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From 1997 the Observatorio Nacional has been observing the Sun with anastrolabe in order to study variations on the solar semi-diameter. Till nowmore than 20.000 observations were done. The observations taken in the year of2001 were special because, for the first time, they can be compared with otherequivalent series, and because the Sun passed by a second maximum of its cycleof activity.  The observed data were at first selected, by leaving out the values relativeto unsuccessful measurements. We dealt with the errors in the observed valuesoriginating from the instability of the objective prism, which increases theeast values and diminishes the west ones. Then, it was possible to remove theerrors by comparing the East and West series. Next, the observationalconditions were investigated: the air temperature, the Fried's parameter, andthe dispersion on the solar limb adjustment. Their influence on the results wasaccounted for. Finally we considered the errors arising from leveling defects,modeled by a dependence to the azimuth of observation. All the corrections aresmaller than the standard deviation and the mean semi-diameter values stayunaffected. This means that the series have the same pattern with smallerdeviations. After the correction of the prism instability the East and Westseries become more similar, what reinforces that they aim at the same target.  The corrected series were compared with other observations of solarsemi-diameter from the CERGA, Cote d'Azur Observatory in France and fromTubitak Observatoty in Antalya, Turkey. The comparison reveals alike features,which deserve further, detailed study. Also, the series was compared to thedaily mean Sun spots number. It exhibited three maxima along 2001, thatcoincide on time with three maxima of the correct series delayed by 50 days.Again, this requires a further study.

Analysis of a series of measurements of solar semidiameter variations during 2001 at the astrolabe of Rio de Janeiro