Dozens of mysterious structures across the Northern Hemisphere - some nearly 5,000 years old - align precisely to frame the rising and setting Sun during midwinter's shortest day. What motivated people to construct these solar-calibrated masterpieces?
The winter solstice, which usually falls on 21 or 22 December in the Northern Hemisphere each year, marks the moment that one yearly cycle comes to an end and another is born. It is the day with the smallest number of sunlight hours in the calendar, and once it's over, the days lengthen again incrementally until the summer solstice in June.
The significance of this day is manifested in ancient monuments that were designed to acknowledge and celebrate its passing. One example is Maeshowe tomb in Orkney. To the untrained eye this burial cairn, created around 2800BC, looks like a grassy hillock - but it conceals a cuboid, stone-clad sepulchre and a 33ft (10m) long entry corridor oriented to the south-west. During midwinter, three weeks either side of the winter solstice, the setting Sun aims directly down the corridor and emanates its light into the tomb.
When the sky is cloudless, the light seems to carve a golden aperture into the tomb's rear wall - a sacrament of pure light. These days of radiance are interrupted by the solstice itself, when blackness temporarily takes over. But daylight reappears soon after, to blaze for another few days as if in celebration of the promise of nature's rejuvenation in spring.