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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Lars Bergman
Date: 2018 Nov 1, 07:16 -0700
William Porter wrote: "Where am I wrong?"
I am not sure, but my own calculations are as follows, for 18h46m47s GMT on October 19, 2018:
GHA Aries 309°53.1'
SHA Altair 62° 4.7'
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GHA Altair 11°57.8' This is the "modern" nautical way of doing it, GHA star = GHA Aries + SHA star.
Working in time, an alternative solution is
GST 20h39m32s
RA Altair 19h51m41s
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GHA Altair 0h47m51s
This result is equal to that above. GHA star = GST - RA star, where Greenwich (apparent) siderial time GST is the time equivalent of GHA Aries.
I am not sure where Willam got his GST from, is it the value at GMT=0h corrected for acceleration? Anyway, it does not agree with my value. Also the RA is somewhat off.
My GHA Aries and SHA are from Omar Reis's online almanac, GHA Aries is linearly interpolated between hourly values. GST is just GHA Aries converted to time. RA is calculated from (360 - SHA)/15. This explains why the two results do agree!
Lars, 59°N 18°E
