A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: John Brown
Date: 2010 Nov 9, 02:51 -0800
Frank Reed wrote:
"The Rude Star Finder is a good but primitive tool for the task.........It became a traditional component of "apex celestial navigation" (this is the name I use for the highly standardized, exquisitely optimized, and even ritualized form of celestial navigation that was practiced in the latter half of the 20th century, particularly after the year 1958, especially on American and British vessels."
Here is one recollection of the deep sea navigational routine on board a typical British merchant ship in the early 1960s. Verniers on sextants had given way to micrometers. Chronometers were still powered by springs. The common method of sight reduction involved a longhand solution of the cosine-haversine formulae using 5-figure logs. Towards the end of the decade these methods were cheerfully made redundant by inspection tables such as AP3270/HO249. Various short method tables had been available for some considerable time, but were not popular because of the relatively complicated rules for their use and marginal time savings in computation.
I joined Royal Mail Lines Limited, of London, as an indentured deck cadet in 1961, promising, amongst other things “not to frequent Taverns or Alehouses, unless upon (my Master’s) business”. The general cargo ships of that long defunct company traded between the UK, mainland Europe, the West Indies, South America and the Pacific coasts of the USA and Canada.
My first ship had a complement of three deck officers, who kept bridge watches and shared the navigational tasks. The 2nd Mate was responsible to the Master for passage planning and the upkeep of all charts and navigational publications carried on board. Sextants and nautical tables were the personal possessions of each officer and these were stowed on narrow shelves in a chartroom locker. Chronometers and plotting instruments were supplied by the ship, as were the charts, almanacs and sailing directions.
The day at sea, then as now, was split into six four-hour watches, commencing at midnight with the 2nd mate’s watch. He was relieved at 0400 by the Chief Officer (The Mate), who was in turn relieved by the 3rd Mate at 0800. Zone time was not formally kept on board, the clocks being advanced or retarded as necessary to locate lunchtime roughly in the middle of the day.
Taking over the watch at midnight, the OOW’s first task was to wind the chronometer, inverting it in its gimbals and giving the key precisely seven half turns. Lest this important task be forgotten, the message “wind chron” was often be found written in soap on the washbasin mirror in the 2nd Mate’s cabin. If the ship was on a great circle track, a new initial course to the destination might be calculated using ABC or Burdwood’s azimuth tables, with the latitude of the destination substituted for declination and the difference in longitude entered in the LHA column. On each night watch, an azimuth of a bright star or planet of fairly low altitude was obtained to determine the errors of the gyro, and the magnetic standard and steering compasses.
With the coming of dawn on the next watch the navigational tasks stepped up a gear. The OOW calculated the time of civil twilight and the LHA of Aries for that time, setting up 2102-D and selecting five or more moderate altitude stars, well distributed in azimuth. Star sights began as soon as horizon was distinct, with the sextant pre-set to the approximate altitude and aimed on the appropriate bearing relative to the ship’s head, or across a bridge wing gyro repeater. The sequence of sights began with fainter stars in the eastern sky and finished with the brighter stars towards the west. Each sight was timed, by a cadet, if available. He was alerted by the call of “standby” a few seconds before the instant of observation, which was marked by a second call of “stop”. If no timekeeper was available the OOW counted seconds on the short walk from bridge wing to chronometer. A few people were equipped with stopwatches. Sextant altitudes and chronometer times were noted in the navigator’s workbook and sight reduction began, using the Marc St Hilaire method with the cosine-haversine formula for calculated altitude and ABC tables for azimuth. All computation was carried out using the Nautical Almanac and five-figure logs from either Norie’s or Burton’s Nautical Tables. Until the advent of inspection tables the same DR longitude was used for each sight, with the resulting LOPs graphically advanced or retarded to a convenient common time, usually to the nearest hour and some convenient multiple of six minutes. A five star fix, including LOP plotting, generally took about half an hour. With the fix on the passage chart, a mercator sailing calculation provided the distance run and course made good since the previous evening’s fix.
At sunrise, an amplitude of the sun gave yet another check on the ship’s compasses. Next, when the sun had risen to a reasonable height, observations yielded position lines to be advanced to noon. For the rest of the day estimated positions and running fixes were worked up by calculation, using the traverse table. The intercept and azimuth from a morning sun sight was used to determine the “through” position, or the position closest to the DR through which the LOP passed. The noon DR or EP was then based on the run from this position. The difference between the observed latitude at noon and the DR or EP latitude was then multiplied by the “C” factor (cot az sec Lat) to find the “longitude correction”. The simple but somewhat arcane process is explained in Burton’s Notes to the Tables, sixth edition.
During the afternoon watch, further position lines were obtained from the sun and perhaps Venus or Jupiter, if available – rarely the moon. In the evening a star fix was obtained using the same process as in the morning, with a similar but reversed sequence of observations, commencing with the brighter and western stars.
On the homeward passage, in the SW approaches to the English Channel, we cadets manned the Kelvin deep sea sounding machine in search of the 100 fathom contour, marking the edge of the continental shelf with its concentrations of fishing vessels and the prospect of heavier traffic. Once a sounding of 100 fathoms had been found and reported to the bridge, the watches were doubled up, and coastal navigation began in an atmosphere of pleasant homecoming anticipation, known to generations of British seamen as “the channels”.
How very far away it all seems now.
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