About the solar clock

The practice of dividing the day into twenty-four hours is millennia old, but before the medieval invention of mechanical clocks, those hours were not of standard length. Rather, each day, as defined from sunrise to sunset, was divided into twelve hours, which could be read by observing the position of the sun in the sky. The sixth hour of the day was noon, the third mid-morning, the ninth mid-afternoon. As day length varied with the seasons, so did the length of the hours: an hour in a northern European winter might be only half the length of an hour in summer.

This idea survives in the canonical hours laid out by St. Benedict and still used by religious orders to mark times of daily prayer. They take their names from Latin: Prime for the first hour of daylight, Terce for the third, Sext the sixth, Nones the ninth. Vespers is said at sunset. Indeed, medieval efforts to improve timekeeping were meant firstly to permit more precise arrangements of religious life. Using spherical geometry and astronomical observation, scholars developed means of reading the hours of the (solar) day very precisely indeed.

This clock emulates the precision of the late medieval monastery. Based on given latitude and longitude, it uses spherical geometry to calculate times of sunrise and sunset for the present day, then divides the resulting day into twelve hours and displays the current time as the hand on a clock.

But why?

Why use a solar clock in 2022 to observe that it a few minutes before Sext, when the laptop on which it runs will already tell me that it is 1:18PM EDT? Because “1:18PM EDT” tells me next to nothing about where and when I am in the world. N.B.: As I write this, on the vernal equinox, that standardized time, based on atomic vibrations, the positions of orbiting satellites, and government fiat, tells me that “noon” occurred an hour and twenty-two minutes before the sun is due to reach its zenith. Standard time is fine for sending messages around the world, but useless for knowing where I am in my day, here.

You may wish to use this clock to pray the canonical hours. You may also use it to remind yourself just how little concern the global has for the local, to disconnect somewhat from the former and reconnect with the latter. Or you may just find it (as I admit I did, at the start) an interesting mathematical problem.

Using the clock

For privacy reasons, my clock does not determine your location dynamically but merely asks for coordinates. (The default is Raleigh, North Carolina.) Click change next to the current location to enter new latitude and longitude. You can obtain this information from any online mapping service (e.g. Google Maps). Once you set your location, you can bookmark the resulting web page to view your custom clock at any time.

The clock refreshes automatically once a minute as long as the page remains open in your browser.

Known issues

The blackletter font for the clock loads remotely from Google Fonts, rather slowly, and so you may see plain lettering when the clock is first drawn. When it refreshes after a minute the blackletter labels should appear. I’m working on this.

The fine print

The clock runs on Javascript in your browser; no personal information is communicated back to the server or collected in any way. The code is licensed Creative Commons: you may use it and improve on it as you wish so long as you credit me and don’t sell it.

I may make incremental changes to this clock without notice, but they won’t alter basic functionality.

For more about medieval timekeeping and mathematics, I recommend Seb Falk’s excellent book The Light Ages, which inspired this clock.

Thanks for stopping by.

David Walbert