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Author
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Topic: StarFinder Epoch
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John Pinto
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posted July 26, 2021 05:15 AM
I just purchased a StarFinder from StarPath. What Epoch are the star locations drawn for? How many years is it good for after the Epoch before it needs to be updated for Precession? I know standard astronomical star charts are revised about every 50 years. Wasn't sure about the StarFinder.
From: Orlando
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David Burch
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posted July 26, 2021 09:42 AM
I would guess this is 1920s, as that was the time of the first patent. They did have an update in the 1940s, but not sure the stars were changed.
One way to figure that out would be use the red template to measure the SHA and dec of a couple stars, then use any online almanac that lets you go back in time and find out when these data were valid... this is essentially the way the Great Pyramids were first dated right after the Rosetta stone was translated to tell us that a small angular shaft in one of them pointed to what was then the North Star (Thuban, Alpha Draconis).
As for how long it will be useful, considering its modest precision to begin with, I would say maybe another 3 or 4 generations. You can also figure that out more precisely from the study above. Just run it forward, and decide what level of uncertainty you will accept. Its base precision considering all latitudes is probably about ± 2º.
From: Starpath, Seattle, WA
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John Pinto
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posted July 26, 2021 10:34 AM
Are there particular navigational stars that are affected the most by Precession? I would check those first to look for the largest discrepancy in SHA and Dec.
From: Orlando
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David Burch
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posted July 26, 2021 12:40 PM
I am not 100% on this; my first guess would be it affects all in the same way—that would be a good positional astronomy question to track down. The experts at Nav-List will be able to tell you that.
There are, however, stars with much larger proper motion than others, and these could be found from google search on "stars with large proper motion."
You can also look in list of extra starts in the back of the almanac (which includes all navigation stars as well) to see which ones are moving the most.
Also, there is the amazing and powerful free Stellarium program and you can use it to go way back and check this. They might even have an output that says just what you want.
From: Starpath, Seattle, WA
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John Pinto
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posted July 27, 2021 10:45 AM
I found a precession calculator online David.
Before doing measurements on the starbase, I'm going to run the 57 navigational stars preprinted on the starbase through the precession calculator using their current 2021 positions and precess them backwards to their 1920 positions to see which ones had the largest changes.
Those will be the ones I will use to confirm if the Epoch on the StarFinder really is 1920 (or not). I'll report back what I find.
Hoping for not too much of a discrepancy to keep it useful "for 3 or 4 generations".
I will also precess the star positions forward to find about when the discrepancies are more than 5 degrees in altitude or direction to determine when it needs an update to a more current Epoch.
From: Orlando
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John Pinto
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posted September 10, 2021 06:59 AM
I've determined the Epoch of the StarFinder is approximately 1950 by comparing the RA of the 20 brightest stars to the 1950 Epoch values for those stars and they match up exactly. Due to precession on average stars change their RA by 1 degree every 72 years. The expectation is we should see something like that in the change in Hc and Zn depending on Latitude and LHA.
I will post some results of comparing the Hc and Zn values for a few of those stars on the Starbase versus those listed in the 2020 Epoch Volume 1 of 249.
From: Orlando
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David Burch
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posted September 10, 2021 01:38 PM
Thanks for that work. Your Hc and Zn comparisons are the main factor as you know, and we look forward to seeing those.
When you make that list, if you can point out the worst case with actual values, then those with a star finder can check that one out to see the issue you raise.
Thanks again.
From: Starpath, Seattle, WA
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John Pinto
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posted September 12, 2021 12:11 PM
Here you go David.
Worst cases only (In Hc and Zn) for each Northern Latitude template I checked for 3 bright stars I can see from here in Florida that had the biggest RA change due to Precession. The stars were Capella, Pollux and Antares.
I will list by the Northern Latitude templates I checked between the StarFinder and Volume 1 of 249.
5N:
Capella:
LHA 012 Vol1: Hc 19-05 Zn 043
LHA 012 StFi: Hc 20-00 Zn 042.5
LHA 078 Vol1: Hc 48-58 Zn 002
LHA 078 StFi: Hc 49-00 Zn 000
Pollux:
LHA 111 Vol1: Hc 66-25 Zn 013
LHA 111 StFi: Hc 66-30 Zn 010
LHA 174 Vol1: Hc 31-01 Zn 300
LHA 174 StFi: Hc 30-00 Zn 300
Antares:
LHA 187 Vol1: Hc 23-28 Zn 122
LHA 187 StFi: Hc 25-00 Zn 122
LHA 234 Vol1: Hc 55-52 Zn 158
LHA 234 StFi: Hc 56-30 Zn 160
15N:
Capella:
LHA 016 Vol1: Hc 29-01 Zn 045
LHA 016 StFi: Hc 30-00 Zn 045
LHA 070 Vol1: Hc 57-58 Zn 013
LHA 070 StFi: Hc 58-00 Zn 010
Pollux:
LHA 080 Vol1: Hc 53-42 Zn 063
LHA 080 StFi: Hc 55-00 Zn 062
LHA 151 Vol1: Hc 55-38 Zn 298
LHA 151 StFi: Hc 55-00 Zn 297
Antares:
LHA 181.5 Vol1: Hc 13-33 Zn 122.5 (Interpolated)
LHA 181.5 StFi: Hc 15-00 Zn 122.5
LHA 239 Vol1: Hc 47-41 Zn 168
LHA 239 StFi: Hc 48-00 Zn 170
25N:
Capella:
LHA 028 Vol1: Hc 44-04 Zn 049
LHA 028 StFi: Hc 45-00 Zn 049
LHA 076 Vol1: Hc 68-48 Zn 007
LHA 076 StFi: Hc 68-30 Zn 005
Pollux:
LHA 082 Vol1: Hc 58-58 Zn 077
LHA 082 StFi: Hc 60-00 Zn 077
LHA 161 Vol1: Hc 50-23 Zn 284
LHA 161 StFi: Hc 50-00 Zn 285
Antares:
LHA 205 Vol1: Hc 24-06 Zn 138
LHA 205 StFi: Hc 25-00 Zn 139
LHA 248 Vol1: Hc 38-32 Zn 180
LHA 248 StFi: Hc 38-30 Zn 182
35N:
Capella:
LHA 012 Vol1: Hc 39-03 Zn 056
LHA 012 StFi: Hc 40-00 Zn 055
LHA 108 Vol1: Hc 65-54 Zn 306
LHA 108 StFi: Hc 66-00 Zn 305
Pollux:
LHA 062 Vol1: Hc 43-28 Zn 083
LHA 062 StFi: Hc 45-00 Zn 082
LHA 085 Vol1: Hc 62-15 Zn 096
LHA 085 StFi: Hc 63-00 Zn 095
Antares:
LHA 209 Vol1: Hc 18-29 Zn 144
LHA 209 StFi: Hc 20-00 Zn 144
LHA 256 Vol1: Hc 28-01 Zn 188
LHA 256 StFi: Hc 28-30 Zn 190
45N:
Capella:
LHA 012 Vol1: Hc 44-08 Zn 063
LHA 012 StFi: Hc 45-00 Zn 064
LHA 028 Vol1: Hc 54-31 Zn 069
LHA 028 StFi: Hc 55-00 Zn 070
Pollux:
LHA 069 Vol1: Hc 48-49 Zn 098
LHA 069 StFi: Hc 50-00 Zn 098
LHA 166 Vol1: Hc 47-35 Zn 264
LHA 166 StFi: Hc 47-30 Zn 265
Antares:
LHA 221 Vol1: Hc 14-30 Zn 155
LHA 221 StFi: Hc 15-00 Zn 156
LHA 248 Vol1: Hc 18-32 Zn 180
LHA 248 StFi: Hc 19-00 Zn 182
55N:
Capella:
LHA 014 Vol1: Hc 48-58 Zn 074
LHA 014 StFi: Hc 50-00 Zn 075
LHA 023 Vol1: Hc 54-00 Zn 080
LHA 023 StFi: Hc 55-00 Zn 081
Pollux:
LHA 038 Vol1: Hc 28-57 Zn 082
LHA 038 StFi: Hc 30-00 Zn 082.5
LHA 138 Vol1: Hc 58-52 Zn 218
LHA 138 StFi: Hc 59-00 Zn 220
Antares: (not used in Vol. 1 of 249)
65N:
Capella:
LHA 139 Vol1: Hc 53-15 Zn 268
LHA 139 StFi: Hc 52-30 Zn 270
LHA 218 Vol1: Hc 25-37 Zn 329
LHA 218 StFi: Hc 25-00 Zn 330
Pollux:
LHA 061 Vol1: Hc 39-29 Zn 109
LHA 061 StFi: Hc 40-00 Zn 110
LHA 170 Vol1: Hc 40-23 Zn 248
LHA 170 StFi: Hc 40-00 Zn 250
Antares: (not used in Vol. 1 of 249)
75N:
Capella:
LHA 060 Vol1: Hc 59-49 Zn 152
LHA 060 StFi: Hc 60-00 Zn 155
LHA 143 Vol1: Hc 50-50 Zn 260
LHA 143 StFi: Hc 50-00 Zn 260
Pollux:
LHA 034 Vol1: Hc 28-51 Zn 089
LHA 034 StFi: Hc 30-00 Zn 090
LHA 054 Vol1: Hc 33-56 Zn 109
LHA 054 StFi: Hc 35-00 Zn 110
Antares: (not used in Vol. 1 of 249)
85N:
Capella:
LHA 028 Vol1: Hc 48-59 Zn 124
LHA 028 StFi: Hc 50-00 Zn 125
LHA 078 Vol1: Hc 51-01 Zn 178
LHA 078 StFi: Hc 52-00 Zn 180
Pollux:
LHA 034 Vol1: Hc 28-30 Zn 095
LHA 034 StFi: Hc 30-00 Zn 095
LHA 178 Vol1: Hc 30-17 Zn 244
LHA 178 StFi: Hc 30-00 Zn 245
Antares: (not used in Vol. 1 of 249)
From: Orlando
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