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From: Geoff Butt (no email)
Date: Mon Sep 13 2004 - 07:37:15 EDT
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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