For ordinary mortals, winter begins when rain, wind and low temperatures force the bottom of the wardrobe to change. The National Astronomical Observatory , however, accurately dates the start of the season based on the position of the Earth. The start of winter can occur on three different dates depending on the calendar we use, between December 20 and 23. This year, winter will begin on Sunday, December 22 at 5:48 a.m. (Spanish mainland time). It will last 88 days and 23 hours, and will end on March 20 with the arrival of spring. To determine exactly when we go from one station to another, the researchers use the astronomical method detailed below.
Contrary to popular belief, the seasons do not depend on the distance from the Earth to the Sun , which only varies by about five million kilometers between perihelion and aphelion. The succession of four different seasons is due to the inclination of the axis of rotation of the Earth with respect to the plane of its orbit of translation. If the axis of the Earth did not have an inclination of 23º, the Sun would be found all the year on the equator, all the regions would receive the same amount of solar light and there would be no periods of differentiated climatology.
The beginning of winter is given by the point on the ecliptic where the Sun reaches its southernmost position . The day this happens, the Sun reaches its maximum southern declination (-23º 27 ‘) and for several days its maximum height at noon does not change, and for that reason, this circumstance is known as the solstice (“Still Sun”). of winter. At this precise moment, summer begins in the southern hemisphere.
The day of the winter solstice is also the shortest day of the year. On the other hand, at this time of year the Earth reaches the perihelion of its orbit, although, as we have emphasized previously, this fact does not affect the seasons.
The day after the winter solstice, in the northern hemisphere, marks the beginning of the longest days, leading up to the summer solstice in June.
In the southern hemisphere, the opposite is true. Dawn comes early and dusk comes late. The sun is high and the noon shade is the shortest of the year. In the southern hemisphere, the inhabitants will experience their longest day and shortest night.
So the solstice is the first day of winter?
The answer may vary depending on who you ask. There are two types of winter: astronomical and meteorological. As we have told you before, the astronomical winter begins on December 22, with the winter solstice, and ends on March 20. Meteorological winter, on the other hand, always begins on December 1 and ends on February 28 (February 29 during leap years).
Astronomical winters are determined by the Earth’s orbit around the sun, and meteorological winters are the three calendar months with the lowest average temperatures.
