A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2015 Mar 9, 14:31 -0400
To my understanding, they are for now.
The stars have proper motion, so over time the sha and declination changes. But the change is slow. So the apparent position of the star on the base should change...slowly. So slowly, in fact, that you cannot see the difference if the new position of the star is plotted vs the plot position at the time of issue. At least for now. Eventually, however, it will be wrong.
The second reason why its okay to use a 2102D now is that the result is on the order of degrees. So many degrees of azimuth, so many degrees of elevation. But the overlays are for every 10° of latitude. You pick the closest overlay, implying up to 5° of error. It will be a long time until the proper motion of the star amounts to that.
The subtle issue Ed and I are considering is the shape of the azimuthal equidistant overlay. This new device corrects for the latitude error, but adds a new (and hopefully) smaller az.eq. error. Even with that, the star proper motion is still tiny compared to the base plotted 65 years ago.
So to answer your real question: is it worth $22.50? Sure. This or a 2102D still work fine and produce workable results. The 5° correction in the CU300 should produce a more correct value than the 2102D. In any event, the result is in whole degrees. Star positions are known to hundredths of a second!