This Saturday, December 21, 2024 marks the winter solstice: the shortest day of the year in the northern hemisphere and the longest in the southern hemisphere. Astronomically, it marks the entry into winter north of the equator and into summer south of the latter. (unlike the meteorological winter in the North and the meteorological summer in the South which begin on December 1st)
The shortest day in the Northern Hemisphere
The Earth orbits the Sun every 365 days with an axis of approximately 23.5°. As a result, different regions of the globe do not receive the same amounts of sunlight depending on the time of year. The calendar seasons are thus based on the spring and autumn equinoxes (times when the two hemispheres receive the same amount of sunlight) and the winter and summer solstices (times when the difference in day length is the stronger between the two hemispheres).
Diagram explaining the position of the Earth during the equinoxes and solstices – via Wikipedia
If the Earth rotated in a straight direction (without an axis of inclination): there would be no seasons because the length of the day would not change over the course of the year.
Diagram of the Earth's position at the winter solstice – via calendrier-lunaire.fr
The winter solstice is therefore the shortest day of the year in the Northern Hemisphere because the Sun rises very late and sets very early. It's quite the opposite in the Southern Hemisphere which experiences its longest day of the year. During the summer solstice – which generally falls on June 21 – the situation is reversed with a very long day in the Northern Hemisphere and a very short day in the Southern Hemisphere.
Satellite view of solar light received on Earth this Saturday, December 21, 2024 – via EUMETSAT
The satellite image above clearly shows the significant difference in sunlight received by the Northern Hemisphere (low) compared to the Southern Hemisphere (high) during the winter solstice. We observe that night falls much more quickly in the north of the globe. The fact that the Sun appears low in the sky in winter (and much higher in summer) is also directly related to the Earth's tilt.
Solstice: the polar night!
As we have just seen above, the Earth's tilt axis is approximately 23.5°. This means that all latitudes located less than 23.5° from the pole no longer even see the Sun rise during the winter solstice! In other words, the latitudes located between the north pole (90°N) and the Arctic Circle (66.33°N) did not see the star of the day on December 21, 2024, thus remaining plunged in shadow! This concerns in particular northern Scandinavia, part of Siberia, 2/3 of Greenland and even northern Canada and Alaska.
Duration of sunshine in the northern hemisphere during the winter solstice (December 21) – via Celtabria on Reddit
However, in Scandinavia we cannot yet really speak of a polar “night” because there is still daylight. Tromsø, for example, is the northernmost large city in Europe (76,700 inhabitants) located more than 300km north of the Arctic Circle.
If the Sun does not rise, twilight lasting several hours is visible there every day because the Sun remains close below the horizon line.
It is almost noon in Tromsø when this photo was taken. If the Sun does not rise, we cannot really speak of night – University of Tromso
If the Sun no longer illuminates the surface of the Earth, it is round so the light can still reach the atmosphere located higher up, which in turn diffuses the light towards the ground. It's dusk.
The Sun must therefore reach a certain height below the horizon to limit this lighting. Between 0 and -6° we speak of “civil” twilight, then “nautical” between -6 and -12° and finally “astronomical” between -12 and -18° below the horizon. Below -18°, the true dark night without disturbance by daylight really begins.
Alert, with 82°29'55″, is the northernmost inhabited place in the world, located 817km from the North Pole – It is theoretically possible to see a slight glow appear in the South at the winter solstice (if the sky is clear)
Thus, the “true” (astronomical) polar night is limited to latitudes above 84°34', which corresponds exactly to 18° latitude north of the polar circle, or 5.5° around the North Pole.
Located at 83°40' north latitude, Kaffeklubben Island (the northernmost land in the world) is one of the only land areas in the Northern Hemisphere to be able to observe a “real” polar night..
Jérémie GAILLARD – Forecaster for MétéoCilles
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