Return to Newsletter Index

The Roamin' Gnomon

by Perry Pezzolanella, MVAS

We are all very much aware of the changing seasons here in Northeast U.S. All of us are also aware of the changes in the length in daylight and the sunrises and sunsets as the seasons progress. In a more subtle way we are also aware of the changes of light and shadows of the seasons. We see the shadows reaching out long and far on a frigid winter afternoon and we search far and wide for a small patch of shade in the heat of a summer afternoon. We celebrate the arrival of spring and the birth of the warmer, calmer months ahead when we relax in our favorite room during a March evening to find at long last that it is flooded with bright sunlight after the long, dark winter. Or we realize that the sunlight, which greeted us in the kitchen in the early morning, has shifted away during September leaving everything in dim shadow making us dread the colder, stormy weather ahead.

The changes in the quantity and quality of sunlight are not just solely due to where the Sun is in the sky, but also how far north or south it rises or sets. The point of sunrise and sunset along the horizon changes as the Earth orbits the Sun in its year-long journey. While the Earth is orbiting the Sun, it tilts on its axis about 23.5 degrees, which gives rise to the seasons. When the Earth's Northern Hemisphere is leaning way back it is winter and we see the Sun around here climb to barely 23 degrees above the southern horizon on the winter solstice (December 21-22). The shadows are long and daylight barely lasts 9 hours with sunrise around 7:35 A.M. and sunset around 4:25 P.M. Half an orbit, or half a year later, the Earth's Northern Hemisphere is leaning into the Sun and it is summer. The Sun blazes down nearly overhead at 71 degrees above the southern horizon on the summer solstice (June 20-21). The shadows are short and daylight lasts nearly 15 hours with sunrise around 5:20 A.M. and sunset around 8:45 P.M.

All of this sets the stage for the continuous dance of shadows by objects on Earth, both natural and man-made. Some objects can be used as a gnomon for the passage of time during the course of the day, but all objects can mark the passage of the seasons. It is particularly fun to watch the north side of a building catch the rays of the Sun in the early morning or late evening as it rises and sets near its northernmost point along the horizon near the summer solstice and then during the course of the summer the Sun no longer shines there, a harbinger that winter will soon return. This is so precise that if one were to take special care and had enough clear weather for at least a week, one could mark that date of passage. Then a year later it will indeed occur again on that defined date. In just such a way our ancestors marked the changes of the positions of the Sun, Moon, planets (especially Venus), and shadows within the year and often in their respective cycles, which can repeat in periods exceeding a decade. Sunlight falling into slits within the pyramids and Stonehenge are examples of how time and the seasons were marked.

In a way, we also mark the time of day as we stand in our shadows during the noontime summer months, or observe our lengthening shadows earlier or later in the day or as the winter season approaches. Indeed, if we are very observant, we can use our own shadows to estimate the time of day and the time of the seasons as we go about our lives. This would make each of us a genuine roaming gnomon.