Winter has come. Today (21), precisely at 6:14 am, we officially changed stations. Autumn is over here in the Southern Hemisphere (and summer has begun in the North).
The date is marked by a physical and astronomical phenomenon. Once a year, there is an exact moment when the sun’s rays strike the Earth perpendicularly on the Tropic of Cancer, due to the angle of inclination of our planet. It is the so-called winter solstice (for those on this side of the globe) or summer solstice (for the lucky ones in the Northern Hemisphere).
Thus, the regions below the equator are less exposed to the sun, which explains the drops in temperatures and luminosity. It is not something that can be observed in the sky, but it has great impacts on terrestrial dynamics and on our lives.
Today, we can confirm that we will have the shortest day and the longest night of the year. In Brazil, a tropical country, we are talking about a variation of minutes or a few hours, which becomes evident the further south. See some examples:
Em Recife (PE), the sun rose at 5:32 am and sets at 5:11 pm. The day will be 11 hours and 39 minutes and the night will be 12 hours and 21 minutes. Sao Paulo-SP), the sun rose at 6:48 am and sets at 5:29 pm. The day is 10 hours and 41 minutes and the night is 13 hours and 19 minutes Porto Alegre (RS), the sun rose at 7:21 am and sets at 5:33 pm. The day will be 10 hours and 12 minutes and the night will be 13 hours and 48 minutes.
Likewise, when this phenomenon happens in relation to the Tropic of Cancer in December, the opposite occurs: our summer solstice, with the longest day and the shortest night of the year.
How does it happen?
The Earth’s axis of rotation around itself has an inclination of 23.5°, relative to the orbital plane around the Sun. That is, it does not rotate “retinha”. Thus, the Southern and Northern Hemispheres receive different amounts of light throughout the year.
Earth’s tilt relative to the Sun causes the seasons
Image: Viktoriia Kasyanyuk
It is this tilting motion that gives rise to the days and nights and the seasons. If the axis were at 90°, sunrise and sunset would happen at exactly the same time and place every day, and we would have no variations in weather.
Also notice that the Sun always rises on our horizon in the east direction, and sets in the west, but not precisely at the east/west cardinal point. Over time, it shifts to the left (strictly, to the north, during autumn and winter) or to the right (strictly, to the south, during spring and summer).
Montage shows the sunrise over a year; he is not always born in the east
For all that, the luminous period never lasts exactly half of the 24 hours; they are largest in the hottest seasons and smallest in the coldest, and they also vary according to how far north or south the observer is.
There are only two days when day and night are of equal length, almost exactly 12 hours each: at the so-called equinoxes, which usher in spring and autumn, in September and March. It is when the sun’s rays fall equally between the two hemispheres, aligned with the equator.
* With Time and Date information