A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2021 Jan 3, 10:27 -0800
Sean, you wrote:
"For β Hyi, Simbad gives a proper motion of +2219.54 mas/yr in R.A. and +324.09 mas/yr in dec.
°, and also the individual components "by axes" which are given as "2579 and 599" mas/yr. The two sets are consistent. You could just draw those latter two numbers on graph paper in the x, y directions, connect them with a hypotenuse and even in that simple way you find that the net length and the angle are "just right". But what the hell?? 599 mas/yr for the proper motion in declination?! That's nearly double the best estimates that have been available in the past thirty years. Is it wildly wrong? Is it displayed in some incorrect units but "right on the inside"? Could it be that someone has listed rate of change of dec with precession included (that just occurred to me ...don't know if it makes any sense)? I should add that I mentioned beta Hydri to start because it was the first that I noticed in a sample of three or four stars. Since then I have found other oddities. I don't see a pattern in them yet.
Sean, you added:
"no matter which numbers you use, that is relatively fast."
Yes, it's zipping along! Beta Hydri is a nearby star, so its physical motion translates to larger angular motion.
A challenge for brave coders: What is Stellarium doing? Well, it's open-source, so go grab the code, open it up, and find out! I leave this as an exercise for the reader. Ha ha ha. Actually, I tired this approach a couple of years ago on another puzzle and found it far too much trouble (easier to diagnose from behavior than from code), but it's really the best way to ensure the code is okay.
PS: I'm aware that this is moving rapidly away from navigation, and we will move it back to navigation or shut it down soon.