NavList:

We've discussed this before, but I would like to get back to one interesting issue.

You can get GMT and then longitude by lunar distances, as most NavList followers know, but as many navigators have discovered, both theoretically and practically, it's also possible to get GMT by meausuring the altitude of the Moon along with the altitude of one other body at about the same time and roughly the same altitude (since then any differences in refraction will mostly cancel out). Then if you plot LOPs (for a modern navigator), they will be consistent only if the GMT is correct. Or for a historical navigator, if you calculate local time from the star and then from the Moon's altitude, they will agree only if you started with the right GMT for the Moon. If they don't agree, you adjust until they do.

This "lunar altitude" method only works if the Moon's altitude is changing measurably as a result of the Moon's motion along its orbit. That means that the path of the Moon along the celestial sphere should be more or less vertical from the observer's point of view when the sights are taken. Observationally, this implies that the poles or "horns" of the Moon should be roughly horizontal at the time of observation. Acceptably horizontal is within 45 degrees of horizontal if we go by the same standards of acceptable rates of change implied by the lunar distance tables. Except in high latitudes, this happens more often than you might expect. The night before last, I saw the Moon rising in northwest LA (the other "LA" --Louisiana) with Castor and Pollux above it. Though it was just past full, it was clear that it was rising in a good orientation and there were plenty of stars to choose from.

Now back in the 19th century navigation manuals included lots of special methods that would only work sometimes, only when conditions were "just right" as with the longitude by lunar altitude conditions above. But this method never made it in. Why was that? Just navigational tradition? Actually, I think there's a bigger issue, something practical. Have a look at the data for the Moon available in any 19th century Nautical Almanac. What is included? What is missing, as compared with a modern Nautical Almanac? What sort of problems would this create for a navigator hoping to experiment with longitude (GMT) by lunar altitudes back in that period?

-FER

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