December 21 marks the winter solstice in the northern hemisphere (and the summer solstice in the southern hemisphere). In addition to the onset of winter (or summer), the solstice is the shortest day of the year for people from the north (and the longest for people from the south). But why does this only happen today?
The answer is the same I’ve read for other questions like “Why do the seasons exist” or “Why is it colder in winter if we’re really closer to the sun?” The earth is tilted. Or rather, the Earth’s rotation axis is tilted by 23.4 degrees with respect to the plane of orbit that our planet describes around the Sun.
Since one year represents one revolution around the sun, on every December 21, the Earth is positioned So that the south pole displays its maximum direction towards the star. This causes the light to shine perpendicular to the Tropic of Capricorn, 23.4 degrees south of the equator.
Today, in the northern hemisphere, the sun has reached its lowest point. The word solstice comes from the Latin coup, which means “still the sun”. Ancient astronomers recognized that around this time the sun stops going down and does not appear to change its course for a few days.
In fact, it is about to change direction and begin to soar in the sky until June 20, when the summer solstice occurs. They are the shortest days in the Northern Hemisphere (8½ hours of sunshine in Madrid) and the longest in the Southern Hemisphere (14½ hours of sunshine in Buenos Aires).
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However, solstices and equinoxes are one of those things where a picture is worth a thousand words, like this Here are a couple of animations.
Northern Hemisphere version
Southern Hemisphere version
This animation is the work of planetary scientist James O’Donoghue, who works for the Japan Aerospace Exploration Agency (JAXA). We’ve posted other animations of him before:
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