OCCULTATIONS (Lat. occultatio, a concealment) are neither more nor less than " eclipses:" but the latter term is confined by usage to the obscuration of the sun by the moon, and of the moon by the earth's shadow, while the former is restricted to the eclipses of stars or planets by the moon. Occultations are phenomena of frequent occur rence; they are confined to a belt of the heavens about 10° 14 wide, situated parallel to, and on both sides of the equinoxial, and extending to equal distances n. and s. of it, being the belt within which the moon's orbit lies. These phenomena serve as data for the measurement of the moon's parallax; and they are also occasionally employed in the cal culation of longitudes. As the moon moves in her °thiamin w. to e., the occultation of a star is made at the moon's eastern limb, and the star emerges on the western limb. When a star is occulted by the dasif limb of the moon (a phenomenon which can only occur between new moon and full moon), it appears to an observer as if it were suddenly extinguished, and this appearance is most deceptive when the moon is only a few days old. When an occultation occurs between full moon and new moon, the reappearance
of the star at the outer edge of tlie dark limb produces an equally startling effect. " It has often been remarked," says Herschel, " that when a star is being occulted by the moon, it appears to advance actually upon and within, the edge of the disk before it dis appears, and that sometimes to a considerable depth." This phenomenon he considers to be an optical illusion, though lie admits the possibility of its being caused by the exist ence of deep fissures in the moon's substance. Occultations of stars by planets and their satellites are of rarer occurrence than lunar occultations, and still more unfrequent are the occultations of one planet by another. Occultations are calculated in the same way as eclipses, but the calculation is simplified in the case of the fixed stars, on account of their having neither sensible motion, semi-diameter, nor parallax