A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2015 Dec 20, 13:55 -0800
First step, where do you live (roughly)? Although you can't use your artificial horizon for noon Sun all year round, you can use it whenever the Sun's maximum daily altitude is lower than about 60 degrees. If you're in North America, you can use it right now. In Australia? Well, then not until winter.
Do you have a small lake anywhere near you? You don't need an entire ocean. If you have a lake more than three or four miles wide, you can sit down right at the water's edge and take sights off the horizon of the lake.
There's lots to learn when you try to go beyond the easy noon Sun sight, but the easiest trick is to compare your observations against a simulation, which is, in fact, the basis of many systems of navigation, including what's commonly known as the "intercept method". If you visit this USNO web site, http://aa.usno.navy.mil/data/docs/celnavtable.php, you can enter your location as a latitude and longitude (or, if you were actually navigating, the lat and lon of any nearby point that you choose) along with the exact time in GMT (UT) of your sight. The web app will then calculation the true altitudes, in a column labeled Hc, of all available stars and other celestial bodies. Start with the Sun. You need to learn to convert your observation from raw sextant value to something that can be compared to Hc (by correcting for index error, semi-diameter, refraction and either correct for dip off a water horizon or divide the altitude by two for an "artificial horizon"), and then all you have to do is compare. If your corrected observation is, for example, 5.0 minutes of arc greater than the simulated altitude, then you must be 5.0 nautical miles closer to the place where the Sun is straight up, assuming you've done everything right. The direction to that point where the Sun is straight up is also given in the table in the column labeled Zn. There are lots of ways to move forward from here.
Conanicut Island USA