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Re: Longitude sight, South Pacific, 1864
From: Ed Popko
Date: 2018 May 9, 04:39 -0700

A very nice example. I have never disassembled one of these, Frank has already done it.  I have tried to annotate the lines I think I understand  marking those I don't or can't read with a question marks.

Below the listing are my speculations as to what this ship might have been doing there.

Bowditch 19th Century Logarithm Method

General

1              Dec of Sun    S 16º44’32” might be Nautical Almanac hourly value

2              ?First corr                 6’        seems too much for 17 minute increment past the hour

3              ?Exxx xxx          16º 50’32”  sum lines 1 and 2

4              ? Corr xxx                   4’

5              Second corr    16º 54’32”  sum lines 3 and 4

7              Polar Distance 73º 05’28” = 90º - 16º 54’32”

8              ? Sun’s altitude rounded  16º 34’  Lower limb

9              ? altitude correction, possibly dip combined with lower limb  10’

10           Altitude with corrections applied  16º 44’

11           Latitude rounded 24º 03  (from DR or LAN)

12           Polar Distance rounded down 73º 05’

13           Sum of Altitude, latitude and polar distance   113º 52’ = 16º 44’ + 24º 03’ + 73º 05’

(Sum of lines 10, 11 and 12)

14           Half Sum of  line 13       56º 56’ = 113º 52’ / 2

15           Altitude from line 10     16º 44’

16           Remainder = Half Sum – Altitude       40º 12’ = 56º 56’ – 16º 44’

Local Apparent Time to Local Mean Time

17           Local Apparent Time  05:15:18  =  78.8250º =  2 * 39.4125º   line 25

18           Equation of Time         00:16:04  (fast)

19           Local Mean Time         04:59:14  =  05:15:18 – 00:16:04

20           Logsec(lat)     line 11             03944 = logsec(24º 03’)

21           Logcsc(pDist) line 12             01921 = logcsc(73º 05’)

22           Log Half Sum line 14          9,73689

23           Log Remainder  line 16     9,80987

24           Log total 9,80270  =  logsec(lat) + logcsc(pDist) + logcos(Half Sum) + logsin(Remainder)

(lines 20 21, 22 23)

25           Logsin(LHA/2)   line 24      9,80270   LHA/2 =   39.4125º

Time corrections and conversion to longitude

26           Chronometer time Nautical Day                      01:01:00

27           Chronometer rate compensation                    00:16:24 slow

28           Chronometer corrected time (lines 26 + 27)  01:17:24

29           Correction of 12hr for Civil Day                      +12:00:00 hrs

30           Chronometer time Civil Day GMT                    13:17:24

31           Local Mean Time  line 19                                   04:59:14

32           Meridian Angle, ‘t’ or longitude as time  08:18:10  = GMT  (line30) – LMT (line 19 or 31)

33           Conversion 8 hrs to angle        120º = 08hr * 15

34           Conversion 18 min to angle     04’30” = 18min/60*15

35           Conversion 10 seconds time to angle  02’30”

36           Longitude      124º 32’30” = 120º00’00”  +  0º04’30” + 0º02’30”    sum of lines 33 to 35

Guano Expedition

This ship is located just south of the Tropic of Capricorn. Perhaps they were going to or returning from Ducie Island, part of the Pitcairn Islands. A U.S. government expeditionary ship? Commercial guano miners? In 1867, Ducie Island it was claimed by the United States under the Guano Islands Act. Although Captain Edward Edwards discovered the atoll in 1791, Ducie was not considered a British possession.[Wikipedia] In 1867 Ducie was claimed by the United States under the Guano Islands Act, which established that an uninhabited territory with guano deposits could be claimed as a US possession, so long as it was unclaimed by any other country.

Whaling

Possible this ship was hunting whales though it’s on the southern most boarder of the “Offshore Grounds”. Below is Matthew Fontaine Maury’s famous 1851 Whale Chart (Source: Washington, D.C., Naval Observatory, 1851. Color lithograph. Geography and Map Division).

Ed Popko

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