NavList:

Glenn, you wrote:
"Using lunar distance to get GMT, how is longitude determined from this information?"

The very short answer is "exactly the same way you would get longitude if you took GMT from a chronometer".

To elaborate, if you know the time in Greenwich, then all you need is the local time in your longitude. This local time is just Sun time (corrected using a simple table called the "Equation of Time" to switch from apparent time to mean time). Basically, we need a sundial. Sundials read local time. If it's 2300 in Greenwich and 1700 in our longitude (sundial time, corrected) then we must be six hours behind Greenwich so that's 90 degrees west. Now, of course, sundials don't work at sea. They have to be level and they have to be aligned north-south and we can't count on either of those at sea so instead of using a sundial, we turn our sextant into a sundial by taking a time sight. This is simply an altitude of the Sun measured when it bears more or less east or west. By a short logarithmic calculation (or a few keystrokes on a calculator or a few entries in a spreadsheet), you convert that altitude into local time. Note that this has nothing to do with "lunars". No matter how you get GMT --by lunar distances, by chronometer, by speaking another ship, by hearing it broadcast "over the wireless"-- you need some sort of time sight to get the local time. Also, this local time did not have to be acquired at the very instant that GMT was determined. One could get local time in the morning, set a watch to it, and then compare with GMT later in the day, but then you have to correct for any motion of the ship east or west in the interim.

By the way, there are other sights that will also yield the local time. This "sextant into sundial" time sight was by far the most common means of getting local time historically. One final wrinkle: in a lunar observation, it was common to measure the altitudes of both bodies to "clear" the lunar. If the geometry was convenient, you could use the altitude of the other body (the Sun typically) to work the time sight, but this was neither mandatory nor expected. It was just a convenience.

-FER

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