The regular photographic observations of Galilean satellites has been made with 26″ refractor at Pulkovo since 1975. The relative positions of satellites are obtained by means of “scaletrail” technique. During 1975–1979 over 600 observations of Galilean satellites were carried out. The observed positions were compared with the ephemeris based on Sampson's and Lieske's E-1 theories, the corresponding computations being made at the Institute for Theoretical Astronomy. The standard deviations of one observed relative position “Satellite minus Jupiter” don't exceed ±0″.10. The comparison of Sampson's theory with observations showed some systematic error which can be considered as an error in longitude of about 1 min. Lieske's theory E-1 showed a better agreement with the observations, the absolute deviations not exceeding 0″.3.
A construction of highly precision theories of motion of planet's satellites and of Galilean satellites, in particular, demands appropriately accurate observations (the error not exceeding 0″.10). At present such observations of satellites can be obtained with the help of long-foci telescopes (focal length of about 10 m), using rigorous technique of astrometric reduction.