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Selection of stars in HO249
From: Geoffrey Butt
Date: 2004 Sep 13, 12:37 +0100
From: Geoffrey Butt
Date: 2004 Sep 13, 12:37 +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