An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in
astronomy
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, galax ...
, but can also refer to any situation in which an object in the foreground blocks from view (occults) an object in the background. In this general sense, occultation applies to the visual scene observed from low-flying aircraft (or
computer-generated imagery
Computer-generated imagery (CGI) is the use of computer graphics to create or contribute to images in art, printed media, video games, simulators, and visual effects in films, television programs, shorts, commercials, and videos. The image ...
) when foreground objects obscure distant objects dynamically, as the scene changes over time.
If the closer body does not entirely conceal the farther one, the event is called a ''
transit''. Both transit and occultation may be referred to generally as ''occlusion''; and if a shadow is cast onto the observer, it is called an
eclipse.
The symbol for an occultation, and especially a
solar eclipse, is
file:Occultation symbol.svg (U+1F775 🝵).
Occultations by the Moon
The term occultation is most frequently used to describe lunar occultations, those relatively frequent occasions when the
Moon passes in front of a
star
A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth ma ...
during the course of its orbital motion around the Earth. Since the Moon, with an
angular speed with respect to the stars of 0.55
arcsec/s or 2.7 μrad/s, has a very thin atmosphere and stars have an
angular diameter
The angular diameter, angular size, apparent diameter, or apparent size is an angular distance describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, and in optics, it i ...
of at most 0.057
arcsecond
A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The n ...
s or 0.28 μrad, a star that is occulted by the Moon will disappear or reappear in 0.1 seconds or less 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 easier observation and timing.
The Moon's orbit is inclined slightly with respect to the
ecliptic (see
orbit of the Moon
The Moon orbits Earth in the prograde direction and completes one revolution relative to the Vernal Equinox and the stars in about 27.32 days (a tropical month and sidereal month) and one revolution relative to the Sun in about 29.53 days (a ...
) meaning any star with an
ecliptic latitude
The ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions, orbits, and pole orientations of Solar System objects. Because most planets (except Mercury) and many small Solar System bodi ...
between –6.6 and +6.6 degrees may be occulted by it. Three first magnitude stars appear well within that band –
Regulus,
Spica and
Antares
Antares is the brightest star in the constellation of Scorpius. It has the Bayer designation α Scorpii, which is Latinised to Alpha Scorpii. Often referred to as "the heart of the scorpion", Antares is flanked by σ Scorpii and τ S ...
– meaning they may be occulted by the Moon or by planets.
Occultations of
Aldebaran are in this epoch only possible by the Moon, because the planets pass Aldebaran to the north. Neither planetary nor lunar occultations of
Pollux are currently possible, however several thousand years ago lunar occultations were possible. Some notably close
deep-sky objects, such as the
Pleiades, can be occulted by the Moon.
Occultation of the planet Saturn by the Moon on 3 November 2001., left, 239x239px
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
Selenography is the study of the surface and physical features of the Moon (also known as geography of the Moon, or selenodesy). Like geography and areography, selenography is a subdiscipline within the field of planetary science. Historica ...
. Photoelectric analysis of lunar occultations have also discovered some stars to be very close visual or spectroscopic
binaries. Some angular diameters of stars have been measured by timing of lunar occultations, which is useful for determining
effective temperature
The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...
s of those 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. This was crucial for the unambiguous identification of the radio source
3C 273
3C 273 is a quasar located in the constellation of Virgo. It was the first quasar ever to be identified.
It is the optically brightest quasar in the sky from Earth ( m ~12.9), and one of the closest with a redshift, ''z'', of 0.158. A luminosi ...
with the optical quasar and its jet,
and a
fundamental prerequisite for
Maarten Schmidt
Maarten Schmidt (28 December 1929 – 17 September 2022) was a Dutch-born American astronomer who first measured the distances of quasars. He was the first astronomer to identify a quasar, and so was pictured on the March cover of ''Time'' mag ...
's discovery of the cosmological nature of
quasars.
Several times during the year the Moon can be seen 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. The same mechanism can be seen with the Sun, where observers on Earth will view it as a
solar eclipse. Therefore, a total solar eclipse is essentially the Moon occulting the Sun.
Occultation by planets
Stars may also be occulted by planets. Occultations of bright stars are rare. In 1959,
Venus occulted
Regulus, and the next occultation of a bright star (also Regulus by Venus) will be in 2044.
Uranus's
rings were first discovered when that planet occulted a star in 1977. On 3 July 1989,
Saturn passed in front of the 5th magnitude star
28 Sagittarii.
Pluto
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest k ...
occulted stars in 1988, 2002, and 2006, allowing its tenuous atmosphere to be studied via
atmospheric limb sounding.
In rare cases, one planet can pass in front of another. If the nearer planet appears larger than the more distant one, the event is called a mutual planetary occultation. The last occultation or transit occurred on 3 January 1818 and the next will occur on 22 November 2065, in both cases involving the same two planets—
Venus and
Jupiter.
Jupiter rarely occults
Saturn. This is one of the rarest events known, with the next occurrence on February 10, 7541. This event is visible worldwide since the duo would be positioned almost in opposition to the sun, in the border line between the constellations of
Orion and
Taurus
Taurus is Latin for 'bull' and may refer to:
* Taurus (astrology), the astrological sign
* Taurus (constellation), one of the constellations of the zodiac
* Taurus (mythology), one of two Greek mythological characters named Taurus
* ''Bos tauru ...
. In some areas this occultation cannot be seen, but when viewed through even small telescopes, both gas giants appear to be in the same part of view through the eyepiece. The last one occurred in 6857 B.C.E.
Occultations by smaller bodies
A further set of
occultations are those when a
small Solar System body or
dwarf planet passes in front of a star, temporarily blocking its light as seen from Earth.
These occultations are useful for measuring the size and position of body much more precisely than can be done by other means. A cross-sectional profile of the shape of a body can even be determined if a number of observers at different, nearby, locations observe the occultation. Occultations have been used to calculate the diameter of
trans-Neptunian objects such as ,
Ixion
In Greek mythology, Ixion ( ; el, Ἰξίων, ''gen''.: Ἰξίονος means 'strong native') was king of the Lapiths, the most ancient tribe of Thessaly.
Family
Ixion was the son of Ares, or Leonteus, or Antion and Perimele, or the not ...
and
Varuna. Software for coordinating observations is available for download at http://www.occultwatcher.net/
In addition, mutual occultation and eclipsing events can occur between a primary and its
satellite. A
large number of moons have been discovered analyzing the photometric
light curve
In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular freq ...
s of small bodies and detecting a second, superimposed brightness variation, from which an
orbital period for the satellite (secondary), and a secondary-to-primary diameter-ratio (for the
binary system
A binary system is a system of two astronomical bodies which are close enough that their gravitational attraction causes them to orbit each other around a barycenter ''(also see animated examples)''. More restrictive definitions require that thi ...
) can often be derived.
Examples
Asteroids
* On 29 May 1983,
2 Pallas
Pallas (minor-planet designation: 2 Pallas) is the second asteroid to have been discovered, after Ceres. It is believed to have a mineral composition similar to carbonaceous chondrite meteorites, like Ceres, though significantly less hydr ...
occulted the naked-eye bright
spectroscopic binary star
1 Vulpeculae
1 Vulpeculae is a class B4IV (blue subgiant) star in the constellation Vulpecula. Its apparent magnitude is 4.77 and it is approximately 780 light years away based on parallax.
The primary was discovered to be a spectroscopic binary in ...
along a track across the southern United States, northern Mexico, and north parts of the Caribbean. Observations from 130 different locations defined the shape of about two-thirds of the asteroid, and detected the secondary companion of the bright binary star; these observations, taken together with those of a separate occultation by Pallas in 1979, provided a complete figure for the asteroid eight years before any asteroid was visited by a spacecraft (the much smaller
Gaspra
Gaspra ( uk, Гаспра, officially transliterated Haspra; russian: Гаспра; crh, Gaspra, from Greek hàspra, άσπρα, white) is a spa town, an urban-type settlement in Yalta Municipality in the Autonomous Republic of Crimea. It is loc ...
by
Galileo in October 1991).
[
* On 12 March 2009 nine asteroids ( 85 Io, 247 Eukrate, 1585 Union, 201 Penelope, 70 Panopaea, 980 Anacostia, 2448 Sholokhov, 1746 Brouwer, and 191 Kolga) occulted notable magnitude stars, viewed from given places on the Earth.][
* According to the 1998 European Asteroidal Occultation Results from Euraster, 39 Laetitia was observed by over 38 observatories in one occultation on 3 March 1998, which resulted in many ]chords
Chord may refer to:
* Chord (music), an aggregate of musical pitches sounded simultaneously
** Guitar chord a chord played on a guitar, which has a particular tuning
* Chord (geometry), a line segment joining two points on a curve
* Chord (a ...
being determined. The star Regulus was occulted by the asteroid 163 Erigone in the early morning of 20 March 2014. This was the brightest occultation of an asteroid ever predicted to occur over a populated area. As the main belt asteroid
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
passed in front of the star its shadow swept across Nassau and Suffolk counties, all of New York City and the Hudson River Valley, with the center of the shadow path following a line roughly connecting New York City, White Plains, Newburgh, Oneonta, Rome and Pulaski before crossing into Canada near Belleville and North Bay, Ontario
North Bay is a city in Northeastern Ontario, Canada. It is the seat of Nipissing District, Ontario, Nipissing District, and takes its name from its position on the shore of Lake Nipissing. North Bay developed as a railroad centre, and its airpo ...
.[ Bad weather obscured the occultation.
]
Distant objects
* Preliminary results of a 6 November 2010 occultation by the dwarf planet Eris of a magnitude
Magnitude may refer to:
Mathematics
*Euclidean vector, a quantity defined by both its magnitude and its direction
*Magnitude (mathematics), the relative size of an object
*Norm (mathematics), a term for the size or length of a vector
*Order of ...
1
star
in the constellation of Cetus
Cetus () is a constellation, sometimes called 'the whale' in English. The Cetus was a sea monster in Greek mythology which both Perseus and Heracles needed to slay. Cetus is in the region of the sky that contains other water-related constell ...
placed an upper limit on Eris's diameter of 2320 km, making it almost the same size as Pluto. Due to their slower movement through the night sky, occultations by TNOs are far less common than by asteroids in the main-belt.
* The dwarf planet Haumea
, discoverer =
, discovered =
, earliest_precovery_date = March 22, 1955
, mpc_name = (136108) Haumea
, pronounced =
, adjectives = Haumean
, note = yes
, alt_names =
, named_after = Haumea
, mp_category =
, orbit_ref =
, epoc ...
was observed in a stellar transit on 21 January 2017, identifying a ring.[
* On 3 June 2017, a star was occulted by the Kuiper belt object ]486958 Arrokoth
Arrokoth (minor-planet designation 486958 Arrokoth; provisional designation ), formerly nicknamed Ultima Thule, is a trans-Neptunian object located in the Kuiper belt. Arrokoth became the farthest and most primitive object in the Solar System ...
, the first such occultation detected. The multi-faceted campaign involved cooperation from the Argentinian government (including local governments – a major highway was closed for two hours, and street lights were turned off, in order to preclude light pollution), three spacecraft, 24 portable ground-based telescopes, and NASA's SOFIA airborne observatory in "the most challenging stellar occultation in the history of astronomy," in an effort spanning six months.
Double occultations
The Moon or another celestial body can occult multiple celestial bodies at the same time.
Because of its relatively large angular diameter
The angular diameter, angular size, apparent diameter, or apparent size is an angular distance describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, and in optics, it i ...
the Moon, at any given time, occults an indeterminate number of stars and galaxies. However the Moon occulting (obscuring) two bright objects (e.g. two planets or a bright star and a planet) simultaneously is extremely rare and can be seen only from a small part of the world: the last such event was on 23 April 1998 when it occulted Venus and Jupiter for observers on Ascension Island
Ascension Island is an isolated volcanic island, 7°56′ south of the Equator in the South Atlantic Ocean. It is about from the coast of Africa and from the coast of South America. It is governed as part of the British Overseas Territory of ...
.
Artificial occultations
The ''Big Occulting Steerable Satellite (BOSS)'' was a proposed satellite that would work in conjunction with a telescope
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observ ...
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 proposed satellite would have a dimension of , a mass of about 600 kg, and maneuver by means of an ion drive
An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions using electricity.
An ion thruster ionizes a neutral gas by extracting some electrons out of a ...
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
In celestial mechanics, the Lagrange points (; also Lagrangian points or libration points) are points of equilibrium for small-mass objects under the influence of two massive orbiting bodies. Mathematically, this involves the solution of ...
. 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
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any ellip ...
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
The common sunflower (''Helianthus annuus'') is a large annual forb of the genus ''Helianthus'' grown as a crop for its edible oily seeds. Apart from cooking oil production, it is also used as livestock forage (as a meal or a silage plant), as ...
-shaped coronagraph
A coronagraph is a telescopic attachment designed to block out the direct light from a star so that nearby objects – which otherwise would be hidden in the star's bright glare – can be resolved. Most coronagraphs are intended to view ...
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.
See also
* Astronomical transit
In astronomy, a transit (or astronomical transit) is a phenomenon when a celestial body passes directly between a larger body and the observer. As viewed from a particular vantage point, the transiting body appears to move across the face of t ...
(also for occultations of planets by other planets)
* Conjunction
Conjunction may refer to:
* Conjunction (grammar), a part of speech
* Logical conjunction, a mathematical operator
** Conjunction introduction, a rule of inference of propositional logic
* Conjunction (astronomy), in which two astronomical bodies ...
* GPS occultation
* Grazing lunar occultation
* Grazing occultation
* List of minor planets
* List of Solar System objects by size
* Occultations, transits, and eclipses
* Planetary transits and occultations
* Radio occultation
Radio occultation (RO) is a remote sensing technique used for measuring the physical properties of a planetary atmosphere or ring system.
Atmospheric radio occultation
Atmospheric radio occultation relies on the detection of a change in a radio ...
* Solar eclipse
* Syzygy (astronomy)
* TAOS: The Taiwanese-American Occultation Survey
* Transit of Mercury
* Transit of Venus
References
Further reading
*
*
Marco Peuschel
– Astronomische Tabellen für den Mond von 2007 bis 2016, Mondphasen, Apsiden, Knotendurchgänge, Maximale und minimale Deklinationswerte und Sternbedeckungen sowie ausführliche Ephemeriden für jeden Tag des Jahres, inkl. Mondauf-und Untergänge und physische Daten.
External links
Occultation Studies at Williams College
www.stellaroccultations.info
International Occultation Timing Association (IOTA)
wit
Moon, planets, asteroid occultations
www.occultations.net
Mutual occultations (and conjunctions) of planets
TAU astronomy Club
stevealbers.net
www.space.com, August 2006
''The Starry Mirror'', 2007
Asteroid Occultation Updates
www.asteroidoccultation.com, 2018
www.occultations.net
Combining asteroid models derived by lightcurve inversion with asteroidal occultation silhouettes
arXiv, 2011
''Il cielo ed i suoi femomeni''
(software), Durch Occultation Association
(software), www.lunar-occultations.com
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