  For the future development of Chinese Giant Solar Telescope (CGST) in Western
China, a new sky brightness monitor (SBM) has been produced for the site survey
for CGST. To critically examine the performance and sensitivity of SBM, we used
it in the observation of the annular solar eclipse in Dali City, Yunnan, on 15
January 2010. The observation met good weather condition with almost clear sky
during the eclipse. The SBM measurement translates into the solar illuminance
changes at a level of 2.4 \times 10-4I s-1 during the eclipse. The time of the
minimal sky brightness in the field of view (FOV) is found consistent with the
time of maximum eclipse. Two local sky regions in the FOV are chosen to make
time series of calibrated skylight profiles. The evolution of the sky
brightness thus calibrated also shows good consistency with the eclipse,
particularly between the second and the third contacts. The minimal sky
brightness in each local sky region took place within half a minute from the
corresponding predicted contact time. Such small time delays were mainly caused
by occasional cirri. The minimal sky brightness measured during the eclipse is
a few millionths of I\odot with standard deviation of 0.11 millionths of
I\odot. The observation supports that the singlescattering process (optically
thin conditions) is the main contributor to the atmospheric scattering. We have
demonstrated that many important aerosol optical parameters can be deduced from
our data.We conclude that the new SBM is a sensitive sky photometer that can be
used for our CGST and coronagraph site surveys.
