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Hi Jack,

In TM-1-206 Technical Manual Celestial Air Navigation, March 4, 1941
chapter 56 page 114 sub. d "Limitations of marine type" says:

"The marine type sextant is not suitable for aircraft use except on
those rare occasions when the airplane is flying over the sea at a
very low altitude (less than 1,000 feet). Since the instrument employs
the sea horizon as a reference line, it is of no practical value when
the aircraft is flying over land, over clouds or haze, or at high
altitudes when sky and sea tend to blend into each other and do not
form a clear-cut horizon  line. Even when the sea is clearly visible
from relatively high altitudes, the altimeter does not indicate height
of the airplane with sufficient reliability to determine height of eye
(dip) correction accurately. Furthermore, the marine type sextant
cannot be used at night when the natural horizon is not visible. Some
types of marine sextants are equipped with a bubble leveling
attachment and light, thus enabling a heavenly body to be observed day
or night regardless of whether the natural horizon is visible."

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