NavList:

Backsights are hardly irrelevant Frank.  You can certainly use them in the face of land, but i prefer to use dip short of the horizon tables instead of backsights because they are far less limited, despite the issue of determining distance off land.

 

I have used backsights mostly for things like a rain shower or fog at the body's bearing which obscures the horizon. This is a modern and very useful application for backsights.  Of course you are limited to fairly high-altitude bodies due to the limits of the arc (more so for octants of course).

 

Yes Kermit, the body arcs in a negative direction (like a hill instead of a valley).  I tend to shoot "lower limb" as observed in the scope which yields an UL observation.  For correction from Hs to Ho first I create an Hs' by using 180-Hs.  Then you invert the sign of both IC and Dip to get a standard Ha.  Next a body correction and finally Ho.

 

I shot two during my trip last summer.  One was the moon and the other the sun.  Both came out well, but there was no need to do so.  I just did it as an exercise.

 

Jeremy 

 

Bill B., you wrote:
"If I recall, I have read a mention of backsights being used aboard naval
vessels when other vessels blocked the view they wanted. I do not
recall the specifics."

I can't recall hearing that specific story, but it's possible. The most obvious case where back sights would come in handy is in the early era of exploration when there were still real terrae incognitae (pardon my French) --basically before the 19th century. If you were sailing along the southern coast of some large island, either uncharted or poorly charted, in the South Pacific with the Noon Sun to the north of you, you had few options for getting latitude easily. In those cases, it might be simplest to use the back sight. During the 19th century, at least after about 1825, they were irrelevant since very nearly every latitude was known reasonably well except for the very smallest Pacific and Indian Ocean islands.

-FER

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