Thursday, April 12, 2012


I spent 8 or 10 or 15 hours listening to the audiobook Longitude, and most of that was how much trying to make a clock that worked at sea consumed John Harrison's life. All of this made me itch to take advantage of centuries of development and buy a practically free, practically perfect pocket watch. It takes two clocks to work out longitude. Here is what you do: Keep one set to the time at the Prime Meridian, where it is 0* and they use Greenwich Mean Time. Then, at local noon, set the other clock. After you subtract GMT from local time and divide by twenty-four, multiply your result by 360* and you are all done! The only thing is, Greenwich uses Mean Time, averaging all the noons in a year, and we are using solar time directly. There's an Equation of Time for that, but I haven't learned it. I made a little reference card for myself and put it into the locket watch.

*You can multiply by 15 but then remembering that the sun takes 24 hours to visit 360* of Earth wouldn't help you remember how to calculate longitude.


flyingvan said...

One of the things I teach for copter stuff is finding compass directions using your watch. If your analog watch has on of those bezels you can turn it makes it really easy. You just decide what, on the bezel, will represent south then turn the bezel until it is halfway between the '12' and the hour hand. With the watch horizontal, you turn the whole watch until the hour hand is pointing at the sun. That's it! Now the point on the bezel representing south is pointing south, 180 degrees from that. obviously, north, and so on. These are northern hemisphere directions.

Caitlan said...

I heard about this! It's so cool that you guys use this! I might print your comment to put in my wallet, because the only time I tried this without directions I got it backwards. I was showing Dad and Nick and I thought it was supposed to point North, not South.