NavList:

Ken M wrote:

"In fact this was the preferred method for land-based lunars. Take the
lunar distance and then take a time sight using a star in the East or West
rather than using the altitudes of the moon and other body in lieu of the
time sight. Whether this preference was due to the fact that an artificial
horizon limits you to altitudes below 60° or that you get a better time
sight using a rising or setting star I just don't know (or perhaps there
is some other factor that I haven't thought of)."

 

In all, to analyze a lunar, an observer working alone might want to shoot as many as six altitudes in addition to the lunar distance itself. Two of them would be time sights for determining local time (I say "two" because a careful observer might want to rate his watch and may thus want to do a time sight before the actual lunar observations and another after but if you have a decent watch or the time sight is reasonably close to the lunar observation, one is sufficient). Also we need the altitudes of the Moon and the other body used in the lunar observation in order to clear it. If I'm working alone and can't get the altitudes simultaneous with the lunar distance, I'll need to shoot an altitude of each object before the lunar distance(s) and and I'll need to shoot the altitudes after the distance(s). Of course, if the other body is reasonably far away from the meridian, we can use its altitude for the time sight observations so that could reduce the number of altitudes required to four. And if I have an assistant who can shoot the altitudes simultaneous with my distance, I can reduce the number of altitudes required to two (per distance). On the other hand, if I skip measuring the altitudes at the time of the lunar and calculate those altitudes based on my DR position, I still have to measure at least one altitude to get local time. So, let's see then, the minimum number of altitudes I have to shoot is one --and not necessarily while I'm shooting the lunar, possibly hours earlier or later-- and a "prudent" maximum number could be as high six (ignoring backups, double-check sights, etc.).

 

The preference for calculating the altitudes of the Moon and other body at the time of the lunar observation seems to have been largely a matter of subjective personal opinion and experience. But there is also an objective question of efficiency. When labor is cheap and rapid updating of the longitude is important as was the case aboard many old sailing vessels, it made sense to shoot the altitudes. Then the longitude could be worked up in twenty minutes. When labor was expensive and the longitude was not required immediately as in land survey, mapping, and exploration, it made more sense to calculate the altitudes. But it might take an hour to get the final longitude.