# NavList:

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

**Re: Almanacs, theory and use.**

**From:**Gary LaPook

**Date:**2007 Nov 20, 11:31 -0800

Gary LaPook writes: Attached is a link to site that will print out a version of the Nautical Almanac daily pages. For times between the tabulated values to find GHA you use straight line interpolation, 15� 00' per hour for the sun and planets, 14� 19' for the Moon and 15� 02.5' for Aires. The "v" and "d" corrections at the bottom of the columns for the planets, the top of the column for the moon and the "d" correction only for the sun are also used for straight line interpolation for the declinations of these bodies and for their GHA motion in excess of the 15� 00' standard value for the planets. ( No "v" correction for the sun.) the actual Nautical Almanac has interpolation tables included for every second of time but these are not in this online almanac but are easy to compute with a calculator. Also attached is a link to an online version of The American Practical Navigator refrence work, go to chapter 19. http://www.tecepe.com.br/scripts/AlmanacPagesISAPI.isa http://www.i-DEADLINK-com/bowditch/ On Nov 20, 3:39 am, Isonomiawrote: > We live in Scotland and my mother lives in England, so I thought it > would be pretty simple to prove to my kids using a sextant that the > world was spherical - so I bought an EBBCO on eBay. Whilst I've > proved to myself I can use a sextant to find out where I am, I'm > still to convince the children either that you can, or that you would > want to. > > I started by using some software into which I put the time, > approximate longitude, latitude and sextant reading, and (after > working out I had to subtract half the sun's diameter) I finally got > something average on our location on average about 3 miles from our > location. Unfortunately, as far as a kid is concerned, if you have a > PC, you may as well look up google/streetmap rather than waste time > with a sextant, so I need to find a PC-less way to find out where I > am. > > So, using a bit of trig (with some software from the web) I created my > own single-page weekly tables (the sun don't shine everyday!), giving > altitude and direction of the sun for a given location for each > minute of the day. This allows me to create a table for any given > place which most children who can add two digit numbers, and use a > ruler/protractor could use by themselves (with instructions) to plot > a line giving their location (to within 10miles I hope!), which if > repeated twice in a day should give an "exact" location. > > Now, I know how my "Almanac" works, but even having figures for every > minute of the day, for a known location and interpolating results for > seconds, I will still be pushing it to get tabular errors less than > 1'. From what I have been able to discern about real almanacs they > contains a fraction of this information with only hourly figures for > every location in the world. Although, I've downloaded a few > worksheets to "calculate" the figures, I can't understand how these > are used (I neither have a worked example, nor do I have an almanac, > nor do I have a theoretical explanation for the tables - but I don't > see that as a fundamental problem!) Surely getting from these figures > in the Almanac to one at any time for a particular location but > involve some complex trigonometry and rather hectic sinusoidal > interpolations - neither of which are apparent on the worksheets! > > What I really want to know is how my "almanac" relates to a real > almanac, and how, could and should I make my "almanac" more like a > real almanac and still have it useable by children? I've tried > searching the internet, for any explanation of how to use an almanac > (with the theoretical background) - any help would be greatly > appreciate (remembering I am not familiar with SHA, GHA, and whilst I > learnt spherical geometry at University, I'm a little rusty) --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To unsubscribe, send email to NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---