NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Gary LaPook
Date: 2011 Mar 14, 12:28 0700
H.O. 218 already has dealt with refraction by adding it in to its computed altitudes, This was done to help flight navigators since they would not have to then apply a refraction correction to their sextant altitudes. Bubble sextants normally do not have an index error and since a flight navigator doesn't have to apply any dip or semidiameter corrections, by applying the refraction correction (with reversed sign) to the computed altitude the flight navigator could compare his sextant altitude directly with the values printed in H.O. 218 to determine his intercepts. This obviously saves time (and in a plane traveling at several hundred knots this is very important) and also prevents errors. Look at whatever form you use to correct sextant altitudes to arrive at observed altitude and then erase all the lines except "Hs" and "Ho"! When using H.O. 218 and a bubble sextant, Ho = Hs. If you are using a marine sextant, the way you should use H.O. 218 is to simply not apply the refraction correction to your sextant altitude except when the altitude is less that 13° in which case you subtract just the one minute correction as shown in table IV of H.O. 218 from Hs to compute Ho. (Of course you also need to apply the other corrections, I.C., and dip.) This one minute correction only applies to low altitudes and is not applied to higher altitudes. But then, if you are shooting the sun or the moon, you will have to apply the semidiameter correction. If you are using the sextant altitude correction tables in the Nautical Almanac (which combine the refraction and semidiameter corrections) for your sun and moon shots, and you like them (I don't), then another way to use H.O. 218 is to subtract the "Stars and Planets" refraction correction from the tabulated values in H.O. 218. For low altitudes, requiring the additional one minute correction, subtract that one minute from Hs or add it to the tabulated values in H.O. 218 before subtracting the refraction correction. Run a few trials with made up sextant altitudes and you will see that you end up with the same intercepts as when using H.O. 249 and other computation methods (rounded to one minute accuracy.)  On Sun, 3/13/11, Gary LaPook <glapook.net> wrote:
