For thousands of years, humans have marked specific times of year as especially important, whether for religious reasons, agricultural reasons, or simply out of curiosity. Ancient observers of the sky noticed a yearly cycle of very long, warm days to short, cooler days, followed by a return to longer days. Although the earliest farmers could not know the cause of this effect, they certainly understood its importance to the growth of their plants, as well as for the populations of the animal inhabitants of their land.

Today we know that the 23.5-degree tilt of Earth's rotational axis relative to the plane of its orbit, is responsible for the seasons and the gradual change in the number of sunlit hours each day. On or about December 21 of each year, the Sun appears farthest south in the sky; for northern hemisphere observers, this is the shortest day of the year...