Like every year, a few days before Christmas the winter solstice will take place. But do you know what this term means, and what actually happens on the shortest day of the year?
On Friday, December 22, an astronomical event will take place which is repeated every year and announces the end of autumn: the solstice winter. This date occurs at the time of year when the nights are longest in our latitudes. On the other hand, it corresponds to the summer solstice in the southern hemisphere, which then benefits from long sunny days and enters the hottest season of the year. This year, the winter solstice occurs on Friday December 22 at 4:27 a.m. (Paris time). But this date varies slightly depending on the year and can occur (very rarely) on December 20 or 23 although it usually occurs on the 21st or 22nd of the month.
The solstices are astronomical phenomena which originate from the position of the Earth in its orbit around the Sun. They are in fact caused by the inclination of the Earth which is “leaned” by 23 degrees relative to its axis of rotation. Because of this particularity, twice a year, the Earth orients one hemisphere more than the other towards the Sun. Thus, at the time of our summer solstice, it is the northern hemisphere which is oriented towards the Sun and benefits from long hours of daily sunshine while the southern hemisphere is plunged into winter. Conversely, during our winter solstice, it is the southern hemisphere which is oriented towards the Sun while our days are short and winter sets in.
You can observe this phenomenon for yourself by looking at the height of the sun in the sky at zenith at this time of year. During the winter solstice, it only rises 17° relative to the horizon (compared to 64° at the summer solstice), so there is little time left in the sky and its rays remain very horizontal relative to the horizon. on the ground. Consequences: it is cold, the days are short and the shadows are long unlike what we observe during the summer solstice.
The winter solstice therefore corresponds to the day when the duration of sunshine is the shortest of the year. So on December 22, the sun will rise at 8:42 a.m., rise very little in the sky and set at 4:56 p.m. in Paris, giving us 8:14 hours of sunshine. Of course, this phenomenon becomes more pronounced as we move towards the North Pole. There is thus a difference in sunshine of one hour on the day of the solstice between the cities of Bastia and Lille for example. More impressive, the inhabitants of northern Europe then experience extremely short days like in Helsinki in Finland where the sunshine will only last 5 hours 50 minutes that day.