Science of the Winter Solstice
And how it is the harbinger of winter holidays
As I mentioned in my article on Advent, the Romans, during the later Empire period, celebrated a holiday known as the Saturnalia, beginning on the Winter Solstice. The solstice can occur any time between December 20 to 23, though usually on the 21st or 22nd; but until modern instrumentation, interpolating the exact date with ancient astronomically aligned instruments was imprecise, and the ancient Romans set their Saturnalia on December 25.
Solstice comes from the Latin “solstitium,” meaning “Sun, standing still.” This year it occurs on December 21 at 21:48 GMT (or UTC) and marks the first day of the Winter season in the Northern Hemisphere from an astronomical perspective.
Seasons
Earth enjoys different seasons because the planet is tilted 23 degrees and 27 minutes off the perpendicular to the plane of orbit. This means that the world revolves like a tilted spinning top. The Winter Solstice is the shortest day of sunlight as the Sun is at its lowest arc in the sky, at least in the Northern Hemisphere.
The farther north one is from the Equator, the more pronounced this is in Winter. However, as the Earth continues its orbit, the hemisphere angled closest to the sun changes, and the seasons are reversed.