A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2021 Oct 9, 03:01 -0700
Sean, yes, it depends on how you define the "epoch" of 2021, which is no small matter for astronomers, but quite irrelevant for the practical use of tables like this. There are more options than you're imagining, and the one used historically in these tables is yet another definition.
A major part of the problem here is the awful design choice in the Polaris Q tables. Specifically they are designed as critical tables. This generates an illusion of accuracy which is difficult for most navigation enthusiasts to understand or resist. A difference in the "critical" index (the hour angle of Aries) of one minute of arc at this very high declination, so close to the celestial pole corresponds to an actual positional difference of approximately 0.7 seconds of arc. That is, when four seconds of time elapse, rotating the celestial sphere by one minute of arc at the equator, the position of Polaris shifts by less than one second of arc. It would be simpler in concept and accurate enough in practice to create a direct table listing Q calculated to the nearest minute (or tenth to satisfy the urge for accuracy) for every whole degree of LHA Aries.
The tables also include the hilariously absurd instruction "In critical cases, ascend". In a "critical case" in this table, a navigator is nerding-out over a change in the position of Polaris smaller than a second of arc in order to get an approximate Q correction that's rounded to the nearest minute of arc. Just Plain Nuts.