Occultation of a heavenly body is said to occur when it is hidden from our view by another body; thus the satellites of Jupiter are occulted by the planet when they pass behind it. Since the moon is so large an object in our sky, stars and planets are frequently hidden behind her disc, and such occultations are extremely interesting. They can only occur in a zone of the heavens about 10° 18' wide, since the moon's orbit is inclined at an angle of 5° 9' to the plane of the ecliptic. A star appears to meet the moon at her eastern edge, to disappear for a short time, and then to emerge at her western edge, and, if the moon be full, nothing very remarkable is observed in the phenomenon. When, however, the moon is younger, there is a part of her edge on the eastern side which we cannot see at all, since it is not illuminated by the sun, but which is, of course, quite as effectual in occulting a star as the bright part; the star then suddenly disappears from view when it passes behind this unseen obstacle, and the effect is rather that of the star having mysteriously "gone out."