The northern hemisphere will have the shortest day and longest night of the year on Monday. For many, the winter solstice, which this year came at 02.03 Greek time on Monday, signals the arrival of winter. But what is the winter solstice? The answer is easier to understand if one follows the Earth in its journey around it Helium.
The planet is tilted
The equinoxes, the solstices but also the enchange of the seasons are due to the fact that the axis of rotation of the Earth is not exactly perpendicular to the plane in which the planet moves, the so-called plane of the ecliptic, but has a slight inclination, as in the image below.
On other planets, the axis forms a larger angle, as in the extreme case of Uranus, in which the axis of rotation is almost parallel to the plane of the ecliptic - and creates a strange alternation of "seasons".
Earth's Axial Tilt vs. Other Planets (Wikimedia Commons)
The important thing is that the axis of rotation always looks at the same point of the sky, the so-called celestial pole, regardless of the time and place where the planet lies on its journey around the Sun. This is evident in the picture below.
The alternation of the seasons: in the spring and summer (left) the northern geographic pole is facing the Sun
The fact that the orientation of the axis is fixed means that in summer and spring the north pole of the planet faces the Sun, while in autumn and winter [of the northern hemisphere] the sun disappears from the region of the north pole, while the south pole is bathed in a permanent sunshine.
Officially, the winter solstice is the time when the Sun it reaches its southernmost point in the sky, which is always the case when it is night in the northern hemisphere. This happened this year at 23.30 Universal Time (Greenwich Mean Time) on December 21, or the early hours of December 22 Greek time.
On the day of the sunset in the northern hemisphere, the Sun travels as little as possible to the sky. This means that it is unusually low at midday and the shade that leaves an object at that time is the greatest possible.
This applies to the entire northern hemisphere until the Tropic of Capricorn, an imaginary 23,5 line over the equator. In the Tropic of Capricorn, the Sun reaches exactly at the top of the sky at noon after the solstice.
Slow dawn
You do not need to be an astronomer to notice that these days Sun sets early and slowly rises. Surprisingly, the earliest west and the slowest east do not come on the day of the solstice, but a few days earlier.
This is due to the fact that the time we follow does not exactly coincide with the so-called solar time. The so-called "true solar noon", i.e. the moment when the Sun reaches its highest point in the sky, comes earlier than 12.00 until about the middle of December, while on the day of the solstice it comes a little later.
And the fact that the actual solar noon comes up later on with the passing of the days means that it is the same with the time of sunset and sunrise. The main factor for this deviation is the slope of the pivot axis. A second reason is that the Earth's orbit is not circular but elliptical, and the planet moves faster when it is closer to the Sun, something that happens in early January.
All of this, of course, applies to the northern hemisphere, since the southern hemisphere welcomes the same day the summer solstice. The day has the maximum duration and the Sun follows the highest possible arc, so it reaches higher in the sky at midday. This means that the December solstice marks the coming of summer in the southern hemisphere. But the northern one has reason to rejoice, since, from Tuesday, the days will become bigger.
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