From this Friday, September 23, it is autumn. As in spring, this day of change from one season to another is called “equinox”. In summer and winter, we talk about solstices. This date marks a precise moment in the duration of the insolation from which we benefit. explanations.
We change stations. Summer departure, hello autumn: this Friday, September 23, is the equinox. A special and rare day (there are only two in a year), in which the length of the day is exactly equal to the length of the night.
As of this Saturday in theory, therefore, the days begin a long descent, which continues until December 21, which marks the transition to winter, with the solstice, which therefore corresponds to the shortest day of the year.
Day and night of the same length, really?
Nevertheless, it is not exactly today that the day and night will have an exactly equivalent duration, and this is for two reasons. In the first place because “the sun is not punctual: seen from the earth, it is a disk. The upload is counted as the time this record comes out. But the sun takes two minutes to rise”, explains Eric Lagadec, astrophysicist at the Côte d’Azur Observatory and president of the French Society for Astronomy and Astrophysics. As a result, “we gained two minutes during the day compared to night.”
Another reason,the air deflects the sun’s rays, it is called refraction. So when you see the sun above the horizon…it’s still below! Add a little more time to the day”, adds Eric Lagadec. Consequently, it will be necessary, in fact, to wait until Monday, for the visible day and the visible night to be almost exactly the same length, before the day actually begins to decline.
A matter of tilt
But why exactly this decrease in the duration of sunshine in autumn, when in other regions of the world the length of the day is more or less the same throughout the year, and in other places it can be almost night for several months? in a row
how to explain the weather channel, everything is related to the tilt of the earth in relation to the sun. If it were constantly perpendicular to it, the days would last 12 hours, when the nights would therefore also make 12. “At the end of September, from the equinox, we lose up to 4 minutes of theoretical sun a day“.
“During the northern hemisphere winter, the southern hemisphere faces more directly toward the sun,” he explains. future sciences. As a result, France, like countries at similar latitudes, enjoys less sunshine. The closer you get to the equator, the more similar the sunlight and night are throughout the year.