The winter season arrives this Friday at 23.23 hours (Peninsular time) in the northern hemisphere, while in the southern hemisphere summer will begin. The winter will last 88 days and 23 hours and will end on March 20, 2018, when spring will begin, according to calculations of the National Astronomical Observatory, belonging to the Ministry of Development.
The National Astronomical Observatory indicates in a report on the winter, collected by Servimedia, that the beginning of the seasons is given, by astronomical agreement, for those instants in which the Earth is in certain positions in its orbit around the Sun. In the case of winter, this position occurs at the point of the ecliptic in which the star reaches its southernmost position.
The day this happens, the Sun reaches its maximum south declination (-23º 27 ') and for several days its maximum height at noon does not change. Therefore, this circumstance is also called solstice ('Sun still') of winter. At that moment, summer begins in the southern hemisphere.
The winter solstice day is the least daylight hours of the year. Around this date you will find the day when the Sun rises later and the day when it sets sooner. A circumstantial fact not related to the start of the seasons is also in this period: the day of perihelion, that is, when the Sun and the Earth are closer to each other throughout the year. It is this greater proximity to the star that causes the planet to move more rapidly along its elliptical orbit and, therefore, the duration of this station is the least.
The beginning of winter can occur, at most, on four different dates of the calendar (from December 20 to 23). Throughout the 21st century, this season will begin on December 20 to 22 (official Spanish date), with its earliest start in 2096 and the latest in 2003.
The variations from one year to the next are due to the way in which the sequence of years fits according to the calendar (some leap years, others not) with the duration of each Earth orbit around the Sun (duration known as the tropical year).
The day of lower sunlight
If it is colloquially called duration of the day to the time that elapses between sunrise and sunset in a given place, this Friday will be the shortest day. As an example, in Madrid this duration will be 9 hours and 17 minutes, compared to the 15 hours and 3 minutes that lasted the longest day (which in 2018 was on June 21).
There are almost six hours difference between the shortest and longest day, which depends a lot on the latitude of the place, since it is zero at the equator and extreme (24 hours) between the polar circles and the poles. Precisely, it is in the Antarctic where some days a year around December 21 there is the phenomenon of the midnight sun, in which the star is visible above the horizon during 24 hours a day.
One might think that the shortest day of the year will also be the day when the Sun comes out later and gets there sooner, but it is not because the Earth's orbit around the Sun is not circular but elliptical and because the axis of the planet is tilted in a direction that has nothing to do with the axis of that ellipse.
It also causes a sun clock and our watches, based on a fictitious Sun, to be out of tune. The day when the Sun set sooner was December 8, while the day it will be released later will be January 5, 2019.
At this time there is also the maximum annual approach between Earth and the Sun, a phenomenon known as perihelion. On this occasion, the closest approach will be on January 3, with a distance of just over 147 million kilometers, about five million less than at the time of aphelion or greater distance (July 5, 2019).
During the winter, the evening sky will be dominated by Mars, Jupiter and Saturn, and the evening sky by Uranus, to which Venus will join in mid-February. There will also be two eclipses: a total of Moon on the night of January 31, which will be visible in Asia, Australia and North America, and a partial one of Sun, which can be seen in Antarctica and southern South America.
During the winter, the sky at dusk will be dominated by Mars and at dawn by Venus, Jupiter and, from mid-January, by Saturn, which will be hidden by the Moon on February 2.
Between the 5th and 6th of January 2019 there will be a partial solar eclipse that will be visible in Northeast Asia and the North Pacific Ocean. On January 21 there will be a total eclipse of the Moon that will be visible in America, Europe and Africa, so it can be seen in Spain.
On February 2, 2018 there will be an occultation of Saturn by the Moon that will be partially visible from the Iberian Peninsula and the Balearic Islands.