NavList:

Henry H, you wrote:

"it would appear that you both ignore the fact that one of the observations in the Lunar set is also intended for the purpose of establishing the Local Apparent Time"

 

Not ignored, Henry. I mentioned this a couple of days earlier in message # 1677. However, it should be said that it is not required that the other altitude be used for getting Local Apparent Time. In the early 19th century, it was quite common for LAT to be determined when convenient by a time sight of the Sun. This time could be carried on a watch, and watches sufficient for the task were very common.

 

So the daily observations might be something like this:

At noon, determine latitude by Noon Sun. Call out the moment, and reset all pocket watches to noon. This is a good way to get latitude, but an inferior way to get local time, at least for navigational purposes. Later in the afternoon, take a time sight for local time and correct the navigator's pocket watch or at least note the error of the watch at that time. Whenever convenient (but probably only a few times a month), in the afternoon, the middle of the night, the following morning, if the Moon was available you would do a lunar observation, derive Greenwich Apparent Time and compare with the time on the pocket watch, adjusted for the dead reckoning change in longitude. Sometimes, depending on the circumstances, the Sun's altitude taken during the lunar observation would be used to calculate LAT once more instead of using the watch and the DR change in longitude, but this was not essential.

 

So as far as altitude accuracy is concerned, the altitude of the "other body" should be accurate to +/-6' in most cases, less if it's very high overhead, more accurate if the lunar distance is very short. This is required for the lunar clearing process to make sure that errors larger than 0.1 minutes are not incurred. But if the other body is being used for local time, also, then the altitude should be as accurate as possible.