An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy and can also be used in a general sense to describe when an object in the foreground occults (covers up) objects in the background. In the general sense, occultation applies to the visual scene from low-flying aircraft and in Computer-Generated Imagery (CGI) technology, where foreground objects obscure distant ones in a dynamic way as the scene changes. Astronomical events. These include transits and eclipses. The word transit refers to cases where the nearer object appears smaller in apparent size than the more distant object, such as transit of syzygy. Every time an occultation occurs, an eclipse also occurs. Consider a so-called "eclipse" of the Sun by the Moon, as seen from Earth. In this event, the Moon physically moves between Earth and the Sun, thus blocking out a portion or all of the bright disk of the Sun. Although this phenomenon is usually referred to as an "eclipse", this term is a misnomer, because the Moon is not eclipsing the Sun; instead the Moon is occulting the Sun. When the Moon occults the Sun, it casts a small shadow on the surface of the Earth, and therefore the Moon's shadow is partially eclipsing Earth. So a so-called "solar eclipse" actually consists of an occultation of the Sun by the Moon, as seen from Earth, and a partial eclipse of Earth by the Moon's shadow. By contrast, an "eclipse" of the Moon is in fact a true eclipse: the Moon moves into the shadow cast back into space by Earth, and is said to be eclipsed by Earth's shadow. As seen from the surface of the Moon, Earth passes directly between the Moon and the Sun, thus blocking or occulting the Sun as seen by a hypothetical lunar observer. Again, every eclipse also entails an occultation. The term occultation is most frequently used to describe those relatively frequent occasions when the Moon passes in front of a star during the course of its orbital motion around the Earth. Since the Moon has no atmosphere and stars have no appreciable angular size, a star that is occulted by the moon will disappear or reappear very nearly instantaneously on the moon's edge, or limb. Events that take place on the Moon's dark limb are of particular interest to observers, because the lack of glare allows these occultations to more easily be observed and timed. The Moon's orbit is inclined to the ecliptic, and any stars with an ecliptic latitude of less than about 6.5 degrees may be occulted by it. There are three first magnitude stars that are sufficiently close to the ecliptic that they may be occulted by the Moon and by planets -- Regulus, Spica and Antares. Occultations of Aldebaran are presently only possible by the Moon, because the planets pass Aldebaran to the north. Neither planetary nor lunar occultations of Pollux are currently possible. However, in the far future, occultations of Aldebaran and Pollux will be possible, as they were in the far past. Within a few kilometres of the edge of an occultation's predicted path, referred to as its northern or southern limit, an observer may see the star intermittently disappearing and reappearing as the irregular limb of the Moon moves past the star, creating what is known as a Grazing lunar occultation. From an observational and scientific standpoint, these "grazes" are the most dynamic and interesting of lunar occultations. The accurate timing of lunar occultations is performed regularly by (primarily amateur) astronomers. Lunar occultations timed to an accuracy of a few tenths of a second have various scientific uses, particularly in refining our knowledge of lunar topography. Photoelectric analysis of lunar occultations have also discovered some stars to be very close visual or spectroscopic binary stars. Early radio astronomers found occultations of radio sources by the Moon valuable for determining their exact positions, because the long wavelength of radio waves limited the resolution available through direct observation. Several times during the year, someone on Earth can usually observe the Moon occulting a planet. Since planets, unlike stars, have significant angular sizes, lunar occultations of planets will create a narrow zone on earth from which a partial occultation of the planet will occur. An observer located within that narrow zone could observe the planet's disk partly blocked by the slowly moving moon. Stars may also be occulted by planets. In 1959, Venus occulted Regulus. Uranus' rings were first discovered when that planet occulted a star in 1977. On the evening of July 2-July 3, 1989, Saturn passed in front of the 5th magnitude star 28 Sagittarii. Pluto, which was re-designated as a "dwarf planet" in 2006, occulted stars in 1988, 2002, and 2006, allowing its tenuous atmosphere to be studied. It is also possible for one planet to occult another planet. However, these mutual occultations of planets are extremely rare. The last such event occurred on January 3, 1818 and will next occur on November 22, 2065, in both cases involving the same two planets -- Venus and Jupiter. Technically speaking, when the foreground planet is smaller in apparent size than the background planet, the event should be called a "mutual planetary transit." When the foreground planet is larger in apparent size than the background planet, the event should be called a "mutual planetary occultation." Twice during the orbital cycles of Jupiter and Saturn, the equatorial (and satellite) planes of those planets are aligned with earth's orbital plane, resulting in a series of mutual occultations and eclipses between the moons of these giant planets. These orbital alignments have also occurred artificially when unmanned spacecraft have traversed these planetary systems, resulting in great photographs. The terms "eclipse," "occultation" and "transit" are also used to describe these events. A satellite of Jupiter (for example) may be eclipsed (i.e. made dimmer because it moves into Jupiter's shadow), occulted (i.e. hidden from view because Jupiter lies on our line of sight), or may transit (i.e. pass in front of) Jupiter's disk. It is possible that the moon or another celestial body can occult multiple celestial bodies at the same time. Such events are extremely rare and can be seen only from a small part of the world. The last event of such type was on April 23rd, 1998 when the moon occulted Venus and Jupiter simultaneously for observers on Ascension Island. In rare cases, one planet can transit in front of another. The next time this will happen (as seen from Earth) will be on November 22, 2065 at about 12:43 UTC, when Venus near superior conjunction (with an angular diameter of 10.6") will transit in front of Jupiter (with an angular diameter of 30.9"); however, this will take place only 8 west of the Sun, and will therefore not be visible to the unaided/unprotected eye. When the nearer object has a larger angular diameter than the farther object, thus covering it completely, the event is not a transit but an occultation. Before transiting Jupiter, Venus will occult Jupiter's moon Ganymede at around 11:24 UTC as seen from some southernmost parts of Earth. Parallax will cause actual observed times to vary by a few minutes, depending on the precise location of the observer. There are only 18 mutual planetary transits and occultations as seen from Earth between 1700 and 2200. Note the long break of events between 1818 and 2065. 19 September 1702 - Jupiter occults Neptune 20 July 1705 - Mercury transits Jupiter 14 July 1708 - Mercury occults Uranus 4 October 1708 - Mercury transits Jupiter 28 May 1737 - Venus occults Mercury 29 August 1771 - Venus transits Saturn 21 July 1793 - Mercury occults Uranus 9 December 1808 - Mercury transits Saturn 3 January 1818 - Venus transits Jupiter 22 November 2065 - Venus transits Jupiter 15 July 2067 - Mercury occults Neptune 11 August 2079 - Mercury occults Mars 27 October 2088 - Mercury transits Jupiter 7 April 2094 - Mercury transits Jupiter 21 August 2104 - Venus occults Neptune 14 September 2123 - Venus transits Jupiter 29 July 2126 - Mercury occults Mars 3 December 2133 - Venus occults Mercury The 1737 event was observed by John Bevis at Greenwich Observatory - it is the only detailed account of a mutual planetary occultation. A transit of Mars across Jupiter on 12 Sep 1170 was observed by the monk Gervase at Canterbury, and by Chinese astronomers. In addition, an occultation of Mars by Venus was observed by M. Mstlin at Heidelberg on October 3, 1590. LINK: http://www.wikipedia.org/