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 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 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.
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.
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).
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.
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.
This table lists occultations and transits of bright stars and planets by solar planets.
|Day||Time (UT)||Foreground planet||Background object||Elongation|
|December 9, 1802||07:36||Mercury||Acrab||16,2° West|
|December 9, 1808||20:34||Mercury||Saturn||20,3° West|
|December 22, 1810||06:32||Venus||Xi-2 Sagittarii||11,1° East|
|January 3, 1818||21:52||Venus||Jupiter||16,5° West|
|July 11, 1825||09:10||Venus||Delta-1 Tauri||44,4° West|
|July 11, 1837||12:50||Mercury||Eta Geminorum||17,8° West|
|May 9, 1841||19:35||Venus||17 Tauri||9,2° East|
|September 27, 1843||18:00||Venus||Eta Virginis||3,2° West|
|December 16, 1850||11:28||Mercury||Lambda Sagittarii||10,2° East|
|May 22, 1855||05:04||Venus||Epsilon Geminorum||37,4° East|
|June 30, 1857||00:25||Saturn||Delta Geminorum||8,4° East|
|December 5, 1865||14:20||Mercury||Lambda Sagittarii||21,0° East|
|February 28, 1876||05:13||Jupiter||Acrab||97,6° West|
|June 7, 1881||20:54||Mercury||Epsilon Geminorum||21,2° East|
|December 9, 1906||17:40||Venus||Acrab||14,9° West|
|July 27, 1910||02:53||Venus||Eta Geminorum||31,0° West|
|December 16, 1937||18:38||Mercury||Omicron Sagittarii||11,6° East|
|June 10, 1940||02:21||Mercury||Epsilon Geminorum||20,1° East|
|October 25, 1947||01:45||Venus||Zuben-el-genubi||13,5° East|
|July 7, 1959||14:30||Venus||Regulus||44,5° East|
|September 27, 1965||15:30||Mercury||Eta Virginis||2.6° West|
|May 13, 1971||20:00||Jupiter||Beta Scorpii (both components)||169,5° West|
|April 8, 1976||01:00||Mars||Epsilon Geminorum||81,3° East|
|November 17, 1981||14:27||Venus||Nunki||47,0° East|
|November 19, 1984||01:32||Venus||Lambda Sagittarii||39,2° East|
|December 4, 2015||16.14||Mercury||Theta Ophiuchi||9,6° East|
|February 17, 2035||15:19||Venus||Pi Sagittarii||42,1° West|
|October 1, 2044||22:00||Venus||Regulus||38,9° West|
|February 23, 2046||19:24||Venus||Rho-1 Sagittarii||45,4° West|
|November 10, 2052||07:20||Mercury||Zuben-el-genubi||2,8° West|
|November 22, 2065||12:45||Venus||Jupiter||7,9° West|
|July 15, 2067||11:56||Mercury||Neptune||18,4° West|
|August 10, 2069||20.25||Venus||Zavijava||38,4° East|
|October 3, 2078||22:00||Mars||Theta Ophiuchi||71,4° East|
|August 11, 2079||01:30||Mercury||Mars||11,3° West|
|October 27, 2088||13:43||Mercury||Jupiter||4,7° West|
|April 7, 2094||10:48||Mercury||Jupiter||1,8° West|
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.
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.
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.