occultation, in astronomy, eclipse of one celestial body by another, e.g., when the moon lies between a star and the earth. Occultations of stars by the moon are important in astronomy. Since stellar positions are very accurately known, the time and position of an occultation can be used to determine the position of the moon. Alternatively, an observer can determine his or her longitude by comparing the time at which he observes an occultation with a table listing the universal time at which the occultation occurs.