The northern hemisphere winter solstice occurs today, 21 December 2010 at 2338 GMT.
But surely the winter solstice is just the shortest day. How can it “occur” at a specific instant?
That’s because we astronomers define the solstice as the instant when the Sun gets to its furthest south below the celestial equator. Or to put it another way, the instant when the north pole of the Earth is tipped directly away from the Sun.
And this happens at exactly 2338 GMT on 21 December 2010.
It’s important to remember though that while we are in the midst of a deep, cold winter, the southern hemisphere are celebrating their summer solstice, and their longest day.