the International Astronomical Union adopted a new constant value in 2000, and new computation methods and polynomial expressions in 2003 and 2006; the accumulated precession is: pA = 5,028.796195T + 1.1054348T2 + higher order terms, in arcseconds, with T, the time in Julian centuries (that is, 36,525 days) since the epoch of 2000. The rate of precession is the derivative of that: p = 5,028.796195 + 2.2108696T + higher order terms. The constant term of this speed (5,028.796195 arcseconds per century in above equation) corresponds to one full precession circle in 25,771.57534 years (one full circle of 360 degrees divided by 50.28796195 arcseconds per year) although some other sources put the value at 25771.4 years, leaving a small uncertainty. The precession rate is not a constant, but is (at the moment) slowly increasing over time, as indicated by the linear (and higher order) terms in T. In any case it must be stressed that this formula is only valid over a limited time period. It is a polynomial expression centred on the J2000 datum, empirically fitted to observational data, not on a deterministic model of the solar system. It is clear that if T gets large enough (far in the future or far in the past), the T² term will dominate and p will go to very large values. In reality, more elaborate calculations on the numerical model of the Solar System show that the precessional rate has a period of about 41,000 years, the same as the obliquity of the ecliptic. That is, p = A + BT + CT2 + … is an approximation of p = a + b sin (2πT/P), where P is the 41,000-year period. Theoretical models may calculate the constants (coefficients) corresponding to the higher powers of T, but since it is impossible for a polynomial to match a periodic function over all numbers, the difference in all such approximations will grow without bound as T increases. Sufficient accuracy can be obtained over a limited time span by fitting a high enough order polynomial to observation data, rather than a necessarily imperfect dynamic numerical model. For present flight trajectory calculations of artificial satellites and spacecraft, the polynomial method gives better accuracy. In that respect, the International Astronomical Union has chosen the best-developed available theory. For up to a few centuries into the past and future, none of the formulas used diverge very much. For