In astronomical usage, an occultation occurs when an object of larger angular size passes in front of an object of smaller angular size. A frequent example is the passage of the Moon between an observer on Earth and a star, producing an occultation of the star. The rarer occultations of stars by planets, satellites, or asteroids provide opportunities to probe the properties (atmosphere, size) of the occulting bodies. Such observations led to the discovery of the rings around the planets  Uranus and  Neptune. When the Earth passes through the equatorial plane of  Jupiter or  Saturn (which correspond to the orbital planes of the major satellites of these planets), mutual occultations among these satellites can be observed. Occultation events have also been observed from unmanned space probes in the Jovian and Saturnian systems. The case when the foreground object has a smaller angular size than the more distant object is called a  transit.