A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Hewitt Schlereth
Date: 2011 Mar 14, 21:35 -0400
What you've done is point this out - and eliminated several opportunities for an arithmetic blunder. Thank you.
So now, what a sea-level marine navigator should do when using 218 is ignore the refraction correction altogether, except when working between 10° and 15°. Then he is to apply -1 to Ho.
No, you have that exactly wrong.
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 semi-diameter 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 semi-diameter correction.
If you are using the sextant altitude correction tables in the Nautical Almanac (which combine the refraction and semi-diameter 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 Mon, 3/14/11, Hewitt Schlereth <hhew36---.com> wrote:
From: Hewitt Schlereth <hhew36---.com>
Subject: [NavList] Re: Mary Blewitt vs M.J. Rantzen CORRECTION to correction
Date: Monday, March 14, 2011, 7:29 AMGary, to make this clear to myself: To any 218 Hc, I should:
1) Add the NA refraction value for that Hc, (this means reversing the sign of the NA value); then:
2) Apply the value for "Height in feet 0" as printed (i.e., -1') in Table VI in the back of 218 - Adjustment to Refraction?
Further, looking down the NA refraction table, it appears that for 218 altitudes above 45° 34' the net of these two steps goes from positive to negative for surface navigators.
Example: For altitudes between 60° 26' and 65° 06', the NA refraction is -0.5'. Reversing the sign makes it +0.5'. Applying 218 Table VI value of -1' would give a net correction of -0.5'.
Have I got this right?
HewittOn Mon, Mar 14, 2011 at 1:37 AM, Gary LaPook <glapook---.net> wrote:
Never mind. I sent the last message without access to the tables and I had forgotten that there was the expected differences between H.O. 249 and H.O. 218 tabulated Hcs.
I did a more thorough comparison using H.O. 214 to look for the effect of rounding in both H.O. 218 and 249. Using all of the combinations for latitude 34 and declination of 19, I added the refraction correction from the Nautical Almanac to the values from H.O. 214 (which which are tabulated to one-tenth of a minute of arc) and found for all cases with Hc greater than 13 that they then totaled to the Hcs tabulated in H.O. 218 as expected. For Hcs of 10 to 13 adding the additional one minute of arc as required in the auxiliary table produced the correct value.
So it appears that H.O. 218, when the refraction correction is applied properly, produces the same computed altitudes as H.O. 249 which are also correct as rounded from H.O. 214.
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