# NavList:

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

**Re: Bowditch Table 15**

**From:**Jim Thompson

**Date:**2005 Jan 24, 12:29 -0400

I look forward to the explanation too. I have never been comfortable with Table 15. I just spent the morning working paper examples and searching the Nav-L archives for prior discussions about Table 9, but it remains unclear. My muddle is posted here, but I will fix it when I understand what is going on, so expect this page to evolve: http://jimthompson.net/boating/CelestialNav/CelestNotes/Bowditch15.htm Is the entering value (H-h) supposed to be the difference between the height of the object above sea level (say a 100' lighthouse) and the height of eye? When I do drawings, the vertical angle that an observer would shoot between the top of an object over the horizon and the horizon has to increase for decreasing height of eye as the observer climbs down a 100' mast, but the table suggests that vertical angle increases (keep "Distance by Vertical Angle" constant, and change [H-h] appropriately as you look at Table 15). The solution ""Distance by Vertical Angle" cannot be the geographic range, or distance between observer and object over the surface of the earth: I ran some examples by adding distance to the horizon for an observer at various heights of eye, to the distance from horizon to a 100 ft lighthouse. But "Distance by Vertical Angle" cannot be the distance from horizon to the 100 ft lighthouse either, according to my examples. To further confuse matters, see the difference in definition for the entering value height of object in the NGIA online calculator at http://164.214.12.145/NAV_PUBS/APN/Tables/T-15.pdf. I emailed NGIA to obtain their underlying formula for that calculator. Jim Thompson jim2@jimthompson.net www.jimthompson.net -------------------- Outgoing email scanned by Norton Antivirus > -----Original Message----- > From: Trevor J. Kenchington > ...I am having trouble with Table 15...