Today, September 22, 2020, is the autumnal equinox, a term you have certainly heard many times before. But do you know what exactly an equinox is and what changes this phenomenon entails? Light on one of the astronomical events on which the mechanism of the seasons of our planet is based.
An equinox is a time in the year when the Sun crosses the equatorial plane of the Earth; its light thus “switches” from one hemisphere to the other. This happens twice a year, in the spring and in the fall. Result ? Each hemisphere then completely changes climatic conditions for the next six months:the one closest to solar radiation begins the warm seasons, while the other begins the cold seasons.
Tuesday, September 22, 2020 at 1:30:40 p.m. UTC (i.e. at 3:30:40 p.m. in French legal time), according to the Institute of Celestial Mechanics and Ephemeris Calculation, it is the exact moment of the autumn equinox this year. Summer will then officially end after 93 days, 15h 46m 58s. At this time, the geocentric latitude of the center of the Sun will be -0.18″, its right ascension will be 11h59min59.995s and its declination will be -0.16″. In other words, the Sun will be at the zenith of the Equator.
For us in the northern hemisphere, this equinox heralds the arrival of autumn and the cool temperatures that characterize it. The weather is less clement, the days are getting shorter. Conversely, in the southern hemisphere, it is the dawn of spring, the arrival of fine weather and warmer temperatures.
To illustrate the phenomenon and allow us to fully understand what is happening, Dr. James O'Donoghue, planetary specialist at the Japan Aerospace Exploration Agency, has created an animation that perfectly summarizes the operation of these events, as well as their relation to the solstices. See instead:
The equinoxes, like the solstices, are due to the axial tilt of the Earth. Indeed, compared to the Sun, the axis of rotation of the Earth is tilted by about 23.5° (as can be seen in the animation). It is therefore easy to understand why, in our hemisphere, we are cold in winter and hot in summer:note that in winter, the Earth is tilted so that the northern hemisphere "moves away" from the Sun and vice versa, the northern latitudes are tilted towards the Sun in summer. This is what the mechanism of the seasons is based on.
At the time of the two solstices, the phenomenon is maximum:these are the two days of the year when one side of the planet is tilted as far as possible from the Sun, while the other is closest to it. Result:on December 21, the northern hemisphere receives less than nine hours of daylight, while the southern hemisphere receives more than fifteen! In summer, sunlight is most intense because it only has to pass through a short column of atmosphere, O'Donoghue says. That is why it is usually hot during this season.
The term "equinox" comes from the Latin æquinoctium (meaning “equal night”). Indeed, at this time of year, day and night have the same length. Because during the equinoxes, the axis of rotation of the Earth is not tilted in the direction of the Sun (nor in the opposite direction). You can clearly see this in the animation above. Therefore, the Sun's rays strike both hemispheres equally.
In other words, each point on the globe enjoys 12 hours of sunshine and 12 hours of darkness that day. Likewise at the time of the equinox, the Sun rises almost exactly in the east and sets almost exactly in the west (which is not the case the rest of the year). After this time, the days will gradually shorten until the winter solstice (at least, in the northern hemisphere).
If you were lucky enough to be right on the equator this Tuesday, September 22 at 1:30 p.m. UTC, your shadow would be almost non-existent for a short time, because the Sun would appear almost directly above your head (at the zenith). However, the moment would be short-lived, because let's not forget that the Earth moves around the Sun at a speed of about 107,000 km/h!
Related:What colors are sunsets on other planets?
Each year, a first equinox takes place between March 19 and 21 (the spring equinox for the northern hemisphere, autumn for the southern hemisphere); the second takes place between September 21 and 24 (autumn for us). Why does the event not always occur on the same day of the year? This range of dates is due to the fact that the Earth orbits the Sun in 365 days, 5 hours and 46 minutes (and not exactly in 365 days), which causes a shift from one year to the next. Fortunately, this shift is largely compensated by the existence of leap years, which have 366 days; this makes it possible to compensate for the time difference between a calendar year and a solar year, otherwise the seasons would gradually shift…
