Shadow Patterns: A Year-Long Journey Following the Sun

If you follow the Astronomy Picture of the Day (APOD) like I do, you may have seen the December 2024 entry showcasing a photographer in Colorado who used a security camera to track a shadow on their driveway at the same time each day for nearly a year. As the seasons changed, this shadow traced out a distinctive figure-eight pattern known as an analemma. I found the simplicity and ingenuity of using readily available technology to capture such a complex phenomenon fascinating.

Video from APOD: Driveway Analemma

What caught my attention wasn't just the clever use of a security camera to track the sun's movement, but how it connected ancient and modern ways of marking time. Most of us have seen sundials in gardens or parks, where shadows mark the time of day. Fewer people are aware of solar calendars, like the Noon Mark on the south face of the Royal Observatory in Greenwich, which uses carefully placed markers to track a shadow's position at noon throughout the year, forming the same figure-eight analemma shape. In the video above, here was someone using modern technology to capture the same astronomical pattern that astronomers and timekeepers have been tracking for centuries.

Inspired by this APOD, I decided to capture a shadow analemma using technology myself. I'll dive into the details of that in my next post, but first, I’d like to share a bit of historical and scientific context.

Analemmas: Where Shadow Meets Sky

Dial at Brou Monestary

The word analemma comes from ancient Greece, where it originally referred not to the figure-eight pattern itself but to the mathematical techniques used for calculating the Sun’s position in the sky. It was derived from the Greek word analēmma, which referred to the support or pedestal of a sundial. In astronomical terms, however, analēmma described the methods used to account for the variations in the Sun's apparent motion throughout the year. This early use of the term was central to creating accurate solar calendars and tracking celestial events.

In modern usage, analemma now refers to the distinctive figure-eight pattern traced by the Sun's position in the sky when observed at the same time each day throughout the year. This pattern arises from two astronomical phenomena: the tilt of the Earth's axis and its elliptical orbit around the Sun.

The Earth's axial tilt of 23.5 degrees is the easy part of the pattern to understand. Because of this tilt, the Sun appears lower in the sky during winter and higher during summer in the Northern Hemisphere, with the opposite occurring in the Southern Hemisphere. If this were the only factor, the analemma would form a vertical line rather than a figure-eight, as the Sun's position in the sky would rise daily between the winter and summer solstices and descend daily between summer and winter.

However, because the Earth's orbit around the Sun is an ellipse, it is closer to the Sun around January and farther from the Sun around July. According to Kepler's Second Law, an orbiting body moves faster as it is getting closer to the body it orbits and slower as it is moving farther away. When the Earth is speeding up, the Sun appears a little ahead of its average position in the sky (i.e., where it would be if Earth's orbit were a perfect circle) and, as it is slowing down, it appears a little behind its average position. This, combined with the Sun's changing elevation through the seasons, creates the figure-eight pattern.

Long before cameras could record the Sun's position in the sky, ancient civilizations from Egypt to China used a down-to-earth approach to track time: observing shadows cast by the Sun—patterns that, over time, reflected the effects of the analemma. They built elaborate structures specifically designed to mark the passage of days and seasons by the position of shadows. These structures were not just timekeeping tools; they were woven into religious and cultural traditions, guiding agricultural cycles and celebrations.

APOD: Analemma over
the Callanish Stones

In the modern era, photographers have found a new way to capture the Sun's path: sky analemmas. By mounting a camera in a fixed position and taking photographs of the Sun at precisely the same time each day throughout the year, they create composite images showing the Sun's position tracing a distinctive figure-eight pattern across the sky. These images have become iconic astronomical artworks, requiring dedication and precise timing to create. However, they offer a more abstract way to visualize the Sun's movement—one made possible by photographic technology.

Shadow analemmas are particularly intriguing because they translate this celestial pattern into something tangible. While sky analemmas show where the Sun is, shadow analemmas reveal the effect of the Sun's position on our immediate environment. By following the Sun's path through the shadows it casts on familiar surfaces, we can experience the same astronomical phenomena that guided ancient timekeeping. Shadow analemmas can bridge historical traditions and modern science.

In this project, I seek to unite ancient astronomical practices with modern technology by developing an automated system to capture a year-long shadow analemma. Throughout this series, I'll explore the technical details of my setup, the challenges I've encountered, and the insights I've gained. Join me on this journey to explore the intersection of astronomy, photography, and the rhythms of everyday life.