Tag Archives: Light

Summer Time

by James Morris

At Saint Sulpice, a magnificent church in Paris, there is a famous gnomon made up of three different parts. Against one of the walls of the church is a tall obelisk. Running down the obelisk and across the floor of the church is a brass line set in white marble. And, in a window opposite the obelisk, there is a small hole that lets in a shaft of light on sunny days.

The gnomon of St. Sulpice

These three pieces – the obelisk, line, and pin hole – work together as a kind of astronomical calendar. The shaft of light crosses the line at noon every day, but it doesn’t hit quite the same spot. When the sun is at the highest point in the sky on the summer solstice, the light hits the line farthest from the obelisk; when it’s the spring or fall equinox, it hits the mid-point of the line; and when it’s at its lowest point on the winter solstice, it falls on the obelisk itself.

I recently visited Saint Sulpice right around the summer solstice and saw the beam of light shining down on the floor of the church, crossing the rose-colored line (popularized in Dan Brown’s The Da Vinci Code). I was intrigued by the ingenuity and engineering it took to build such a calendar in the 1700s.

But something else struck me as well. When I watched the circle of light on the floor, it didn’t seem to move. However, if I turned my back for a moment and looked at it again, I could see that it had moved slightly. Or, if I noted where it was relative to some reference point, like a tile on the floor, I could see that it moved ever so slightly compared to the fixed line.

I began to think about all types of things that move at this pace – too slowly to notice, but easy to see after a few moments. That is, you can’t see it move, but you can see that it has moved.

Of course, there are all kinds of things that move too slowly for us to see at all – think of the evolution of amphibians from fish, the gradual movement of the continents across the surface of the Earth, or the uplift of the ground to build the majestic Himalayan mountains. Evolutionary and geologic processes often move breathtakingly slow, well beyond our perception. And there are things that move much too quickly for us to perceive, such as light and chemical reactions.

But what about things that move just beyond our ability to detect them, just a bit too slowly, in that small window beyond our immediate perception? I’m thinking of the minute-hand of a clock, for example, not the second hand (too fast) or the hour hand (too slow).

Or the setting the sun. It’s in one place at one moment, and a different place the very next. The only time you might be able notice that it moves is when it starts to settle below the horizon. The moon and stars move at this same rate, as the apparent motions of the sun, moon, and stars all relate for the most part to the rotation of the Earth on its axis.

These kinds of slow changes were brought to life for me as a child. Every summer, we camped for a week in Small Point, Maine, along the coast. Each day, we noticed the tides moving in and out, in and out. It was impossible to see the water move, but if we went away and came back, it was dramatically different from where it was, the whole view changing dramatically. My father thought it would be fun to make a time-lapse film of the tide coming in, speeding up what is too slow for us to easily see. I had visions of a film showing the water rushing in. The reality wasn’t what I imagined (lots of panning back and forth, with large gaps between shots), but it nevertheless was a fun idea.

The garden is another place where things change slowly, just outside of our ability to notice them. Day to day, it basically looks the same, but go away for a week or two, and it’s a different place entirely – daisies replacing peonies, and weeds seemingly everywhere.

Age is like that too – growing older is imperceptibly slow on a minute-by-minute, day-by-day, week-by-week, even month-by-month basis, but over the years, the changes are dramatic. Kids grow up “just like that.” We give our children all kinds of advice, until one day they are giving it to us. And it’s often said that we all become our parents and “suddenly” we are saying things that sound just like them.

And what about movement that is just a bit too fast? The snap of a finger. The rapid back-and forth of a punching bag. The movement of drumsticks on a snare drum. The swing of a golf club or baseball bat. The beating wings of a bee or hummingbird.

Without some sort of technology to slow these movements down, we are not able to see them. But for things that move just a bit too slowly, there is something we can do: we can pay attention, watch closely, and attend to them moment by moment. We can take our time, slow down, and, in the words of William H. Davies, stand and stare, so that we notice what is going on. Or we will miss what’s happening right in front of our eyes.

© James Morris and Science Whys, 2018