Occultation

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Occultations

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 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 Moon's orbit), 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 curently possible. However, in the far future, occultations of Aldebaran and Pollux will be possible, as they were in the far past.

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.

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 occulted stars in 1988, 2002, and 2006, allowing its 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).

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 X-Ray Occulting Steerable Satellite (XOSS) 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 70 m × 70 m 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 600 kg.

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 between 1800 and 2100

This table lists occultations of bright stars and planets by solar planets.

Day	Time (UT)	Occulting planet	Occulted object
December 9, 1802	7:36	Mercury	Acrab
December 9, 1808	20:34	Mercury	Saturn
December 22, 1810	6:32	Venus	Xi-2 Sagittarii
January 3, 1818	21:52	Venus	Jupiter
July 11, 1825	9:10	Venus	Delta-1 Tauri
July 11, 1837	12:50	Mercury	Eta Geminorum
May 9, 1841	19:35	Venus	17 Tauri
September 27, 1843	18:00	Venus	Eta Virginis
December 16, 1850	11:28	Mercury	Lambda Sagittarii
May 22, 1855	5:04	Venus	Epsilon Geminorum
June 30, 1857	0:25	Saturn	Delta Geminorum
December 5th, 1865	14:20	Mercury	Lambda Sagittarii
February 28, 1876	5:13	Jupiter	Acrab
June 7, 1881	20:54	Mercury	Epsilon Geminorum
December 9, 1906	17:40	Venus	Acrab
July 27, 1910	2:53	Venus	Eta Geminorum
June 10, 1940	2:21	Mercury	Epsilon Geminorum
October 25, 1947	1:45	Venus	Zuben-el-genubi
July 7, 1959	14:30	Venus	Regulus
September 27, 1965	15:31	Mercury	Eta Virginis
May 13, 1971	20:00	Jupiter	Beta Scorpii (both components)
April 8, 1976	1:00	Mars	Epsilon Geminorum
November 17, 1981	14:27	Venus	Nunki
November 19, 1984	1:32	Venus	Lambda Sagittarii
February 17, 2035	15:19	Venus	Pi Sagittarii
October 1, 2044	22:00	Venus	Regulus
February 23, 2046	19:24	Venus	Rho-1 Sagittarii
November 10, 2052	7:20	Mercury	Zuben-el-genubi
November 22, 2065	12:45	Venus	Jupiter
July 15, 2067	11:56	Mercury	Neptune
October 3, 2078	22:00	Mars	Theta Ophiuchi
August 11, 2079	1:30	Mercury	Mars
October 27, 2088	13:43	Mercury	Jupiter
April 7, 2094	10:48	Mercury	Jupiter

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 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 Sep 1702 - Jupiter occults Neptune
20 Jul 1705 - Mercury transits Jupiter
14 Jul 1708 - Mercury occults Uranus

04 Oct 1708 - Mercury transits Jupiter
28 May 1737 - Venus occults Mercury
29 Aug 1771 - Venus transits Saturn
21 Jul 1793 - Mercury occults Uranus
09 Dec 1808 - Mercury transits Saturn
03 Jan 1818 - Venus transits Jupiter
22 Nov 2065 - Venus transits Jupiter
15 Jul 2067 - Mercury occults Neptune
11 Aug 2079 - Mercury occults Mars
27 Oct 2088 - Mercury transits Jupiter
07 Apr 2094 - Mercury transits Jupiter
21 Aug 2104 - Venus occults Neptune
14 Sep 2123 - Venus transits Jupiter
29 Jul 2126 - Mercury occults Mars
03 Dec 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. Möstlin at Heidelberg on October 3 1590.

External links

External references

Meeus, Jean: Astronomical Tables of the Sun, Moon and Planets. Richmond, Virginia: Willmann-Bell, Inc., 1995, ISBN 0-943396-45-X.

Astronomical events

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