NavList:

I took the data provided by Wolfgang (112572.wales 4 august 1777.pdf)and utilized it directly with Frank's online Lunar calculator.

To Re-Iterate the data from the log, we find
Date 4 Aug 1773
TIME 01-18-59
Lunar Distance (Sun) 155 degrees 13 minutes 7 seconds
Sun's Alt LL 5 degrees 41 3/4 minutes
Moon's Alt LL 10 degrees 31 7/8 minutes
DR Longitude 227 degrees 40 minutes 30 seconds
DR Latitude S20 degrees 49 minutes

As the alitudes are given as lower limb, we should add 15.8 minutes to the sun's altitude and 15 minutes (?? don't know if this is correct) to the moon's altitude. This yields Sun's alt 5 degrees 57.5 minutes and the Moon's alt 10 degrees 47.8 minutes

Firstly, I injected all terms directly from the logbook into the online Lunar calculator. The response was that there was more than 2 degrees of error in the lunar distance and about 194 degrees of error in longitude. That didn't seem reasonable, so I checked the "predicted" lunar distances for the day.

Aha! The distance, at DR, was 155 degrees 39 minutes at 15-00 and was 155 degrees at 16-00.

Conclusion #1: The time recorded in the log may be the time on the face of the watch. It may be UNCORRECTED!

Next, I manipulated the time injected into the clearing of a lunar online, until the sun's altitude was spot on at 5 degrees 57.5 minutes. Why? Because the motion of the sun would have been know better than that of the moon. Using a time of 15-48-12, the sun's altitude is correct and the moon's altitude is given as 10 degrees 40.2 minutes (about 7 minutes of error in altitude). The online calculator then states that the lunar distance is in error by less than 0.1 minutes and the error in longitude to be 1.6 minutes.

Conclusion #2: Back to the time piece, it showed 01-18-59 while we can calculate 15-48-12. Wow, that's a chronometer error of 14-29-13!!

Question #1: Is it fair to assume that the motion of the moon wasn't well enough understood in 1773 to be able to calculate moon altitudes? 7 minutes seems like a large amount, although that is dependent on my estimate of 15 minutes semi diameter for date.

Question #2: Is the chronometer error that far off or is it a matter of the astronomical day vs the sea day?

Conclusion #3: After a quick look at the USNO site, the moon was full on 2 August at 16-26 GMT. As the date in the log is 4 August, the moon is indeed very near to full, no matter how we account for the astronomical vs sea vs civil day. Most interesting then is that the sun would set (due to phase) before the moon would rise, preventing this from being an afternoon observation. The moon is about to set and the sun has risen, making this an early morning observation. At least that is the way I think about it at this time!

Question #4: In the first column of the log, the navigator is giving the astrological "signs" and planet "signs". What do these mean?

Best Regards
Brad

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