Starship Asterisk*

Today is the winter solstice — specifically, it occurs at 23:38 GMT, or 6:38 p.m. Eastern (US) time. Technically what this means is that, at that moment, the center of the Sun is at the lowest declination of the year.
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And that’s where we are today. The declination of the Sun has been getting lower ever since last June, and today it reaches its farthest point south. At 11:38 p.m. GMT, the Sun’s movement south stops (solstice literally means "Sun stops/stands still"), and it starts to slowly creep back north again. That’s why there is an actual moment, a point in time, for the solstice. In June it’ll get as far north as it can, and the process reverses. Incidentally, the times halfway between the solstices when the Sun’s declination is exactly 0 are called the equinoctes (the singular is equinox). Just so’s you know.