A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Henry Halboth
Date: 2009 Apr 16, 20:15 -0700
I previously raised the possibility of limitations or restrictions to the use of Equal Altitudes for the purpose of obtaining the Lat/Long at Noon, and have several times been quoted to this effect in subsequent Posts. I hasten to emphasize that these limitations and restrictions are essentially quotations from the noted sources, i.e., Bowditch (1909), Raper (1909) and Lecky; they do not originate with me and seem to primarily concern the slowness of high Latitude altitude change as well as a larger error introduced by an error of only 1 min (arc) in the observed altitude. These limitations or restrictions do not necessarily negate use of the method to obtain an approximate position – it just depends on the degree of approximation the user may find acceptable, as well as perhaps the expertness of the observer.
Bowditch (1909) states ...”the interval form the meridian being not greater than 10 min (time) and the altitude not less than 75 degrees.” ...and, “... if observations be taken when the body bears not less than 80 degrees from the meridian, the time of meridian passage may with accuracy be regarded as equal to the mean of the times of observation, no matter what course may have been steered by the vessel in the interval.”
Lecky (22nd Ed) states ... “even in moderately high Latitudes the change in altitude near noon is very slow. Inversely, an error of only 1’ in the altitude means a large error in the time of Longitude.” Although Lecky obviously does not favor this method utilizing the Sun, he speaks in favor of its use with Stars, and provides a clear rendition of Raper’s approximate correction for change in Latitude between observations.
My use of Equal Altitudes for determination of Lat/Long at noon and Chronometer Error, both at sea and ashore, has been primarily between Latitudes 40 N and S, as well as in the tropics, with no practical high Latitude experience. I have always found it to be reasonably accurate. even when corrected for a difference of Latitude between sights. Much depends, of course, on the quality of the sea horizon available and the observer’s reaction time in accurately calling the time of the descending limb contact – this being extremely important to the success of the method, no hesitation or uncertainty in recognizing and acting on this later contact is allowable.
At my stage of the end game, I certainly have no objection or criticism regarding the reinvention of navigation, yet must state a degree of perplexity at the effort being expended in making what was an historically simple procedure into the complexity of analysis here seemingly evidenced and elevating it to an all encompassing system of Celestial Navigation.
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