Slow Motion

Barry Perlus

Those in the Northern Hemisphere who have recently stepped outside just after nightfall to view Comet Neowise know that the window of time in which to view this current marvel is limited to a few hours before it disappears below the horizon. The apparent movement of celestial bodies—galaxies, constellations, stars, planets, and our sun and moon—has fascinated sky-watchers through the ages. But the motion we ascribe to celestial bodies is rarely seen as motion the way we see a bird gliding through the air or a Ferris wheel slowly turning. It is usually perceived as a change of position over time. We know that the motion is there; it is just so slow that we don’t perceive it as motion.

Celestial motion isn’t easily perceivable without a reference. We watch a moonrise or sunrise, and our horizon, be it a line of trees or a city skyline, serves as a reference to allow us to see, moment by moment, the grand appearance of our subject. One of the ways the ancients marked the movement of celestial bodies was to establish alignments with natural features like mountain peaks, or constructions such as temples and standing stones. Any earth-based feature may be used as a reference to discern the motion of the stars. Looking again into the night sky, we see that stars and planets can also be references for one another. Their relative positions, changing over time, provide evidence of their and our motion through space and are the basis of the ancient practice of astrology.

We know that the sun appears to move across the sky throughout the day, but except for sunrise and sunset, when the horizon provides a reference, its radiance masks all else that would give evidence of its motion. There is, however, an indirect effect that reveals the sun’s motion—its shadow.

A few years ago, on August 30, 2017, a solar eclipse was visible from our location in Ithaca, NY. I was teaching that day and rushed home in time to set up a time-lapse video of shadows moving across the flagstones of our entry path. During the time lapse I marked the edge of the shadow on the path with chalk at five-minute intervals as a way to make note of the sun’s motion and create a very accurate (but temporary) sundial.

Figure 1. Chalk markings show the time the leading edge (left edge) of the shadow passed that point on the stone path.

Sundials, no matter what form or size, give us the local time by virtue of this moving shadow. Though we tend to think of sundials as decorative objects or devices for telling time, a sundial, if large enough, will make the motion of earth and sun palpable. This is what took me by surprise one morning in December 2001 while I was making a time-lapse video at the Samrat Yantra, the great sundial at the Jantar Mantar in Jaipur, India.

Jantar Mantar is the name given to a group of four astronomical observatories built by Maharaja Sawaii Jai Singh II in the early 1700s in India. Jai Singh based his observatory designs on naked-eye sky observation, and the masonry instruments he built for that purpose are large-scale works of architectural design and precise engineering. Combining geometric forms in unique ways, these structures are exotic and beautiful and were one of the reasons I was there to photograph them.

Figure 2. The Samrat Yantra at the Delhi observatory. Like the Samrat Yantra in Jaipur, the gnomon is approximately seventy feet high.

The Samrat Yantra is what is called an equinoctial dial. It uses a gnomon, a triangular wall like the vertical triangle of a garden sundial, to cast a shadow on curved surfaces called quadrants on either side of the gnomon and set at an angle parallel to the earth’s equator. A feature of the equinoctial dial is that the shadow of the gnomon moves across the quadrant an equal distance for each unit of time no matter where on the quadrant the shadow is falling.

Figure 3. Aerial perspective of the Samrat Yantra at the Jaipur observatory.

The Samrat Yantra at Jaipur is large. With a gnomon some seventy-four feet in height it is an imposing structure. Each quadrant has a radius of nearly fifty feet and is just over nine feet wide. The marble surface is inscribed with units of time in multiple gradations of one hour, twenty minutes, five minutes, one minute—down to the smallest interval—two seconds! In practice, an observer notes the position of the shadow on the quadrant scale to determine the local time. When local time is converted to standardized time, it turns out that Jai Singh’s sundial is absolutely accurate to within two seconds.

Figure 4. A thin twig is moved slowly left and right within the soft edge (penumbra) of the gnomon’s shadow. The point where the twig’s shadow appears/disappears represents the center of the penumbra and gives a precise measurement of the local time.

What captured my attention that day was the enormous scale of the instrument. It was about 11:00 am, and the sun’s shadow was close to the base of the gnomon. On my right, the solid wall of the gnomon, tall in its north-south orientation. In front of me, the tilted marble scale of the quadrant. I hadn’t set up the camera yet and was studying the details of the marble scale, examining the different rows and types of inscribed lines that indicated units of time, and the shadow of the gnomon that fell across them. The question rose up in my mind: “was it possible to actually see the shadow in motion?” I stood still, very still. The observatory was relatively quiet that day, and at that moment there were no visitors within sight. I became quiet. Everything around me seemed to become quiet too. I looked closely at the scale indicating two-second increments and then began to see the motion, the gradual quenching of light on each two-second marking as the shadow progressed across the quadrant. As I stayed there, patient, watching, aware of myself in front of this great scale, I felt a part of this great movement, profoundly connected to the earth and its living rotation in space.

It was experiences like this, along with the unique architectural forms of Jai Singh’s creations, that led to my thirty-year interest in presenting the observatories to a wider audience.

Barry Perlus is associate professor emeritus of art in the College of Architecture, Art, and Planning at Cornell University. He has taught courses in photography since 1984 and received support from the Graham Foundation for Advanced Studies in the Fine Arts for his creative work. You can learn more about Jai Singh’s observatories and explore them visually at

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Featured Image: Photo by Barry Perlus

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