The great year is known very well with good accuracy, as for
all other motions of Earth. There will be small oscillations but they are insignificant for our purposes. The Stellarium software is perfectly adequate.
The very epitome of scientific complacency in prologue to the demise of an unsuspecting civilization...
Since the discussion gained momentum toward the rate of precession, particular perspective on the matter may be informative... Earth rotates retrograde at consistent speed, however, the observation thereof via nocturnal means incurs a (circle -VS- ellipse) paralax: although the co-ordinates of stars change, the 3D movement respective changing obliquity is not conveyed celestially...
Other than advancing the equinox, precessional revolution changes the orientation of Earth's axis - this registers in true Polar wander, in turn attested by the alignment with Polaris being on the wane - two dimensionally. The apparent rate at which this occurs in 2D however, does not account directionally for 3D change in the slant of Earth's axis.
Observing distance discrepancy side-on, by means of Sunrise -- over the course of a SolarYear -- line of sight along the ecliptic and at perpendicular to the horizon, whereby the two Solstice locations, with Equinox in-between, is depicted. (Present era obliquity 24°, the velocity of Earth and the parabolic path to which it is held by the Sun is consistent, therefore the time in which one orbit is completed, also). Perceiving the Sun disc from azimuth perspective, mid-winter to mid-summer & repeat, the time and distance to the stand-still/turn-around points on the horizon are consistent - relative to an era... (introduce the Zodiac)
When the obliquity of Earth's axis was 14° (1250 BC) Earth's orbital time and eccentricity wrt the Sun was also consistent with the present, however, the lattitude of the Tropics was reduced -i.e. the perceivable distance of travel, one Sunrise at a time -- viewed against the horizon -- is much shorter: the apparent rate at which the Sun progresses from one Solstice to the other registers accordingly.
Observed comparatively, Earth tilted over on its side -- 58°obliquity LGM 14210 BC (next 11710 AD) -- its orbital velocity and eccentricity remain unaltered, however, the exaggerated slant of the polar axis cause the separation distance of the Tropics to be greatly increased -i.e. the perceivable distance travelled by the Sun against the horizon is far greater, therefore the apparent rate of progression between summer; & winter Solstices, requires such to be speedier...
Both events occur in the usual Jun.& Dec. time-slot, hence the Sun needs to arrive on time, regardless of it seemingly covering more ground - it's an illusion. Any noticable change in the rate of precession, therefore, is an illusion. Perhaps once satisfaction concerning speed is ascertained, it may be a good idea to investigate other dynamics, of which the Great Year is in fact a product.
Edited 2 time(s). Last edit at 15-Aug-19 15:00 by hendrik dirker.