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We occasionally see celestial observations posted here, including the
sextant reading and the index correction. While I think it's good
practice to show all your numbers, that may be going too far since I
never know if person understands the difference between "error" and
"correction".

Some years ago on these pages I pointed out to someone that he had the
terms mixed up. He replied that other people do the same, and seemed
satisfied with his understanding.

A helpful mnemonic is to remember the goal is to remove error from your
observations. Remove = subtract. Therefore, error is subtracted. A
correction is added. For instance, the Almanac altitude correction
tables have negative numbers for dip and star / planet refraction since
the corrections make altitude smaller.

I adjust my sextant so the index error check always produces an angle
"on the arc." Hence, index correction is always negative. It's one less
opportunity to blunder. Also, if the IE check is sometimes on the arc,
sometimes off, you have to read the vernier backward in the latter case
(if you use the vernier — I don't). E.g., at a micrometer setting of
-0.2′, the 0.8′ vernier graduation is in coincidence. This isn't
difficult — you read the micrometer drum backward too — but a mental
lapse is always possible.

--
Paul Hirose
sofajpl.com

   

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