  From 1997 the Observatorio Nacional has been observing the Sun with an
astrolabe in order to study variations on the solar semi-diameter. Till now
more than 20.000 observations were done. The observations taken in the year of
2001 were special because, for the first time, they can be compared with other
equivalent series, and because the Sun passed by a second maximum of its cycle
of activity.
  The observed data were at first selected, by leaving out the values relative
to unsuccessful measurements. We dealt with the errors in the observed values
originating from the instability of the objective prism, which increases the
east values and diminishes the west ones. Then, it was possible to remove the
errors by comparing the East and West series. Next, the observational
conditions were investigated: the air temperature, the Fried's parameter, and
the dispersion on the solar limb adjustment. Their influence on the results was
accounted for. Finally we considered the errors arising from leveling defects,
modeled by a dependence to the azimuth of observation. All the corrections are
smaller than the standard deviation and the mean semi-diameter values stay
unaffected. This means that the series have the same pattern with smaller
deviations. After the correction of the prism instability the East and West
series become more similar, what reinforces that they aim at the same target.
  The corrected series were compared with other observations of solar
semi-diameter from the CERGA, Cote d'Azur Observatory in France and from
Tubitak Observatoty in Antalya, Turkey. The comparison reveals alike features,
which deserve further, detailed study. Also, the series was compared to the
daily mean Sun spots number. It exhibited three maxima along 2001, that
coincide on time with three maxima of the correct series delayed by 50 days.
Again, this requires a further study.
