NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Clarification of Question regarding LAN
From: Chuck Taylor
Date: 2004 Aug 4, 08:36 -0700
From: Chuck Taylor
Date: 2004 Aug 4, 08:36 -0700
--- Andrew Corlwrote: > Is there a way to use the GHA for the sun from the > daily page from one > of the online almanacs to calculate noon GMT, and > then use the table > "Convert Arc to Time" to calculate LAN? Is that > possible? By definition, LAN occurs when GHA Sun equals your longitude. If you know your longitude, you can go into the nautical almanac to calculate the time at which LAN occurs at your location. Figuring LAN this way, and taking a noon sight of the sun at that time to calculate your latitude is a common practice. In theory, if you had an accurate timepiece and could observe the exact instant that the sun crosses your meridian (when it reaches its highest point), you could then calculate your longitude by converting time to arc. In practice, it is very difficult to tell the exact instant when the sun reaches its highest point, so this method is impractical. You can get an approximation good enough for "lifeboat navigation" by the following procedure: 1. Record the altitude of the sun a few minutes before LAN and note the time. 2. Observe the maximum altitude of the sun (to get latitude). 3. Set your sextant to the altitude recorded in step 1, and note the time when the sun descends to the same altitude. 4. The time of LAN is then the average of the times noted in steps 1 and 3. From this you compute longitude. This assumes you didn't move very much between the times of steps 1 and 3. The more traditional way of determining longitude was to use a time sight at the time the sun crosses the Prime Vertical (i.e., the time at which the sun is due east or due west of you). This procedure is described in Bowditch and elsewhere. It requires that you know your latitude, which you can get from a noon sight or from an observation of Polaris. Chuck Taylor North of Seattle __________________________________ Do you Yahoo!? New and Improved Yahoo! Mail - 100MB free storage! http://promotions.yahoo.com/new_mail