When we use the phrase “shortest day” what we actually mean is the shortest daytime or the shortest time period for which the sun is in the sky in an entire year. Naturally, the shortest daytime also means that when the day would be over, it would be followed by the longest night of the year as well. The shortest daylight hours are observed on the day of the winter solstice, when the earth is tilted at 23° 26′. At this point the axial tilt of the earth lies at the furthest point from the sun possible. The dates of the winter solstices differ depending on which hemisphere we are talking about. The winter solstice occurs on the 21st or the 22nd December in the Northern Hemisphere and at that exact time, the Southern Hemisphere experiences the summer solstice. The Southern Hemisphere on the other hand experiences the winter solstice at the exact time when the Northern Hemisphere experiences its summer solstice, that is on the 20th or 21st June.
Apart from marking the shortest day and the longest night of the year, the winter solstice also marks the beginning of winter. Although it may not be practically accurate at all times because winter often comes much earlier than 21st December, but from the astronomical point of view at least, the winter solstice marks the beginning of the winter season. Meteorologists find it easier and more appropriate not to depend on the solstices or the equinoxes as it helps them to keep a better record and calculate data if they mark the first day of a month as the beginning date for a new season.
The solstices had been recognized from times as ancient as the Neolithic period, although, the date might not have been pin point due to lack of technology. Even the earliest of the civilizations managed to recognize the change through natural occurrences like the arrival of the mating season for animals as well as the harvest season for crops. Recognizing the winter solstice was important too for them because without the facilities that we have today, they had to prepare for the cold winter by preserving food especially, as otherwise, the entire population would die out of hunger.