A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Joshua Hunley
Date: 2021 Mar 1, 09:02 -0800
While reading through old articles in The Journel of the Institute of Navigation I ran across a technique for calculating GHA Aries requiring only simple arithmetic and knowledge of its value at 0000UT on 31 Dec (alternately, 0000UT 1 Jan). I have not seen the method mentioned elsewhere but it may well have been. The method presented is:
"If one knows GHA ARIES for 000 GMT, 0 January of any year (31 December of the previous year), then the GHA ARIES for 000 GMT on any day of that year may be found simply by adding 1° per day of the year to this value, subtracting 1’ for each day, and adding 1’ per week. Adding 2.5’ per hour plus 15° per hour plus 15’ per minute will then give the desired GHA ARIES for any shot time without the necessity of recourse to an almanac. The resultihg answer has been within 1’ of the almanac value for all cases tried."
-- Survival Position location Using Star Sighting; EH Sharkey; Journal of The Institute of Navigation vol 6 iss 8 pp 516-525; 1957; doi 10.1002/j.2161-4296.1959.tb01979.x
Example using 01 Jan 0000UT as initial GHAA and 1 Jan = Day 0. If you use 31 Dec 0000UT of the year prior then one's count of days of the year would start with Jan 1 as Day 1. Either will work.
27 Feb 2021 1615UT = Day 57
GHAA 01 Jan 2021 0000UT -> 100°51'50"
57° - 57' -> 56° 3'
57/7' -> + 8' -> + 56°11'
= ~157° 3'
1615UT 16(2.5') -> + 40'
16(15°) -> + 240°
15(15') -> + 3°45
GHAA = ~41°28' ezAlmanac shows 41°27.6' for the same moment
The article itself is should be interesting to those who are interested in terrestrial navigation and field expediant devices. It describes the construction and alignment of a 30/60/90 triangle from fishing line to use as a sighting device to obtain one's position using Polaris and stellar transit times.