NavList:

It's mostly precession that invalidates the five-yearly editions of volume 1. You asked about the limitation on the calculation of GHA Aries in the tables. That's mostly a matter of saving space and "paper". It won't really extend the validity of the tables if you extend the GHA Aries calculation beyond the limits.

For historical reasons, the correction for precession is referred to as "precession and nutation" or "P&N" but nutation, physically, is a type of precession, so it's fair to say "it's just precession". Precession is fundamentally a rotation of the celestial sphere about an axis through the ecliptic poles: one pole is in Draco, roughly halfway between Polaris and Vega; the other, in the deep southern sky, is near the Large Magellanic Cloud. That axis is perpendicular to the ecliptic. The sphere of stars rotates about that axis at a rate of one degree in 72 years (remember it as "a degree in a human lifetime"), which is about 50" or 0.8 minutes of arc in one year. Proper motion is also an issue for a handful of rapidly-moving stars like Arcturus.

If we ignore proper motion (and also aberration, which is a smaller annual phenomenon), the rotation of the celestial sphere due to precession (P&N) is a "rigid" rotation. The whole celestial sphere of stars rotates slowly to a new orientation, but the relationships among the stars, the angles between them, the shapes of the constellations, etc. are not changed by precession. Since the sphere of stars is unchanged "internally", a position derived by lines of position built from out-of-date astronomical data can still be a "tight" fix with LOPs crossing in a small area. The fix looks good, but it's in the wrong place. That's why the P&N correction tables in Pub.249 tell us to shift our fix by a certain number of miles in some direction in proportion to the number of years away from the primary date of the tables. The fix is good, and the lines of position from various stars are in their correct relative positions, but the celestial sphere is rotated as a whole.

A given edition of Pub.249, like the 2025 edition, can be used for +/-5 years around its primary date, but it's safer to limit this to +/-2.5 years, assuming that a new edition will be available two and a half years before its nominal date. In any case, if the date of the fix is a year or two from the date of the tables, the precession correction should be applied to the fix. Some independent (generally downloadable PDF) versions of Pub.249 Vol.1 are now released annually. For these annual editions, or indeed for a five-year edition within six months of its nominal date, the P&N correction is small enough to be ignored.

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA

PS: Precession is a rotation of the celestial sphere that grows without limit. We have tables for that? But what about aberration of starlight? Aberration is ignored in Pub.249 Vol.1 as standard practice, but you can visualize the effect of aberration by imagining the celestial sphere as a slightly malleable surface, like a rubber ball with the stars drawn on it. Aberration takes the rubbery surface on which the stars are drawn, grabs it by the middle and squeezes it towards the "dead-ahead" spot in the Earth's motion around the Sun. Stars perpendicular to the dead-ahead/dead-astern axis of the Earth's motion are deflected in the direction of motion (toward the dead-ahead spot by about 20 seconds of arc. If we call the angle from a particular star in the sky to the dead-ahead point, theta, θ, then that star's position is shifted towards the dead-ahead spot by 20·sinθ seconds of arc. So where's dead-ahead? Where is the Earth "going" in its orbit around the Sun? It's helpful to remember that most meteor showers are stronger in the pre-dawn hours because the Earth is then facing "forward" in its orbit. Ignoring orbital eccentricity, which is a very small correction, the dead-ahead spot is in the morning sky 90° away from the Sun right on the ecliptic. Stars are very slightly pushed together towards that spot by aberration on any given date. Similarly in the evening sky, stars are slightly spread away from the dead-astern spot on the ecliptic which is also 90° from the Sun.