Midwinter celebrations originated with events to mark the solstice, the point at which the days finally start to become longer. But lighter does not mean warmer. An old proverb warns, “As the day lengthens, the cold strengthens.” Why should this be so? More daylight means more warmth from the sun each day. But the overall energy balance is still negative, with more heat lost at night than is regained during the daytime. The Earth is a big heat sink, and it cools down and heats up slowly. “The cold of winter will continue to increase for some time, though the sun has withdrawn from the winter tropic and begins to dart his rays more perpendicularly on us,” as an 1821 astronomy textbook charmingly puts it. This is why average temperatures in the UK are higher in dark December than in January, which is lighter. And February, when the days are even longer, has the lowest average minimum of all. The opposite effect makes August warmer than June. However, the monthly average difference is slight, with larger variance from year to year. But while the sun may not bring much actual heat, the pleasure of seeing it still in the sky, rather than plunging into darkness before 4pm, provides some much-needed spiritual warmth.