How an analemma draws the Sun's yearly path into one picture

Take a photograph of the Sun from the same spot at the same time of day, every few days, for a year. Stack the frames and the Sun doesn't sit still — it traces a long, narrow loop, a shape called an analemma. NASA's Astronomy Picture of the Day describes exactly that method, and its September 20 image was assembled from shots taken every few days at noon near the village of Callanish in the Outer Hebrides, Scotland.

What makes the loop instead of a straight line is that two things are varying at once over the year: the Sun's north-south height in the sky and the time it crosses your local meridian. The result is the tilted figure-eight that photographers have been building since long before anyone could composite images digitally.

The photograph's foreground is the Callanish Stones, a circle built around 2700 BC, in the Bronze Age. Whether the stones' placement ever carried astronomical meaning is, as NASA puts it, not known — a useful reminder that a striking alignment in a picture is not the same as a documented purpose.