It is said that the person who introduced mechanical clocks to Japan was none other than Francis Xavier. A little surprising, isn’t it? But the Japanese reaction was different from Europe’s. Japan didn’t adopt the fixed-hour system. Instead, Japanese clockmakers kept experimenting with ways to make clocks work with their existing unequal-hour system. And that was the beginning of the wadokei.

Perhaps people were simply too accustomed to hearing temple bells mark the traditional hours. Imagine being told to ring the “Akemutsu” dawn bell when the sun hadn’t even risen yet. Around the winter solstice, that might have seemed rather strange. Or perhaps Buddhist temples weren’t particularly enthusiastic about adopting a new system brought in by a foreign missionary. Or maybe it was simply because Japan was—and perhaps still is—rather conservative when it comes to adopting new ideas. And since the difference between daytime and nighttime hours in Japan was only about 40 minutes at most, perhaps the traditional system was practical enough. Whatever the reason, Japan went its own way. And Japanese clocks evolved into their own little Galápagos.

Now, let’s take another look at the definition of a wadokei. It can be described like this: “Sunrise (Akemutsu) is the exact Hour of the Rabbit, and sunset (Kuremutsu) is the exact Hour of the Rooster. The period from Akemutsu to Kuremutsu is divided into six daytime hours, and the period from Kuremutsu to the next Akemutsu is divided into six nighttime hours. The exact hour is defined as the midpoint of each hour.”

Can you spot the contradiction? The key is this part: “The exact hour is defined as the midpoint of each hour.” Day and night are normally different in length. That means, except around the spring and autumn equinoxes, sunrise and sunset cannot actually be the exact Hour of the Rabbit and Hour of the Rooster while also being the midpoint of their respective hours.

In other words, the wadokei was a rather approximate—and slightly contradictory—clock from the very beginning.

The defining feature of the wadokei is its unequal-hour system, in which daytime and nighttime hours change in length. Clockmakers in the past tried all sorts of ingenious ways to deal with this problem. Some clocks used dials that changed with the seasons. Others allowed the hour markers to be moved. But even then, creating a truly perfect mechanical wadokei was impossible. Fortunately, today we have software.

The clock face and hour divisions can now be adjusted freely, making it possible to create a perfect—or at least nearly perfect—wadokei. Unequal-hour systems are most practical at low to middle latitudes. At high latitudes they become so impractical that such systems apparently never really developed there in the first place. But software doesn’t have to worry about that. So yes, you can even use this wadokei in regions with the midnight sun or polar night.