Precession-nutation estimates

249

Figure 1. Celestial pole o sets from optical astrometry OA99.

In addition, we derived also a set of auxiliary `station' parameters, giving the

systematic deviations of individual instruments and the rheological parameter

= 1 +

k

;

l

(responsible for nonrigid tidal variations of local verticals). The

list of the instruments used in the solution is given in Table 1.

3.

Celestial

p ole

o

sets

and

their

analysis

Because we used the standard IAU models of precession IAU1976 (Lieske et

al.

, 1977) and nutation IAU1980 (Seidelmann, 1982) to calculate the apparent

positions of the observed stars, the celestial pole o sets derived from our solution

express the displacements of the Earth's pole from the celestial ephemeris pole

whose position is de ned by these models. The results are plotted in Figure 1,

the values themselves in the upper part (crosses), their formal uncertainties

below (lines), both in the same scale. It is necessary to say that

"

i

,

i

,

referred to the same epoch

t

i

, are strongly correlated (their error ellipses highly

elongated), due to only a partial coverage of observations during a day (for a

detailed explanation of this e ect see Vondrak et al. (1992). The error ellipses

are always pointed towards the Sun (the observations being centred around

local midnight) and they thus rotate in the celestial reference frame with an

annual period that causes an annual change in rms errors of

"

and

(phase

shifted by 90 ). This e ect became much weaker after 1956, when latitude

observations (the only type of observations used before that date) were combined

with Universal Time observations. The gure shows also that the noise level

substantially diminishes after 1956, thanks to a larger number of participating

observatories (and, consequently, also observations).

The series of celestial pole o sets was subject to analysis, in which we used

a classical least-squares method to estimate corrections to precession plus the

nine most signi cant nutation terms (OA99/1). Since the resolution of the series