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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

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