The observatories from Bucharest , Cluj - Napoca and Timisoara have participated in the international campaign '' PHEMU - 91 '' , for observation of the mutual events of the galilean satellites during the 1990 - 1991 period. This program has been intiated by the '' Bureau International des Longitudes, Paris ''.
The first predictions tables for the movements of the galilean satellites were carried out by Galilei since 1612. In 1788 Laplace elaborated a complete theory for the satellite movements. Further, the prediction tables have been improved by Souilard (1880), Sampson (1921), Lieske (1977) and Arlot (Arlot, 1984).
In Romania, Prof. G. Petrescu dealt with the verification of the satellites position given in 1910 by the Sampson's tables. The results have been published in his doctoral thesis entitled ''Corrections of the orbital elements for the first four satellites of the planet Jupiter, deduced by photographic observations carried out at the Bucharest and Paris observatories''. The thesis was held on the 7-th of February 1939 at the Science Faculty '' King Ferdinand '' from Cluj. Petrescu's observations performed in Bucharest (1934) and Paris (1936) were used by NASA in 1974, for the preparation of the Voyager mission. The data were sent for NASA disposal.
On the base of Lieske and Arlot predictions, one calculates the satellite ephemeris which are published in '' Connaissance des Temps ''. These are utilised in the forecasting of the satellites eclipses and mutual events. The space mission towards the galilean satellites require a good knowledge of their position.
There are two types of phenomena which involve the galilean satellites : the classic occultations (eclipses and transits) and the mutual events. The last ones consist in phenomena between the satellites themselves twice each Jovian year of 11.8 years. The mutual events are occultations of one satellite by another for a terrestrial observer, or eclipses of one satellite by the shadow of another. Because of the small sizes of the satellites and the very small inclination of their orbits to the planet equator , mutual phenomena do not occur for each geocentric conjunction (for occultations) or heliocentric one (for eclipses) during the favorable period. This ones occurs when the jovicentric declination of the Earth and the Sun are smaller than a defined quantity.
The photoelectric methods are most indicate for mutual event observations. The observations consist on the recording of the magnitude drop during the event. The relative velocities of the satellites are large (= 10 km/s) requiring a very good magnitude and time estimations. Observing the classical satellite phenomena by not large instruments , the position errors will be of 600 km on an average. In the case of the mutual phenomena observed from city places , an error about 48 km is expected for a 40 cm instrument. By increasing the instrument diameter and in good seeing conditions, the errors drop close to 8 km.