# The Sun stands still today!

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.

Um, what?

Astronomers measure the positions of objects on the sky using a coordinate system based on longitude and latitude on Earth, but to make it more confusing we call it Right Ascension and declination. The first measures the east/west position, and the second north/south. So just like an island might have coordinates of so-and-so degrees longitude and this-and-that degrees latitude, a star has an RA and a dec (if you want to sound cool and use astronomy slang).

OK. But what if an object moves? Well then, the coordinates change with time, too (just like the lat and long of a ship steaming across the ocean has changing coordinates). As the Earth orbits the Sun, the Sun’s position against the background stars changes, and therefore the coordinates change. I won’t go into details – you can read about it here, for example – but the Sun makes a sine wave across the sky over the course of the year, sometimes farther north, sometimes south. The time when the Sun’s position (actually, the center of the Sun’s disk) is at its farthest south – -23° declination – is called the winter solstice.

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.