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I think an important point is being lost, in this discussion of determining
lunar distance from a thin crescent Moon, by both Frank and Jim. It boils
down to Frank's sweeping-statement - "Celestial navigation back then was a
daytime activity."

If a navigator chooses to restrict himself to daytime lunars (with the Sun)
then yes, the dimness of the Moon crescent against an illuminated sky
presents a real challenge in perceiving exactly where its edge should be,
because of low contrast. However, this is just the situation for which
night-time star-lunars were intended.

If the crescent being observed is shortly after New Moon, all the observer
has to do is to wait until the Sun has set, and the sky background has
darkened, when the Moon outline will then show up, clear and sharp as ever
(in the case of a waning crescent, he must get up before dawn). And that's
the time when the horizon is visible for measuring altitudes. And it's also
the time for measuring a star-lunar, ideally choosing a star, near the
ecliptic, that's makes a good angle with the Moon, somewhere around 90
degrees.

For the rest of the month, Sun-lunars are available, and indeed preferred by
many observers. But for just those days when the Moon is a thin crescent,
star-lunars provide a way of shortening the dead-period, around New Moon,
when lunars are impossible.

Frank writes, in another sweeping-statement- "That's really all that was
necessary since dead reckoning for longitude was "good enough" for ten days
or more ...", regarding ten days without an observed longitude with
equanimity. I doubt whether any real navigator, approaching an unseen coast
after a week or more against contrary winds, under square rig, would share
Frank's confidence in his DR. He would, by then, be desperate to get a
longitude, and that's what a star-lunar could often provide.

George Huxtable.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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