  We present measurements of the large-scale circulation (LSC) of turbulent
Rayleigh-Benard convection in water-filled cylindrical samples of heights equal
to their diameters. The orientation of the LSC had an irregular time
dependence, but revealed a net azimuthal rotation with an average period of
about 3 days for Rayleigh numbers R > 10^10. On average there was also a
tendency for the LSC to be aligned with upflow to the west and downflow to the
east, even after physically rotating the apparatus in the laboratory through
various angles. Both of these phenomena could be explained as a result of the
coupling of the Earth's Coriolis force to the LSC. The rate of azimuthal
rotation could be calculated from a model of diffusive LSC orientation
meandering with a potential barrier due to the Coriolis force. The model and
the data revealed an additional contribution to the potential barrier that
could be attributed to the cooling system of the sample top which dominated the
preferred orientation of the LSC at high R. The tendency for the LSC to be in a
preferred orientation due to the Coriolis force could be cancelled by a slight
tilt of the apparatus relative to gravity, although this tilt affected other
aspects of the LSC that the Coriolis force did not.
