One core principle I recite like a litany to my students is that history happens in space and time. Taken together, the where and the when of an event create for it a unique historical context. By understanding an event’s context, we can appreciate its significance, both as it happened and as it affects us in the here and now.
For example, epidemic disease runs right through the story of human history, but in premodern times it was far more prevalent in Eurasia than in the Americas simply because humans in Eurasia domesticated and lived alongside a wider array of animals and were therefore exposed to more animal pathogens. Within the long history of epidemic disease, the Black Plague traveled further and faster in the early fourteenth century than ever before because of major new routes and technologies of human travel that had opened in the preceding decades. These included the Mongol conquests of much of Asia and the intensification of Mediterranean maritime trade. In other words, the Black Death was a uniquely fourteenth-century Eurasian phenomenon: it could not have happened in fourth-century Eurasia any more than it could have happened in fourteenth-century North America. (Of course, when Europeans brought their animal pathogens to North America two centuries later, the results were catastrophic, but that is the subject for another time.)
This is the first of a series of posts where I explore the unique historical context—the where and the when—of the Prague astronomical clock, including its specific location and the age of its creation. Of course, history will play into pretty much everything I write for this project, but this series of posts will explicitly feature spatial and chronological context. In later entries, I will write about European society in the late fourteenth and early fifteenth century, when the clock was made. That was a watershed period in the human understanding of time itself, and it is no accident that the technology of clockmaking took great strides during it. For this post and one to follow, however, I will focus on the location of Prague and how it affects the design and use of the astronomical clock. This will help us understand some of the basic principles of celestial timekeeping. I will also start introducing some of the graphics I am creating for a planned eventual animation of the clock, to help illustrate things as I go.
Time and space
There are many types of clock in the world. The most modern are those that track time by measuring quantifiable characteristics of certain materials. These include clocks attuned to the resonant frequencies of crystals (typically quartz) or atoms (typically cesium), but there was a time when the oscillations of alternating current provided the rhythm that ran electric clocks. Before these, there were other physical mechanisms that helped people delineate time: springs, pendulums, the flow of sand or water, the burning of candles, oil, or incense. All of these made timekeeping portable and flexible, freeing it from some of the limitations of the oldest clocks of all: sticks and stones casting shadows on the ground.
Whatever the mechanism, we tell time by the Sun and, in some societies, by the Moon. Incense, pendulum, and atomic clocks all approximate divisions of the regular rising and setting of the Sun and the waxing and waning of the Moon. Underlying all clocks, therefore, is an astronomical understanding of time. When I talk about “astronomical clocks,” I refer specifically to timekeeping devices that depict the movement of heavenly bodies themselves. The Prague clock is an astronomical clock because it shows the location of the Sun and Moon against the zodiac and the horizon of the Earth. A sundial relies entirely on sunlight to be of any use, but it will never show you where the Moon is.
Even with this definition, though, things get tricky. As we will see, even the familiar round clock face is designed with an eye to the course of the Sun. Indeed, celestial timekeeping permeates our lives every day in ways many of us don’t recognize. The division of the day into twenty-four hours dates back to Babylonian mathematical methods for measuring the sky. The count, name, and ordering of days of the week is rooted in observations of the seven visible planets.
Without getting too sidetracked (for now) by the details of planetary motion or Babylonian mathematics, celestial timekeeping is based around two physical facts: first, that the Earth spins on an axis while simultaneously orbiting the Sun, and second, that the axis of the Earth is titled relative to the plane of orbit but is mostly stable relative to the surrounding universe. As a result, most parts of the Earth tilt either towards or away from the Sun at different times of the year, but more distant stars appear to rotate along constant circular paths in the sky.
The tilt of the Earth’s orbit gives us seasons, and it makes it appear from a point on Earth as if the Sun changes its course through the sky relative to the background of stars. The change in the Sun’s course during the year becomes more extreme the further one gets from the equator, and so telling time purely by the course of the Sun and stars requires knowing one’s location on Earth. This brings us to the specific location of the Prague astronomical clock.

The clock stands on the southwest corner of the Old Town Square of Prague. In the lower right of the image above, you can maybe just read a set of coordinates for the clock: 50°05’13.22” N 14°25’14.61” E. (In another form of annotation, you might see this written as 50.087, 14.421. The difference between the two systems—Babylonian hexadecimal and modern decimal annotation—might feature in a different post, if I ever get that far into the weeds.) These are measurements of latitude and longitude: the Prague clock sits just over 50 degrees north of the equator and almost fourteen- and one-half degrees east of the Prime Meridian, which is defined by the Greenwich Observatory in southeast London.
The system of latitude and longitude dates back well over 2000 years, to efforts during the Hellenistic period to apply observations of the heavens to the effort of mapping the Earth. (It is no accident that the type of mathematics required for this—geometry—literally means “measure of the Earth.”) The core concept of a coordinate system was described at least as early as the third century BCE by Eratosthenes, the great librarian of Alexandria, but it was Ptolemy, four centuries later, who formalized latitude and longitude largely as we know them.
Lines of longitude run north to south and intersect at the poles, like the divisions between sections of an orange. Lines of latitude run east to west, staying parallel to the equator and therefore never crossing one another. Ever since Ptolemy, latitude has been measured north or south of the equator. Longitude has been measured east or west from different points at different times, from the Canary Islands to the legendary city of Kandakes in East Asia. Greenwich only came to define the Prime Meridian in 1884, when British influence internationally was at its peak. This is another case of how the context of a historical decision continues to impact our lives today.
A schematic representation of the system of latitude and longitude appears at the center of the Prague astronomical clock in the image of a globe tilted towards the viewer. This schematic globe was added to the clock during renovations done in 2017-2018. The previous renovation, undertaken to repair damage sustained during World War II, showed a map of the earth, with south facing up, the north pole just below the center of the circle.

On the post-World War II map face, the main clock dial emerged from Central Europe. The new schematic globe is tilted in just such a way that the main clock dial sits right around fifty degrees north of the equator. It is no accident that both the old and new faces are designed so that the main dial corresponds to the location of Prague. Between the two, however, the new schematic globe image is the more appropriate one exactly because it is non-specific.
Designing an astronomical clock requires knowing one’s precise latitude north or south of the equator, as this determines how the divisions of day and night are depicted on the clock face. (I will post a short technical note to explain this in more detail, for those of you who want it.) However, a clock made for a specific latitude would work equally well at any point along that same latitude, since every point along that latitude has the same position relative to the tilt of the Earth’s axis, which is the key variable in celestial timekeeping. Therefore, the Prague clock, made for fifty degrees north latitude, might be moved to Krakow or Frankfurt or Amiens, or even Kharkiv or Winnipeg and it would work just as well. The same cannot be said about longitude, the more challenging coordinate to measure. We will tackle that question in the next post.
Fascinating! I walk past this clock regularly and have heard multiple explanations and stories about it, but never one told from this angle (literally). I'll tilt my head a little differently next time I walk past.
Not easy for me to follow. But my brain is not very scientific. But I did get some of the basics of time keeping. 🙏, Stefan!