"Earth's tilt is the reason for the season. Our planet orbits the sun while tilted at an average of 23.5 degrees, so the Northern and Southern Hemispheres receive unequal amounts of sunlight. This causes both the solstices and the seasons.
Each hemisphere's cooler half of the year happens when it's tilted away from the sun, and its winter solstice (December in the north, June in the south) marks the point when that half of the globe is tilted away from the sun at its most extreme angle.
Lack of exposure to the sun's rays makes the winter solstice the darkest day of the year but it's not the coldest. That's still a month or more away, depending on your location, because oceans and landmasses are slow to lose the heat energy they absorbed during the warmer months."
“Everything You Need to Know About the Winter Solstice.” Science, National Geographic, 3 May 2021, https://www.nationalgeographic.com/science/article/151221-winter-solstice-explained-pagans.
And this year, 2021, the Solstice Sunrise will be live streamed from Newgrange!
"In line with the revised Government Guidelines, Brú na Bóinne Visitor Centre and the site at the Newgrange Monument – though not the passage tomb itself – are currently open to visitors. We will once again live-stream the Winter Solstice on this page at 08:45 UTC on the 20th, 21st and 22nd of December, so make sure to join us to view the spectacular phenomenon and learn some fascinating facts.
"The Winter Solstice is an astronomical phenomenon that marks the shortest day and the longest night of the year. In the Northern Hemisphere, the Winter Solstice occurs on 21st or 22nd of December, when the sun shines directly over the tropic of Capricorn. At sunrise on the shortest day of the year, for 17 minutes, direct sunlight can enter the Newgrange monument, not through the doorway, but through the specially contrived small opening above the entrance known as the ‘roof box’, to illuminate the Chamber."