It is believed that some numerical technique must be employed for the determination of the system parameters of a visual binary or a star with a planet because the relevant equations are not only highly nonlinear but also transcendental owing to the Kepler's equation. Such a common sense, however, is not true; we discover an analytic inversion formula, in which the original orbital parameters are expressed as elementary functions of the observable quantities such as the location of four observed points and the time interval between these points. The key thing is that we use the time interval but not the time of each observation in order to avoid treating the Kepler's equation. The present formula can be applied even in cases where the observations cover a short arc of the orbit during less than one period. Thus the formula will be useful in the future astrometric missions such as SIM, GAIA and JASMINE.

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