Our planet’s north pole leans its farthest away from the Sun. It reminds me of when a ketch leans way over to starboard and all hands have to lean out over the port rail – should we all rush sunward to save the Earth from rolling over?
The moment of greatest tilt comes today at 23:03 Universal Time, which is 11:03 PM in Britain, 6:03 PM in eastern North America, 2:03 PM on the west coast.
You can see why, in our north-hemisphere countries, sunlight strikes at the lowest angle, and daytime is shortest. The sunlit half of the globe reaches up to the Arctic Circle and no farther; north of there, there is no sunlight for the full 24 hours.
The Sun is overhead at the southernmost latitude it can be, and that is called the Tropic of Capricorn: “tropic” because that means turning-point, Capricorn because on this date the Sun enters the constellation Capricornus – or used to. Precession has changed that, and the Sun really is still in Sagittarius.
23:03 UT is the geometrical instant of the solstice, but there isn’t a discernible difference between it and 23:02 or 23:04, or for that matter Dec. 20 or 22. It’s the summit of a smooth curve in a graph. It’s the moment when, because a gradual change is going into reverse, the change from instant to instant is slowest.
If we could show the Moon in our picture it would be back along the beam of light coming from the Sun. For it is New only an hour and a half after the solstice. A 30-day cycle of the Moon from invisible to invisible begins at virtually the same moment as a cycle of the Sun from lowest to lowest.
St. Lucy was the saint of “the year’s midnight” (http://universalworkshop.com/guysblog/2014/12/12/saint-lucy-and-her-squibs/) when that was December 13, but that was before the calendar changed. Every day in the year has at least one saint. On December 21 the Catholic church celebrates St. Peter Canisius, hard-working Dutch Jesuit who was sent in the 1550s to hold the line against Protestantism in Germany. Earlier the saint associated with the day was the apostle Thomas. He is called both Thomas Didymus and Doubting Thomas, so that a lot of people, including I’m sure me, have supposed that Didymus means “doubting”; but it means that he was a “twin.” December 21 is still his day in the churches of England, also of India, where he is said to have gone preaching, and his name was given to those born on this day, such as St. Thomas à Becket of Canterbury.