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As the Immortal Bard said:

"What's in a name--a rose by any other name would smell as sweet."


The terminology used for navigation suffers the same fate as others in 
English of having meanings shift with time and some words becoming the 
general description for a whole group of objects. For example, the brand 
name for a particular copying machine, Zerox, has come to be applied to 
all copying machines no matter who makes them. "To Zerox" also became 
the generally accepted verb for photo-copying.

There is a distinction between sextants, octants and quadrants. Because 
of the double reflection operating principle of the marine instruments 
it is possible to measure an angle of  90�, one quarter of a circle, 
with an instrument having a calibrated arc of only one-half that, 45�, 
one-eight of a circle, an instrument called an octant.
 If the arc is one-sixth of a circle, a sextant, you can measure angles 
up to 120� and if the arc is one-quarter of a circle then you can 
measure an angle of half of a circle, 180�, and you then have an 
instrument called a quadrant.

Quadrants must be made larger than sextants which are larger than 
octants. An octant is sufficient to measure any angle from horizontal to 
straight up so can be use for normal celestial navigation and this is 
how they are used in aircraft. Sextants and quadrants allow a greater 
range of measurement which might be useful for lunar distance 
measurements, horizontal sextant angles for coast wise navigation and in 
the rare case of a body within 30� of the zenith with an obstructed 
horizon below it but a clear horizon in the opposite direction in which 
case the navigator could turn his back to the star and use the opposing 
horizon for the sight. (I don't know if this was ever actually done in 
real life.) Since these types of sights are never taken from an aircraft 
it is never necessary to have more than an octant on an aircraft. My 
tamaya sextant actually measures up to 125� and my SNO-T goes all the 
way to 140� but these are both sextants.

Aviation instruments have confusing nomenclatures.
The Pioneer/Bendix A-7 identification plate calls it an "octant" and it 
goes only to 90�.
However, even though the A-10 also goes only to 90� its name plate calls 
it a "sextant." The same is true of the Kollsman instruments, the 
periscopic and the hand held models, the MA-1 and the MA-2 are all 
labeled as "sextants" even though they only read to 90� and are actually 
"octants." So at this point in time "sextant" has shifted its meaning to 
refer to all navigational instruments used to measure the altitudes of 
celestial bodies for navigation even if they are actually "octants."

Notice, octant does not refer to just bubble instruments since there are 
marine octants also.

In the early days of airborne celestial navigation many attempts were 
made to use marine instruments but there are severe limitations on the 
use of these instruments in the air. Then many attempts were made to add 
a bubble artificial horizon to the marine sextant and these usually were 
devices that replaced the normal sextant telescope so they were easily 
removed and the telescope remounted. There were very expensive devices 
and still are, $950.00 in the latest Celestaire catalog. Then various 
instruments were invented that incorporated the bubble (and sometimes a 
pendulous mirror) to use as the horizontal reference and some of these 
instrument also allowed the use of the natural horizon such as the A-7. 
When high speed aircraft came along there was too much of a drag penalty 
from the astrodome so the periscopic sextant (actually an octant) was 
invented consisting of a 1 1/2 inch diameter tube that extends through a 
hole in the top of the fuselage about two inches to allow the navigator 
to take sights from inside the usually pressurized cockpit.


gl
 

The Pioneer


Jackie Ferrari wrote:
> Dear List,
>  
> I understand that sextants as opposed to bubble octants were used to 
> navigate aeroplanes. I was wondering therefore how prevalent this was 
> and what were the advantages and disadvantages.
>  
> Regards,
> Jackie Ferrari.
>
> >


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