NavList:

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From: Frank Reed <NoReply_FrankReed@fer3.com>
To: luabel@ymail.com
Sent: Friday, November 14, 2014 2:46 PM
Subject: [NavList] Re: Practical Use of Lunar Distances in the 18th Century.

Well, all of that will be covered in the class I am teaching this weekend at Mystic Seaport. You should fly over!

The short answer is that it was neither practical nor necessary to shoot lunars with the sort of ritual regularity that we associate with the "day's work" of nautical astronomy in the modern era. They shot lunars "occasionally", typically for a few days around first quarter and last quarter to check on the dead reckoning longitude in the earlier period, and to check on the single chronometer longitude in the later period. There's no specific date when this changed over, and it varied from one navigation culture to another. On American commercial vessels, I usually cite 1835 as a tipping point. Certainly before 1830, relatively few American commercial vessels were equipped with chronometers, but they all knew how to keep a good position "by account" or "dead reckoning" (synonyms for the same thing). And they trusted their DR contrary to what you'll read in most modern histories. A vessel could sail for a week or more and not be too far off in its longitude by account. Then shoot a lunar and see if there's much difference. If there's a substantial difference, well then you have a decision to make... After 1840 nearly all American ocean-going vessels were carrying chronometers, but usually only one and of unknown quality. Lunars were still used for a decade to double-check those mysterious blackboxes. After roughly 1850, lunars nearly disappear, and logbooks often reference multiple chronometers, which of course could serve as checks upon each other, rendering lunars superfluous. Lunars were taken even after 1850, but by then they drew attention precisely because they were such rare and unusual sights.

By the way, you asked if they used "normal astro" on most days. Yes, they did, and that consisted mostly of latitude by noon Sun and longitude by time sights. The sort of "astro" that you might recognize as such, with crossed lines of position, was considered "fancy navigation" by many navigators even through the early twentieth century.

As for Bligh's lunars, those would have been "independent" determinations of time. Many navigators also kept separate positions by account after shooting lunars. That is, if the dead reckoning differed substantially from a lunar longitude, they might then carry forward two positions by account. When in doubt, you can often tell which ones are "fresh" lunars by looking at the change from day to day. If the change in longitude matches the standard DR change to the minute, then it's probably a lunar brought forward via DR.

Here's a NavList message I posted in April of 2010 discussing some of this and including a map showing the dates and frequency of lunars aboard the Bounty: http://fer3.com/arc/m2.aspx/g12816.

-FER