Short answer up top: an analemma is a thin, figure-eight-shaped curve you get when you photograph the Sun from the same spot, at the same time of day, every few days for a full year, then combine every frame into a single image. NASA's Astronomy Picture of the Day for September 20, 2026, credited to photographer Giuseppe Petricca, is exactly that: a year of noon photographs of the Sun, taken over the ancient stone circle at Callanish, in Scotland's Outer Hebrides, layered into one composite finished near the December solstice.
How do you actually build one?
The method is simple to describe and hard to pull off. You pick a fixed camera position, a fixed direction, and a fixed clock time — in this case, noon — and you go back to that exact spot every few days, rain or shine, for roughly a year. Each frame captures the Sun a little higher, lower, or off to one side compared to the last. Stack all of those frames together and the individual dots of sunlight trace out a loop that looks nothing like a straight line: it closes into a lopsided figure eight, with one loop noticeably bigger than the other.
Why a figure eight, and not a straight line or a circle?
If the Sun's daily position only depended on the seasons, you'd expect something closer to a simple vertical line — higher in summer, lower in winter. The reason it bulges out sideways into a figure eight comes down to two separate effects working together, as NASA's own explanation puts it: the tilt of Earth's axis, and the ellipticity of Earth's orbit around the Sun.
The axial tilt is what drives the vertical spread — the Sun climbing higher in the sky in summer and sitting lower in winter, the same tilt responsible for the seasons themselves. The orbital ellipticity is what adds the sideways lean and the lopsided sizes of the two loops: because Earth's orbit isn't a perfect circle, our planet moves faster when it's closer to the Sun and slower when it's farther away, which means the Sun doesn't cross the local meridian at a perfectly even pace throughout the year. Combine an up-and-down motion from the tilt with a side-to-side wobble from the changing orbital speed, and the result is a figure eight rather than a line or a loop.
Why photograph it over a Bronze Age stone circle?
The Callanish Stones, where this particular analemma was framed, were built around 2700 BC, during humanity's Bronze Age — making the circle roughly four and a half millennia old by the time Petricca's yearlong exposure was finished. It's a striking pairing: one of the oldest deliberately placed structures in that part of Scotland standing beneath a phenomenon caused by orbital mechanics nobody on Earth could have measured with that kind of precision at the time. NASA's own note on the image is careful on this point: it is not known whether the placement of the Callanish Stones had any astronomical significance. The stones raise the question; they don't answer it. What the analemma image does answer, independently of any ancient intent, is exactly how the Sun's position drifts through the sky over a year — a question modern orbital data settles precisely, tilt and ellipticity included.
See the two ingredients for yourself
Both effects behind that figure eight — Earth's tilted axis and its elliptical, uneven-speed orbit around the Sun — are the same real orbital mechanics the real-time 3D simulator at Solar System Live tracks continuously. You won't see a photographic analemma there, but you can watch Earth's tilt and its current distance from the Sun as it actually stands right now, the same two numbers that, tracked patiently at noon for a year, produced the loop photographed over Callanish.
Next time you're outside at the same hour on a clear day, take a quick look at exactly where the Sun sits against the horizon. Do it again next month, and you'll already be a few frames into your own analemma.