Time Compass
Sundials are a beautiful and elegant application of some fairly straightforward science. The sun rising and setting happens because the whole sky appears to spin overhead, but the object that is actually spinning is the Earth, (sort of) obviously. Since it’s so big, the rate of spin is regular, and on average it takes 86,400.013 seconds to complete a single turn.
That extra 0.013 seconds comes from when the second was defined, by the way: it was found by taking a fraction of the average day length between 1750 and 1892. As I mentioned previously, the length of a day is getting longer, and by now that has added up to thirteen extra milliseconds per day. If that sounds interesting, go look up Universal Coordinated Time: you’ll soon find your interest waning, I’m sure.
Anyway, the Earth spins on its own axis, bringing the sun into view on a regular schedule, and with a very consistently varying bearing throughout the day. At midday in the northern hemisphere, the sun is due south; at midnight it is due north. Unless you’re above the arctic circle in the summer months, you won’t be able to visually check the latter, but take it from me, it’s true.
This means that you can take your compass rose, divide the circle into twenty-four segments, and tell the solar time by which segment the sun is in. That’s the basis of the sundial, and is why it is the only type of clock which actually tells the time and not just how long it has been since it was set. Of course, life is never as simple as that, so you have to adjust for the equation of time, and for any daylight savings time if it’s that time of year.
All of this is interesting, but the part that really grabs my attention is the idea of using this in reverse: if we can use direction and the sun to tell time, we can use time and the sun to tell direction. I can use the watch that I talked about yesterday to work out which way I’m headed, whether that’s when I’m in a city I’ve never visited before or maybe when I’m traversing woods or jungle come the apocalypse. It sure beats lugging a compass everywhere with me.
The premise is that we work out which direction the sun should be in at the current time, and align ourselves with that. I like to visualise it in a very esoteric fashion, where I imagine an extra hand on my watch that is always halfway between the hour hand and midnight. This would mean it is pointing to the six at midday, and completes one rotation every twenty-four hours. If that imaginary hand is pointing at the sun, the twelve on my watch dial is pointing north. Obviously that’s completely insane, but it works for me.
If you’re less crazy than me, the following algorithm might help: point the hour hand at the sun, and then picture the arrow being made by the hour hand and the twelve: if you’re in the Northern Hemisphere and it’s between six a.m. and six p.m., that arrow points due north. In the Southern Hemisphere, it points due south. If it isn’t immediately obvious, this only works if your watch isn’t on daylight savings time: you’ll have to imagine moving the hour hand an hour back if it is.
So, the arrow formed by the hour hand and the twelve points north if the hour hand is facing the sun.
Now your watch has one more function.
This is from the