Abstract :
The use of some Arabic medieval solar and lunar eclipse records for the determination of secular changes in the Earthʹʹs rotation is critically reviewed. The published results derived from these data suggest a non-uniform decrease in the Earthʹʹs rotation rate over the last 27 cy. There is, however, up to this day no sound physical explanation for the deduced lsquonon-tidal oscillationsrsquo, with an apparent period of about 1500 yr and a semi-amplitude of some 4 ms in the l.o.d., which overlayed to a constant secular tidal change in the Earthʹʹs rotation rate produce a net non-uniform deceleration of the Earthʹʹs rotation. In this paper we discuss a set of observations, which were executed by professional Arabic astronomers. We show by our analysis the way in which the non-uniform deceleration of the Earthʹʹs rotation was constructed. A correct reading of the Arabic medieval observations shows that they do not contradict a secular constant decrease in the Earthʹʹs rotation rate of nearly -4.6 sdot 10-22 rad s-2. This value is in accordance with other similar ones derived from ancient eclipse records and from satellite tracking data.
