NavList:

Brad, you wrote:
"I think I may have the answer to the index error for backsights. Consider NAV1268 at the National Maritime Museum in Greenwich. It is a backsight Octant. As is typical of Octants, it is a vernier type, with the arc from -2 degrees to +101 degrees. Due to the length of the vernier itself, however, the octant can only measure to 95 degrees. The measurement beyond 90 degrees is the key. Using a FORESIGHT, measure the altitude of a star whose apparent altitude is greater than 85 degrees. Why greater than 85 degrees? There is a doubled region between 85 degrees and 95 degrees, in which we can measure the altitude of a star with EITHER a foresight or a backsight observation. Since it is possible with either method, we must perform the observation with BOTH methods. In knowing what the altitude is with a foresight observation, we therefore know what the vernier must read for a backsight observation, given the same star. Set the octant's vernier to the arc for a backsight observation and adjust the backsight horizon mirror until the altitude is correct. "

That would work, but it wouldn't be easy, and good luck getting a star in the right place! If you haven't already said so in a post I've missed, this problem of getting the index correction for the back sight is actually identical to the problem of finding the arc error at some odd spot on the arc of a normal sextant. And therefore any of the solutions that we've discussed would also work. So, for example, you could do a star-star sight for any angle in range for the back sight. Or, if you have three "lighthouses" in a row (far enough out on the horizon so that they fall on a great circle), let's call them A, B, and C, and you can measure the angles less than 90 between A and B and then the angle between B and C. Then of course the large angle between A and C is known. So you measure it as a back sight and any error is the index correction.

-FER

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