A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2017 Apr 2, 04:54 -0700
Mike you wrote:
Just learning and wondering if anyone has the Mary Blewitt book and would care to pose a question for a single LOP based on any HB. Question would need to consider I only have Mary Blewitt for reference in terms of selected page from Almanac etc. I will give an answer in Zn & Intercept in due course.
Mike. The reason you’re getting ancillary advice without anyone attempting to back-work a question for you to solve is twofold. Firstly the corrections involved depend so much upon the circumstances. E.g. for a marine sextant using a sea horizon – allow for dip & probably observe an upper or lower limb. For a marine sextant using an artificial horizon – no dip correction is necessary and only observe a limb if you can be sure you know if it’s upper or lower. For a bubble sextant – it’s probably best to observe the middle of the body. For a pendulous reference sextant – it depends upon the design of the graticule. For a Smith’s London Underground sign – a centre observation is probably best. For a Kollsman cross – a limb observation might give better results. Then, if you’ve got cloudless skies for hours on end you and no other distractions likely, you could observe only on the hour or an exact ten minutes and get GHA without further increments required. If you’re plagued with identifying only the moon through occasional and unpredictable gaps in the cloud then you’re in for a much longer calculation. Authors try to cover every possibility and heavenly body, so you end up with extremely complicated prompting tables. In reality, you can often strike out a lot of it, but it does make setting worked examples for beginners difficult and time consuming.
Secondly, the tables in the rear of Mary Blewitt cover only the pages the reader needs to follow the examples in taht edition of the text. E.g. GHA for only 6 days in a single year. Pages differ between editions. Also increments for time adjustments are given for only 56 and 57 minutes past the hour (Appendix C). Furthermore, only one page is given from AP3456 Vol 1 (Appendix G), and for some reason, at least in my 12th edition, the names of the stars it gives Hc and Zn for have been left out.
Considering the differences between using the Nautical and the Air Almanac, in the air you’re usually stuck with a bubble or pendulous reference sextant and are travelling much faster, which means you have to allow for Coriolis and aircraft accelerations and possibly rhumb line steering. The net result is that if you’re consistently getting LOPs within 3nm of your real position, you’re either very good or very lucky. Therefore, why worry about the odd point one or two of a minute of arc simplification in the Air Almanac if using it enables you to get a fix on your chart minutes of time sooner when you could be racing towards danger at a rate of eight miles a minute.
The best advice is ‘ “Why”, said the Dado, “the best way to explain it is to do it” – Alice in Wonderland’ (Dickie Richardson AP1234, ChVII, 1941Edn). Don’t try and cover absolutely everything. Look to your own anticipated situation and set up a question for yourself or find a book or paper on the web which gives someone’s dates, times, observed altitudes, and DR positions and check it out for yourself. There are a number of places on the web where you can get GHA and declination going back hundreds of years e.g. This site/Tools/Nautical Almanac Data and other sites and programmes which will do the calculation for you as a check (I use ‘Navigator’ but there are many others). Hope this helps. DaveP