How did ancient societies use sundials as a way of tracking time?

No living person can definitively state when early humans began using the sun, moon, stars, and other celestial objects to measure time. In today’s world, relying on these heavenly bodies might seem outdated, but many modern timekeeping practices and calendars can be traced back to ancient systems. In those times, accurate timekeeping was crucial for survival—activities such as planting and harvesting crops were closely tied to the seasons.

The earliest timekeeping techniques were based on simple observations of nature, such as placing sticks in the ground and tracking the movement of shadows. This experiment would have been the foundation of the gnomon (pronounced nom-on), the part of the sundial that creates the shadow. It's easy to envision how this practice evolved into the use of obelisks, pillars, and other large-scale clocks and calendars. While some of these monumental structures can be considered sundials, we’ll focus on the smaller, often portable, sundials of the ancient world in this article.

The invention of the sundial is credited to the ancient Egyptians. Although obelisks date back to around 3500 B.C., the earliest portable sundials that have survived, often called Egyptian shadow clocks, gained popularity around 1500 B.C. These T-shaped or L-shaped devices, featuring a raised end bar, were used to track the morning hours as the sun moved across the sky. Later, they could be flipped to measure the afternoon hours.

It's crucial to remember that the Egyptians weren’t developing timekeeping methods in isolation. The concept of tracking time—whether by minutes, hours, days, seasons, years, or even longer periods—was a fascination for many ancient civilizations. Some of these cultures achieved remarkable precision. The Sumerians, Babylonians, Egyptians, Mayans, Greeks, and Chinese all created clocks and calendars that still align with our current numerical systems in various ways.

On the following page, we’ll explore why even the simplest sundials can carry hidden complexities.

The History of Sundials

Sundials originated as straightforward tools, but over time, they grew increasingly sophisticated. Let’s take a moment to revisit some fundamental astronomy.

The Earth orbits the sun in an elliptical (oval) path, not a perfect circle. This means that when the Earth is closer to the sun, the apparent speed of the sun's movement across a sundial increases. Additionally, the Earth's equator does not align with its orbital path; it’s tilted at an angle of about 2 degrees.

Considering the first of these facts, we realize that the time shown by a sundial (often referred to as solar time or sun time) can differ from the time displayed on a wristwatch (known as clock time or mean solar time). This discrepancy can vary by up to about 15 minutes at different points throughout the year. This wasn’t a significant issue before mechanical clocks, but in the early days, sundials were used to reset these timepieces when they ran down, which led to slight modifications in sundial design to accommodate this.

Another key astronomical factor to consider is Earth's rotation as it moves along its orbit. It may be tricky to visualize, but think of it like this: you’re standing in your backyard, counting the days. As Earth completes each 24-hour rotation, it is also slowly moving along its orbit. This means that the view of stars from the nighttime side of the planet changes slightly each evening. This phenomenon, called a sidereal day (in contrast to a solar day), is often associated with the zodiac signs, as it was in ancient times, and it offered clues to how the sky could be used to predict earthly events.

The tilt of the Earth creates two challenges for sundials: longitude and latitude. Regarding longitude, most sundials need to be aligned exactly with the planet's axis to work correctly. For instance, in the Northern Hemisphere, the gnomon must be pointed toward the North Pole. But be cautious when using a compass, as the magnetic North Pole shifts over time. An alternative method to find true north is to use the stars; Polaris, the North Star, is located at the north celestial pole and can guide you.

Latitude presents another issue if a sundial is used at a different latitude than where it was designed. Chances are it will need to be carefully tilted to work correctly. Even more complicated are the time markers on the dial, which may not be spaced accurately for the new location—this requires trigonometric calculations to resolve.

On the following page, we will explore how time was measured in ancient civilizations and the progression of sundial technology throughout history.

The Evolution of Sundials

Sundials might conjure images of primitive, outdated technology or charming garden decorations, but their story doesn’t end there. Even after the advent of mechanical clocks, sundials continued to serve as accurate time-keeping tools well into the modern age. Their lasting presence was partly due to the necessity of resetting mechanical watches. Ultimately, it was the enduring fascination and admiration from people worldwide that has helped preserve this ancient device’s place in history.

Now, let’s take a step back. In the early days, sundials measured what were known as seasonal hours. The day was divided into 12 hours, but in winter, those hours were shorter than in summer because of the varying lengths of daylight. Near the equator, this difference was less noticeable, but towards the poles, timekeeping fluctuated significantly, especially for those living in extreme climates. This gives us an interesting contrast to the fixed time structures that define modern life.

Sundials played a crucial role in ancient civilizations, helping to track not just time, but also the seasons, solstices, and equinoxes. Originating with the Greeks, sundials such as the hemispherical sundials or hemispherium employed a hollowed bowl and a pointed gnomon to measure time and seasonal changes. The gnomon’s point indicated the time of day, while the shadow’s size revealed the time of year. Occasionally, one half of the bowl was removed, creating what might be called a hemicyclium or hemicycle, but both versions essentially functioned in the same manner.

As mentioned previously, the tilt of the Earth complicates sundial design and placement, but it also provides a way to determine seasonal information. By aligning the gnomon with a curved surface, it’s possible to trace lines on the dish corresponding to the summer solstice, winter solstice, and the equinoxes (which share the same path).

Throughout history, a variety of sundial models were crafted across different cultures, with creativity being the only limit. For example, during the European Dark Ages, Muslim scholars applied principles of trigonometry to create the flat circular sundials we commonly see today. They are also credited with introducing the concept of equal-length hours, which were often marked by prayer times. This idea of equal-length hours spread, but despite the arrival of mechanical clocks in the 1300s, seasonal hours remained in use for many years, eventually giving way to mean solar time and, later on, time zones.

Through these innovations, ancient civilizations were able to record past events and plan for the future. They could formalize government, religion, and societal activities with a standardized schedule — a practice that has carried over into our modern world, now structured with the precision of cesium atomic clocks.

On the next page, you'll find links to a wealth of information about astronomy and clocks, along with intriguing questions about the ancient world that will be answered.