In the northern hemisphere, the winter solstice has arrived – marking the first day of winter in the astronomical calendar. For those in the southern hemisphere, it is the first day of summer. In school, you’re taught the make-up of the seasons, with December-February often considered winter, March-May spring, June-August summer and September-November autumn. In this sense, the rotation of Earth around the sun is marked with four key natural phenomena: the spring equinox, the summer solstice, the autumnal equinox and the winter solstice. So today (21 December) the winter solstice is here, marking the day in 2016 with the fewest hours of sunlight.