A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2015 Dec 21, 11:54 -0800
Lee Sublett, you wrote:
"I am wanting to learn to find my position the old school way with nothing more than a sextant, an almanac, and a watch."
I believe you wrote this comment in response to my recommendation of using the USNO (US Naval Observatory) web page which calculates celestial navigation data (here: http://aa.usno.navy.mil/data/docs/celnavtable.php). No matter what sort of navigation you're interested in, you should absolutely learn to experiment with this web app. It will make you a better navigator if you have some sense of how altitudes change over time.
For an example, let's consider the case of observing the noon Sun from your location. On that web app page, we enter enter today's date and for the time we try 18:00:00 UT. That time corresponds to twelve noon according to the clock in the CST zone. Note that this is not the time of local apparent noon, but it's a good place to start. Next enter your latitude and longitude to the nearest minutes of arc (for my sample calculation I'm using 34° 00.0' and 87° 00.0' exactly but you would use whatever is close to your observing location). Hit the Get Data button and see what comes out.
The output data from the USNO web app includes lots of useful data which you can learn as you go, but right off the bat you can understand the altitude and azimuth columns which are labeled Hc and Zn, respectively. For the data as input above, we see that the Hc of the Sun is 32° 28.2' and its azimuth is 183.8°. Now in terms of azimuth in degrees, due South is 180 degrees so this latter value is telling us that we are too late. It's already past noon. If you try the time again a few times, you can narrow it down to get a better idea when the Sun was right at local noon and nearly due South. It's not necessary to observe at this exact instant. In fact, the altitude barely changes for several minutes right around that time, and you can experiment with that by looking at the Sun's altitude for a few minutes right around the time when the Sun would be at azimuth 180°. Try it out: can you determine the time when local noon occurs (Sun's Zn nearly 180.0°) for today? What time do you find?
After using the USNO web app, you can easily pre-set your sextant to the correct altitude: double the Hc will get you close enough. This is not "cheating". It was absolutely normal in traditional navigation to pre-set a sextant to an approximate altitude based on the previous day's experience or based on a rough calculation from almanac data and the navigator's estimated position.
You mentioned that you want to learn the "old school way". There are some two hundred years of old school ways available in the history of scientific navigation. All of them include methods and calculation techniques which can get your position within a mile or two using a sextant and a clock. You could choose to learn the last "old school" methods, which were in place just before GPS became dominant. Or you could choose to learn navigation from the era of the Second World War... or perhaps from a century ago... or two centuries ago... Most navigation enthusiasts eventually dabble widely in different eras. Because there is such a rich history, all of which qualifies in one way or another as "old school", you can anticipate many options ahead. And don't forget that the technology available today allow you to learn faster and become much more effective with the tools of traditional and/or historical navigation even if you decide not to employ any modern tools in your personal choice of procedures.
Conanicut Island USA