Occultation

In Islam the occultation is the name given to the disappearance of the Twelfth Imam. See also occult (disambiguation). It is also the devil

An occultation is an astronomical event that occurs when one celestial object is hidden by another celestial object that passes between it and the observer. Compared to astronomical transits and eclipses, an occultation is said to occur when the nearer object appears larger and completely hides the more distant object. In contrast, the word transit refers to cases where the nearer object appears considerably 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 terms are the visible effects of a syzygy.

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. 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. However, in the far future, occultations of Aldebaran will be possible, as they were possible in the far past. Occultations of Pollux by planets are not possible at the present, as they pass Pollux to the south. In the far future and in the far past, such occultations were possible. Occultations of Pollux by the moon were also possible in the far past. They seem not possible any more.

Within a mile or two 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 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.

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 2-3, 1989, Saturn passed in front of the 5th magnitude star 28 Sagitarii.

It is also possible for one planet to occult another planet. However, these mutual occultation of planets are extremely rare. This 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.

BOSS

The Big Occulting Steerable Satellite is a proposed satellite that would work in conjunction with a telescope to detect planets 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.

The currently-planned satellite would have a dimension of 70m × 70m 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 is expected to block more than 99.998% of the starlight. The satellite would have a mass of about 600kg.

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.

Occultations of bright stars (<4 mag) and planets by planets between 1900 and 2100

These events are not everywhere visible, where the occulting body and the occulted body are above the skyline. Some events are barely visible, because they took place in close vicinity of the sun.

See also

• Transit
• Transit of Mercury
• Transit of Venus
• Asteroid occultation

External links

• Internationl Occultation Timing Association (IOTA) (http://lunar-occultations.com/iota/iotandx.htm)
• Occultations of Regulus by planets (in German) (http://www.marco-peuschel.de/Regulusbedeckungen%20durch%20die%20gro%DFen%20Planeten.pdf)
• Occultations of stars by planets (in German) (http://www.marco-peuschel.de/planetenundregulus.htm)
• Occultations of planets by other planets (in German) (http://www.marco-peuschel.de/planetenplaneten.html)
• Big Occulting Steerable Satellite (http://boss.phys.cwru.edu/)fr:Occultation

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