# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: H.O. 218 Vs. H.O. 249
From: Gary LaPook
Date: 2015 Nov 8, 01:21 +0000
Yep, flight navigation practice is to do the computation prior to taking the sight so refraction correction is added to the Hc instead of being subtracted from Hs, you get the same intercept either way.

gl

ing the From: Stan K <NoReply_StanK@fer3.com>
To: garylapook---.net
Sent: Saturday, November 7, 2015 3:02 PM
Subject: [NavList] Re: H.O. 218 Vs. H.O. 249

Ah ha!  The difference is because the computed altitudes include the refraction correction (for a 5,000 foot flight altitude, but virtually the same as at sea level), according to Gary LaPook.  Thanks, Gary.  Nice having the archives.

Stan

-----Original Message-----
From: slk1000 <slk1000---.com>
To: navlist <navlist@fer3.com>
Cc: slk1000 <slk1000---.com>
Sent: Sat, Nov 7, 2015 5:10 pm
Subject: H.O. 218 vs. H.O. 249

A short time ago I mentioned that I had a copy of Volume J of H.O. 218, but I had not done anything with it yet.  Well,  I started looking at it, and I am confused.

218 is similar to 249.  I am working with Part 2 of 218, which corresponds to Volumes 2 and 3 of 249, the ones that do not refer to particular stars.  I am finding what I consider significant differences between 218 and 249.  For example:

Declination 1ºN
Latitude 40ºN
LHA 76º

For 249 and any method other than 218 I have tried, this gives an altitude of 11º20'.  218 gives 11º24'.  The values of d and azimuth agree, it is just the altitude that is off.  Most other sets of inputs I have tried disagree by one to four minutes of altitude, with an occasional one agreeing.

The volume I have is dated 1941.  Is it possible that this edition has issues but later editions have different numbers?  Or do all editions have issues?  Or do I have issues?

Stan

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