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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 (see below) and can also be used in a general (non-astronomical) sense to describe when an object in the foreground Occult s (covers up) objects in the background. In the general sense, occultation applies to the visual scene from low-flying aircraft and in Computer-Generated Image ( CGI ) technology, where foreground objects obscure distant ones in a dynamic way as the scene changes. Astronomical events. These include Transit s and Eclipse s. The word Transit refers to cases where the nearer object appears smaller in apparent size than the more distant object, such as transit of Mercury or Venus across the Sun 's disk. The word Eclipse generally refers to those instances in which one object moves into the shadow of another. Each of these three events is the visible effect of a 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 (i) an ''occultation'' of the Sun by the Moon, as seen from Earth, and (ii) 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''. OCCULTATIONS BY THE MOON 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 (see Orbit Of The Moon ), 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 Binaries . 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. On the 20th of January 2007, an occultation of Venus by the Moon was seen on the southern tip of Africa. On 3rd of February 2007, an occultation of Saturn by the Moon was seen. OCCULTATION BY PLANETS 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 Sagitarii . 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." (See Transit for a list of past and future events). , a moon of Saturn , by another moon, Dione ]] 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 photographs such as the one shown here. 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. DOUBLE OCCULTATIONS 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. OCCULTING SATELLITES The ''Big Occulting Steerable Satellite (BOSS)'' was a proposed Satellite that would work in conjunction with a Telescope to detect Planet s around distant stars. The satellite consists of a large, very lightweight sheet, and a set of maneuvering thrusters and navigation systems. It would maneuver to a position along the line of sight between the telescope and a nearby star. The satellite would thereby block the radiation from the star, permitting the orbiting planets to be observed.1 The proposed satellite would have a dimension of 70 m × 70 m, a mass of about 600 kg, and maneuver by means of an Ion Drive engine in combination with using the sheet as a light sail. Positioned at a distance of 100,000 km from the telescope, it would block more than 99.998% of the starlight. There are two possible configurations of this satellite. The first would work with a Space Telescope , most likely positioned near the Earth 's L2 Lagrangian Point . The second would place the satellite in a highly elliptical orbit about the Earth, and work in conjunction with a ground telescope. At the Apogee of the orbit, the satellite would remain relatively stationary with respect to the ground, allowing longer exposure times. An updated version of this design is called the Starshade , which uses a Sunflower -shaped Coronagraph disc. A comparable proposal was also made for a satellite to occult bright X-ray sources, called an ''X-ray Occulting Steerable Satellite'' or XOSS.2 OCCULTATIONS AND TRANSITS BETWEEN 1800 AND 2100 This table lists occultations and transits of bright stars and planets by solar planets. These events are not visible everywhere the occulting body and the occulted body are above the skyline. Some events are barely visible, because they take place in close proximity to the Sun. MUTUAL PLANETARY TRANSITS AND OCCULTATIONS In rare cases, one planet can transit in front of another. The next time this will happen (as seen from Earth) will be on 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!
transiting Jupiter , as it did on January 3, 1818.]]
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. Möstlin at Heidelberg on October 3 1590 . SEE ALSO
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