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Here's some longitude data from a whaling voyage in the late 1840s. This is 
from a logbook that I am still working on so I don't want give complete 
details on the ship involved at this time. See what you can make of these 
data points...

These are all the cases of longitude by lunars in this particular logbook. 
This is very late in the history of lunars so they are used only as an 
occasional check on the chronomter. "Lon C" indicates longitude by 
chronometer. "Lon L" is longitude by lunar (these are the way they were noted 
in the logbook incidentally). I include the latitude in case you want to 
convert longitude minutes to nautical miles. The vessel leaves Connecticut in 
late 1847. The first recorded lunar observations are taken after crossing the 
equator:

Dec 11, 1847: Lon C: 29.02.45W, Lon L: 29.28.45W, Lat:  5.24.47S
Dec 12, 1847: Lon C: 30.30.00W, Lon L: 31.11.00W, Lat:  7.28.37S

On December 15, 1847, the logbook says:
"Long by Lunar Observations 34.34.15W by it I regulate my Chronometer by 
adding 4'38" & ---- 3&2/3 seconds per day" (the word indicated by "----" is 
illegible but appears to be "losing" with "gaining" written over it).

Dec 31, 1847: Lon C:  49.36.30W, Lon L:  49.20.45W, Lat: 37.22.24S
Feb 26, 1848: Lon C:  88.40.15W, Lon L:  88.28.30W, Lat: 22.52.43S
Feb 27, 1848: Lon C:  89.35.45W, Lon L:  89.39.00W, Lat: 21.53.00S
Feb 28, 1848: Lon C:  90.49.15W, Lon L:  90.51.00W, Lat: 20.34.40S
Mar 25, 1848: Lon C: 144.05.30W, Lon L: 144.07.00W, Lat: 18.22.15N
Apr 23, 1848: Lon C: 149.04.15E, Lon L: 149.00.00E, Lat: 25.10.22N
Apr 25, 1848: Lon C: 148.25.15E, Lon L: 148.38.15E, Lat: 27.32.37N

From May through September they are whaling in the Sea of Okhotsk often in 
sight of land. The vessel's position is recorded nearly every day though 
sometimes the  positions are labeled "about". There is only one recorded 
longitude by lunar in this period:

Jul 24, 1848: Lon C: 152.18.15E, Lon L: 152.16.00E, Lat: 55.52.27N

After October 1 or so, they are bound for Hawaii.

Oct 19, 1848: Lon C: 165.59.11E, Lon L: 166.32.00E, Lat: 35.24.20N
Oct 31, 1848: Lon C: 154.40.45E, Lon L: 155.20.45E, Lat: 26.56.37N

From November 5, 1848 through about December 13 they are at Maui (spelled 
Mowee). One lunar on the voyage from Hawaii to Guam:

Jan  1, 1849: Lon C: 160.33.15E, Lon L: 160.54.00E, Lat: 13.12.10N

They spent most of January at Guam where they witnessed a major earthquake and 
a tsunami on January 24, 1849 noting that the "ground sank about 12 feet at 
the head of the bay". From February through early August they were whaling in 
the Sea of Japan frequently in sight of land. As in the Sea of Okhotsk, the 
position is noted daily but there is little doubt about the longitude since 
they see land often. After about August 12 they are "Homeward Bound" with a 
stop in Hawaii (Sep 30 - Oct 17).

Sep 10, 1849: Lon C: 163.01.30W, Lon L: 162.54.30W, Lat: 38.29.06N
Nov  5, 1849: Lon C: 158.27.45W, Lon L: 158.26.30W, Lat:  7.57.34S
Mar  7, 1850: Lon C:  68.15.00W, Lon L:  68.35.00W, Lat: 28.57.00N

Two weeks later they sight land and return home to Connecticut arriving in a "thick snow storm". 

The use of lunars in this voyage is typical of the late 1840s in my experience 
with logbooks. After this date, lunars rapidly disappear from the logbooks 
(though of course there were exceptions). By the way, I am not sure that the 
exact dates are correct (plus or minus one day is possible). I haven't yet 
determined how the "dateline" is handled in these logbook entries, and I 
don't recall whether he was using "sea days" in the reckoning.

-FER



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