archaic timekeeping devices
Short Version: zomg nomograms!

Long Version:

From time to time I make a post about archaic technology. I was recently out on a field trip looking at some land. As I was hiking around, I starting brainstorming on how one could tell time without strictly knowing what direction you're facing. As the sun rises and sets over the course of a day, the length of a shadow cast by an object will change. Most sundials take advantage of the fact that the angle of a shadow also changes. If you don't know which way is north (i.e., you're walking around a lot), the angle doesn't help you very much.

In its most basic form, measuring the length of a shadow can be used to determine the angle of the sun in the sky. In theory, you could use this to calculate the time of day, but it is mildly complicated. You need to know your latitude as well as the time of year (to compute the sun's declination). With these, you can use the sunrise equation to determine how many hours of daylight you'll have today, as well as determine the highest point in the sky the sun will reach. (which is called noon.) You can then figure out where you are between those two points to arrive at the time of day. After that, you have to convert to railroad time (which requires knowing your longitude) and whether daylight savings in is effect. I haven't finished working out the equation, but when I showed it to yarrowkat she chuckled, wondering how I would run the calculation every time we wanted to know what time it was.

Despite the apparent complexity of the equation, it boils down to running a fairly detailed calculation in the morning, based on the year day and latitude, and writing down that number. Then you measure the length of the shadow of your walking stick, and consult a table to convert that length into another number. This table is constant for a particular walking stick. You then multiply those two numbers together and convert the result to hours and minutes before or after noon.

Certainly one can look up a result in a table and multiply two numbers together reasonably quickly, and working out the problem has so far proved entertaining. Today, however, I'm all a-wiggle over discovering nomograms. I've actually heard of them before, as they are similar to slide rules. But how they were actually useful has escaped me, until I got a chance to see a working example.

I realize now that telling time by the length of a shadow can be accomplished by constructing a nomogram, which would avoid the need for running any calculations at all. You would only need to "dial in" the device with your latitude and the length of your shadow, from which you could then read off the time of day. How cool is that!

One could also construct a nomogram for sunrise and sunset, phase of the moon, or to tell time at night. (Which, incidentally, is called a nocturnal, and I've been wearing the one that yarrowkat got me as a gift since our handfasting. I didn't realize it was a specific example of a neat general principle, however.)

And since role playing games have been on my mind, nomograms apparently see use in wargaming as well. One more point for role playing games as practical learning & practice tools.