The Northern Hemisphere will experience the shortest day and longest night of the year on Monday. For many, the winter solstice, which this year fell at 02.03 Greek time on Monday, marks the arrival of winter. But what is the winter solstice? The answer is easier to understand if you watch the Earth on its journey around the Sun.
The planet is tilting
The equinoxes, the solstices, and the very alternation of seasons are due to the fact that the Earth's axis of rotation is not exactly perpendicular to the plane in which the planet moves, the so-called ecliptic plane, 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 bizarre alternation of "seasons".
The Earth's axial tilt relative to other planets (Adapted from Wikimedia Commons)
The important thing is that the axis of rotation always points to the same point in the sky, the so-called celestial pole, regardless of the season and the point where the planet is in its journey around the Sun. This is evident in the image below.
The alternation of seasons: in spring and summer (left) the north geographic pole is pointed towards the Sun.
The fact that the orientation of the axis is constant means that in summer and spring the north pole of the planet looks towards 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 permanent sunshine.
Officially, the winter solstice is the moment when the Sun reaches its southernmost point in the sky, which always happens when it is night in the Northern Hemisphere. This year, it happened at 23:30 Universal Time (Greenwich Mean Time) on December 21, or early morning on December 22, Greece time.
On the day of the solstice in the northern hemisphere, the Sun travels the shortest distance in the sky. This means that it is unusually low at noon and the shadow cast by an object at that time is the largest possible.
This is true for the entire Northern Hemisphere up to the Tropic of Capricorn, an imaginary line 23.5 degrees above the equator. At the Tropic of Capricorn, the Sun reaches exactly the top of the sky at noon after the solstice.
Late dawn
One doesn't have to be an astronomer to notice that on these days the Sun sets early and rises late. Curiously, the earliest sunset and the latest sunrise don't 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", that is, the moment when the Sun reaches its highest point in the sky, comes earlier than 12:00 until about mid-December, while on the day of the solstice it comes a little later.
And the fact that true solar noon is getting later and later as the days go by means that so are the times of sunset and sunrise. The main factor for this discrepancy is the tilt of the Earth's axis of rotation. A second reason is that Earth's orbit is not circular but elliptical, and the planet moves faster when it is closest to the Sun, which happens in early January.
All of this applies to the northern hemisphere, of course, since the southern hemisphere welcomes the summer solstice on the same day. The day is at its longest and the Sun follows the greatest possible arc, meaning it reaches the highest point in the sky at noon. This means that the December solstice marks the arrival of summer in the southern hemisphere. But the northern hemisphere also has reason to rejoice, since, from Tuesday, the days will become longer and longer.
Watch the video
https://www.youtube.com/watch?v=bR9nbi-PuvY
Science
