NavList:

From: Gary LaPook <glapook@pacbell.net>
Subject: [NavList] A new way to apply sextant corrections
To: "Navlist" <Navlist@fer3.com>
Date: Monday, January 24, 2011, 6:14 PM

I have found it difficult to subtract a series of numbers in a column, it is much easier (and less prone to error) to add a series of numbers. When using the Nautical Almanac, the normal way to correct the sextant altitude (Hs) to obtain observed altitude (Ho) involves both adding and subtracting various correction factors. The index correction (IC) can be either added or subtracted and the same for semidiameter (S.D.) and temperature and pressure (T&P). Dip, refraction, and the 30 minute additional factor for upper limb of the moon are always subtracted and parallax is always added. Normal procedure for flight navigation is to pre-compute the sights and reverse the signs of these corrections and apply them to Hc (computed altitude) to obtain Hp (precomputed altitude) so that no time is lost in figuring the intercept. If you give it some thought you will see that applying sextant corrections, with reversed signs, to Hc will result in the same intercept as when making the corrections in the conventional manner. Using this equivalence, I decided to make a correction form that involves only addition, no subtraction. If the sign of the correction is plus I add it to Hs. If the sign is minus I add it to Hc. I have attached a PDF file with this form. At the bottom you copy the smaller of Ho or Hc beneath the opposite column and then subtract to find intercept. Give it a try. gl