365 days for the Earth to go around the sun. In this respect our Gregorian calendar, established in the 16th century, is far too simplistic. It takes exactly 365.2422 days (365 days, 5 hours and 46 minutes) for us to have traveled completely around the star! We are therefore obliged to add a February 29 from time to time (during leap years) to partially (and only partially) correct this shift. A one-off addition that artificially pushes back the date of spring by one day during leap years. Which explains why astronomers “advanced” it to March 20 last year. And that the situation presents itself again this year.

When Caesar established the Julian calendar in 45 BCE, the spring equinox was set at March 25, based on imprecise observations at the time. But the absence of February 29 in this ancient calendar ended up moving this date to March 11 in the 16th century... It was not until the establishment of the Gregorian calendar (the one we use today) , in 1582, that a date approaching March 21 was chosen.

The date of "common" spring should not be confused with that of meteorological spring, which begins each year on March 1, allowing meteorologists to make their seasonal calculations on the scale of entire months. In meteorology, we consider that spring begins on March 1 and ends on May 31: in this discipline, we characterize spring as a period of warming temperatures (in our latitudes) and an increase in the duration of the day. It is also one of the most difficult seasons to analyze in this discipline. However, it is the equinox that continues to mark the advent of spring in people's minds. This is evidenced by the demonstrations organized around March 20 in France, starting with the Printemps du cinéma.

The word equinox comes from the Latin "æquinoctium" ("equal night") because the most visible phenomenon for man is that the length of the day becomes identical to that of the night. The reason for this phenomenon? The equinox corresponds to the time of year when the Sun crosses the equatorial plane of the earth. The star is then at the zenith of the Equator, which allows day and night to share time equally. And this in both hemispheres, southern and northern. For our part, the days are getting longer and we are halfway between the short days of December and the long days of June. During the equinox, the axis of rotation of the Earth on itself and the axis of rotation of the Earth around the Sun therefore correspond exactly. On our planet, during the equinox, our star appears right in the east at dawn only to disappear right in the west.

This phenomenon is further linked to geometry. The Earth's axis of rotation is naturally inclined by 23.4° relative to the plane of its orbit. In other words, our planet "leans" relative to the plane on which it revolves around the Sun (see diagram below). The star therefore illuminates it differently depending on the time of the year. This phenomenon explains why the days lengthen or shorten between summer and winter. This also gives rise to the seasons, due to the heating or cooling of air masses and oceans depending on the time spent each day under the rays of the star. The distance between the sun and the Earth, however, has no direct link with temperature. Know for example that the Earth reaches the closest point to the sun (perihelion) on January 3, that is to say in the heart of our winter.

The equinox occurs twice a year: between March 19 and 21 (spring or vernal equinox) and between September 22 and 23 (autumn equinox). In spring the duration of sunshine increases at the equinox to reach 16 hours at the end of June, during the summer solstice. Conversely, the autumnal equinox begins a period of daylight reduction that drops to just over 8 hours at the winter solstice, around December 21. During the equinox, however, there is no jealousy: day and night are supposed to last 12 hours sharp each. These data nevertheless vary slightly since the shape of the Earth is not perfectly regular and the atmosphere slightly deflects the sun's rays. So, in Paris, on March 20, last year, the sun rose at 6:52 a.m. and set at 7:03 p.m. The length of the day was therefore very slightly longer than 12 hours. It is also at the time of the equinoxes that the length of the day increases/decreases the fastest in our latitudes.

Spring equinox and pagan festival are closely linked. Thus, various celebrations of the spring equinox around March 21 existed in very ancient times, some of which still survive today. Among them are bonfires that symbolize freedom from the darkness of winter; or even cakes offered to a deity; when it is not a straw mannequin that we burn or put in water, as if to “destroy” winter.