Diameter measurements performed with the solar astrolabe are affected by atmospheric effects. In order to deduce significant astrophysical results, it is necessary to know how these effects contribute to the error of diameter measurement. For this purpose, a numerical simulation was developed leading to a law of error behaviour with observing conditions. In order to validate these results, recorded data obtained at the Calern observatory astrolabe (France) during the period 1996 – 1997 are used. A good agreement is observed with the simulation, which permit salso to give access to daily atmospheric time constants. (Co-author is A. Irbah, Algeria.)