A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bruce J. Pennino
Date: 2014 Jun 7, 06:04 -0700
After reading John Letcher's book, Self Contained Celestial Navigation with H.O.208, Chapter 12 , I collected the data below to reset my watch from a known position. This problem requires calculation of the time increment to reset an incorrect watch. I find that this type of problem forces me to viualize the "basics". The time sight is used to calculate LHA for an Ho, latitude, and declination. When longitude is determined, it is found to be incorrect, compared to the known value, because it is based on the wrong time. From the difference of the longitudes, a time increment is calculated and the watch can be reset. Time conversion is known because the sun "moves" at 15 degrees per hour. The time sight equation for LHA is Cos LHA = (sin Ho - sinLat x sin Dec)/ ( cos Lat x cos Dec). I hope others find this interesting. After a week or so I'll provide the correct answer and my calculated answer. The data is not flawless.
Reset Clock Problem (with a few wrinkles)
Your true and correct location N 41 degr 55.1 ' , W 70 degr 01.8', Indian Neck Beach , Wellfleet, MA, USA. You are facing west toward Great Island. Dip short, use 1.5 miles to horizon. When your watch is working correctly on May 3, 2014 in the early evening, you sight the MOON LL setting about due west . Index correction is off arc 1.3 minutes (add 1.3'). The average of 2 moon sights LL gives HS 36 d 31.8 ' , May 4 (fourth) @ UT 00:13:54. Again dip short, eye height 12 ft. Really don't need this information, but these results give a sense of accuracy with a correct time piece.
On the evening of May 4, 2014, as the sun is about to set, I noticed my watch had stopped. I restarted the watch and guessed the time based on the soon arriving sunset. I know the watch error is less than 30-45 minutes of time. With the same Astra and IC, at Indian Neck Beach facing west to Great Island (dip short), same eye height, the following data was recorded at INCORRECT (I) UT times, on the setting SUN LL. IUT 22:31:00, Hs 8d 12.6' ; IUT 22:32:09 , 7 d 59.8' ; IUT 22:33: 26 Hs 7d 46.6 ' ; IUT 22:34:44, 7 d 32.4 ' . All things considered, what is the watch correction in minutes and seconds, added or subtracted ?
Just to repeat, first data on moon LL, second set of data on sun LL, dip short.