Earth Is (Still) A Clock

by Mary Hrovat

Image of sundial on an external wallBefore the second was defined in terms of the characteristics of the cesium atom, before leap seconds or leap days or Julian dates or the Gregorian calendar, before clocks, even before the sundial and the hourglass, there were sunrise, sunset, and shadows.

I’ve been thinking about timekeeping using shadows because a tulip tree in my backyard casts a shadow that traces a semicircle over the lawn on sunny days and moonlit nights, like the hand of a clock. The shadow is longest and most noticeable at this time of year, when the sun crosses the sky low in the south, and on summer nights around full moon. (The full moon crosses the sky low in the south in the summer, when the sun is riding high in the north.) However, I can see it year-round, given adequate sunlight or moonlight, although its appearance varies depending on the position of the sun or moon. I enjoy seeing this subtle demonstration of daily and seasonal cycles.

The gnomon on a sundial (the part that casts a shadow) was probably inspired by natural objects like this tree that cast useful shadows and roughly indicate the time of day. The first human-made gnomons were vertical poles or towers. For example, Egyptian obelisks, in addition to being tributes to gods or markers celebrating a ruler’s achievements, acted as gnomons, and their shadows marked the time of day for a city.

Sundials of various types were developed as these early timekeepers were refined by aligning the gnomon with Earth’s rotational axis and adding a dial that marks divisions in time. In addition, the natural or solar hours, which vary in length throughout the year if you’re not near the equator, were eventually replaced by hours of equal length. Thus was timekeeping made more useful but also distanced somewhat from its roots in solar time, particularly when clock time had to be coordinated across different regions and eventually across the globe.


Earth is also a calendar. Gnomons have been used to indicate the seasons as well as the time of day. A horizontal pole casts its longest shadow on the winter solstice and its shortest shadow on the summer solstice. On the equinoxes, the shadow reaches the point equidistant between the two. By marking each point, people could identify the time of year from where the shadow fell on any particular day.

This is essentially the way that Upwells, an ingeniously designed outdoor sculpture on the University of Illinois campus in Champaign–Urbana, works. The sculpture includes a spire and three very low fountains in the shape of domes. The shadow of the spire falls on the most distant fountain at the winter solstice, the nearest fountain on the summer solstice, and the one in the middle at the equinoxes. Upwells is sometimes called Beckhenge because it’s near the Beckman Institute.

The name Beckhenge, of course, is a play on Stonehenge, probably the best known structure marking the solstices and equinoxes. However, there are many other such sites around the world. The seasons are important agriculturally, and the observance of the turning points of the year has long been associated with ritual, religious festivals, and sacred sites, in addition to the necessary farming activities. Ultimately, the science of astronomy grew out of observation of the path of the sun throughout the year, among many other things.

The oldest stone structure used to mark the solar extremes may be that at Wurdi Youang in Australia, where important solar dates are marked by approximately 100 stones in a more or less egg-shaped arrangement oriented east–west. The Neolithic Goseck Circle in Germany dates back to around 4900 BCE. An excavation in 2002 revealed holes that once contained wooden posts forming two concentric circles. In 2005, the site was reconstructed; from the center of the enclosure, sunrise and sunset on the winter solstice appear through entrances to the structure. At Chankillo in Peru, 13 short, wide towers of rock can be viewed from two observation points to identify any day of the year (not just the solstices and equinoxes) according to where the sun rises or sets, as the towers cover the entire annual range of sunrise and sunset locations. The site was used for solar observations between 500 and 200 BCE; it may be the earliest such observatory in the Americas. For information on other sites of archaeoastronomical importance, see the Portal to the Heritage of Astronomy.


It’s just luck that the tulip tree happens to lie on the southern border of my property and to stand more or less alone so that its shadow is distinct. But any building that’s not near the equator, however it’s oriented, probably also has interesting patterns of light and shadow that change as the year progresses. Although they may not be tied precisely to the solstices or equinoxes, they still reveal (or come to represent) the time of year, for those who pay attention.

For example, for a few weeks in summer, the sun is far enough north that I get a glimmer of light low through my northern windows late in the day. This relatively rare glow at the bottom of the windows is a reminder of the luxury of abundant light in the summer, but also of the swift passage of time, because the light, welcome as it is, usually fades away without my noticing. There are also two times a year when sunlight comes directly into the kitchen window right around dinner time; I think of these times as golden happy hour, and one of my small domestic rituals is to now and then pour a glass of wine in this blaze of sunlight and drink a toast to time and the sun. When my son’s household included a cat, early and late winter were marked by periods in which a square of sunlight on the kitchen floor was so close to the refrigerator that you had to nudge aside the cat (who enjoyed the warmth) to open the refrigerator. This is probably my favorite example of an inadvertent sun clock/calendar.

These days we have other ways of tracking both the hours and the year, and our livelihoods and religious festivals, by and large, no longer depend on careful observation of the solstices, the equinoxes, or the place of the sun in the sky on any given day. Even so, people still visit Beckhenge and many other sites to celebrate the solstices, and we still note fortuitous events like Manhattanhenge, which occurs when the sun lines up exactly with Manhattan’s street grid twice a year (in late May and mid-July). We still build and use sundials, an extreme example being the Sundial Bridge across the Sacramento River in Redding, California. The bridge is a working sundial, one of the largest in the world, although its size limits its usefulness.

Interest in sundials and informal sun clocks and sun calendars might seem atavistic and redundant. However, I think it’s an essential part of being human. Our lives still depend on the regular round of the seasons (and we may be more aware of this now that the seasons are being reshaped under the pressure of climate change). In addition, Earth’s day/night and seasonal cycles are embedded deep in our most basic bodily functions and, I suspect, our psychology.

For example, the reason that late-night screen time can disrupt your sleep is tied to changes in sunlight throughout the day. Early and late in the day, sunlight is redder because it travels a longer distance through the atmosphere, and the blue light tends to be scattered out. (This helps explain sunrise and sunset colors in the sky, and the incredible orange moonrises you sometimes see when the moon is full.) During the day, the light is bluer.

Our bodies respond to the color of sunlight by producing and releasing hormones that stimulate either wakefulness or sleepiness. In the bluer light of midday, we’re more alert, our physical coordination is better, and we react more quickly. The redder light of evening signals the secretion of melatonin, which ultimately helps us to sleep. Getting more natural light in the middle of the day and reducing exposure to blue light in the evening may make it easier to sleep.

Lighting technology is now sophisticated enough that there’s such a thing as circadian-friendly indoor lighting, which mimics, as much as possible, the natural color profile of sunlight. It seems to me that it would ultimately be simpler, as well as more pleasurable and beneficial in other ways, to spend more time outside or within sight of windows (e.g., to design public spaces and workplaces so that everyone has access to natural light).

There’s some scientific evidence that being outside (even in urban parks or green spaces) is good for our mental and physical health. I think exposure to natural lighting throughout the day and the year may be one reason for this. The pleasure of watching the days and seasons play out in familiar spaces may do much more for us than we realize. If nothing else, it grounds us in the physical world that supports us. It connects us to something larger than us, something that was here long before we were and will go on long after we’re gone. There is a great deal of comfort in that.


Image courtesy of Pixabay.

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