NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Selection of stars in HO249
From: Robert Gainer
Date: 2004 Sep 13, 13:36 +0000
From: Robert Gainer
Date: 2004 Sep 13, 13:36 +0000
Geoff, Does this refer to a choice being made so that the same star will be available and on the list for subsequent nights? All the best, Robert Gainer >From: Geoff Butt>Reply-To: Navigation Mailing List >To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM >Subject: Selection of stars in HO249 >Date: Mon, 13 Sep 2004 12:37:15 +0100 > >Does anyone have any information on how the Selected Stars were chosen for >HO249; particularly with respect to the statement in the Introduction (p >iii) 'Continuity was sought in regard to both latitude and hour angle, >particularly for latitude where changes are not immediately evident by >inspection.' > >I have been entertaining myself by writing some programmes to calculate >daily Almanak-like data from the Meeus algorithms. As I am calculating >data >for chosen days it occurred to me that it would be handy to print out a >selected star diagram for the morning and evening twilight observing >periods. As I was doing that for a specific day I could also plot the >position of any planets along with the selected stars. > >At first the 'rules' for selecting stars seemed fairly simple: >- select stars with altitudes between 15 and 65 deg >- use 1st mag stars in order of brightness unless they were separated by >less than > 20 deg >- if 7 1st mag stars not available then choose suitable 2nd mag stars to >plug the > larger gaps in the azimuth coverage as near uniformly as possible. > >Comparing the results with the selections published in HO249 I get about >60% >of the groups I selected agreeing with HO249. By increasing my number >selected to 9 I get closer to including all the HO249 selection. But >examining the differences I can't spot any additional 'rules' for the HO249 >preference rather than my selections. > >For example, I interpreted the 'continuity .. with regard to .. latitude' >as >meaning 'Don't select low altitude stars lying to the North or South' >(which >would also improve continuity with regard to LHA) - but that doesn't seem >to >explain the differences. > >I also thought about giving preference to stars which were more easily >identified in the sky. There seem to be several examples of pairs of stars >which are relatively close neighbours and the differences between the >selected star lists being the choosing of one rather than the other - but >the preference for either is not biased, so ability to identify isn't the >criterion. > >If anyone has any ideas about how properly to interpret 'Continuity ... ' I >would be very interested to hear them. > >Geoff Butt _________________________________________________________________ Don�t just search. Find. Check out the new MSN Search! http://search.msn.click-url.com/go/onm00200636ave/direct/01/