NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2023 Nov 11, 11:33 -0800
How does this happen?? And how did anyone spot it?!
David Burch posted a note on Facebook yesterday noticing an odd error in the data for the star Adhara. How could this error occur? It happens when the star's declination is almost exactly 29° 00.0' but the official USNO celestial navigation web app shows the declination approaching that value (both before and after) and then suddenly jumping to 27° 00.0'. It doesn't last long, and it only impacts this single star. The error is some sort of display error, only, since it does not impact the computed altitude or azimuth. An easy, quick way to test this visually: go the pole and compare Dec and Hc, as seen in the third image below. They should match, and with the right conditions, the azimuth will match GHA. If they don't match, as is the case for Adhara around today's date, then this presumably means that we're looking at some sort of "display error", as I say. It's weird though! And I can't see conceive of any obvious cause for this bug...
Frank Reed
