# NavList:

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

**Re: Mary Blewitt vs M.J. Rantzen**

**From:**Gary LaPook

**Date:**2011 Mar 13, 16:09 -0700

Hewitt wrote:

Hi Paul -

�I believe Gary has personal experience with HO 218. I have volumes D (15̊ - 19̊ N&S) and F

(25̊ - 29̊ N&S).

The nice thing is they're compact - 6.5"X10"X.75". They're obvious shortcoming is each covers

only 5̊ of Latitude. Initial entry is for declination (whole number), with the pages for each

declination thumb-tabbed. The print is large and crisp. They're well made - thick cream-toned

paper, cloth bindings - so just as books-qua-books, they're lovely. A pleasure to handle.

The star treatment is really slick, though limited to 22 stars. I tried one for 2010 against my

StarPilot and did get a 5NM difference. But I was extrapolating the correction table which ends

for the year 2000.

Hewitt�

I have the entire set. A comparison with H.O. 249 shows that it provides the same level of

accuracy in very compact volumes. The computed altitudes include the refraction correction for a

5,000 foot flight altitude but, since the minimum computed altitude tabulated is 10̊, this doesn�t

cause a problem because for observed altitudes much greater than 10̊ the refraction is the same

at 5,000 feet as it is at sea level (to a one minute of arc accuracy.) Even at 10̊ the difference is

only one minute. An auxiliary correction table is included in the tables to allow for observations

taken at different flight altitudes. I have attached a copy of this table.

So that you can compare for yourselves, I have attached an excerpt from H.O. 218 for latitudes

30̊-34̊ and for declination 19̊ and also the pages from H.O. 249 that covers the same

coordinates.

I have highlighted examples for latitude 30̊, declination 19̊, both same and contrary names, and

hour angles of 15̊, 55̊ and 65̊. For the all the examples except hour angle of 65̊ and

declination 19̊ contrary name, the computed altitudes are exactly the same in both tables. For

that exceptional case, which has a computed altitude of about 10̊, you find what appears to be a

4 minute difference, but, in fact, after the proper refraction corrections are applied, the two

computed altitudes are the same. H.O. 249 gives the computed altitude as 10̊ 34' and H.O. 218

gives 10̊ 38' which includes the refraction correction at 5,000 feet. When refraction correction is

applied to Hc it is added instead of being subtracted from observed altitude. The auxiliary

correction table shows an additional refraction correction for sea level observations of 10̊ to be a

minus 1' to be applied to observed altitude. So to apply this additional correction to the tabulated

Hc in H.O. 218 we add this one minute making it 10̊ 39'. To directly compare the Hc in H.O.

249 to this we must also add the refraction correction from the nautical almanac, 5.3' rounded to

5', to the tabulated Hc of 10̊ 34' which ends up with the same Hc of 10̊ 39'.

I have also attached the refraction correction table from the Air Almanac so you can compare the

corrections for sea level and 5,000 feet.

gl

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