NavList:

Frank, you asked: "What did they call index error back in 1824?"

My oldest textbook of navigation is, by coincidence, from that year: "Navigation, eller en sjömans dageliga handbok uti styrmanskonsten" by Fabian Casimir von Roswall, printed in Stockholm 1824. On page 103 Roswall describes "indexfelet" (the index error) and explains how to handle it. You measure the sun's diameter both ways, the mean of the two readings is the correct zero point, half their difference is the error; which, if the on-arc-diameter is the largest is to be subtracted, but if the largest reading is off the arc, then it should be added to the read altitude.

Around onehundred years later, Alex Thore in his "Lärobok i navigation", Stockholm 1926, tells us that the easiest way to determine "indexfelet" (the index error) is to measure the horizon or a star upon itself. The reading indicates the index error, which is to be added to the read value when the zero point of the vernier is to the right of the arc's zero (i.e. if the reading of the index error is off the arc), but is to be subtracted in the opposite case. "This rule is obvious, when you consider that in the former case you've read too small an arc, in the latter case too large an arc".

Close to mid way between these years, Ekelund and Ljunggren in their "Lärobok i navigations-vetenskapen", Helsinki 1876, write about "indexfelet". But they continue "The magnitude of the error which arises due to lacking parallelism of the mirrors, when the index bar is at zero, is called "indexkorrektionen" (the index correction). It is twice as large as the angle between the mirrors and as it is easier to determine it than to nullify it, and as it often changes, it is usual to include it in the calculations rather than adjust the mirrors." Furthermore, "To find "indexkorrektionen" you do as follows: ... the number of minutes and seconds that this arc contains is the index correction, which becomes positive or negative depending on whether the vernier's zero is to the right or left of the zero of the index bar."

80 years later, Bolling and Holm in "Lärobok i navigation", printed 1956, write "... This is not usually the case, thus the read angle has to be adjusted with a correction for the index error, the index correction. ... If the real zero of the index bar is to the left of the arc's zero point, the read angle will be greater than the true angle and the index correction gets a minus sign."

Above examples show that some authors did make a distinction between error and correction, some did not. But all examples provide instructions on how to handle the issue with non-parallel mirrors. And the use of the word index correction can be traced at least back to 1876.

Lars