Occultation

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In this July 1997 still frame captured from video, the bright star Aldebaran has just reappeared on the dark limb of the waning crescent moon in this predawn occultation. Occultation.jpg
In this July 1997 still frame captured from video, the bright star Aldebaran has just reappeared on the dark limb of the waning crescent moon in this predawn occultation.

An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The term is often used in astronomy, 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) when foreground objects obscure distant objects dynamically, as the scene changes over time.

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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.

This 17 April 2021 time-lapse video of Moon shows Mars reappearing on the bright limb of the crescent moon after the occultation of mars by moon. Moon-Mars Occultation 2021.gif
This 17 April 2021 time-lapse video of Moon shows Mars reappearing on the bright limb of the crescent moon after the occultation of mars by moon.

Occultations by the Moon

Occultation of a star 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 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 of at most 0.057 arcseconds 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) meaning any star with an ecliptic latitude between –6.6 and +6.6 degrees may be occulted by it. [1] Three first magnitude stars appear well within that band – Regulus, Spica and Antares – meaning they may be occulted by the Moon or by planets. [2] 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. Saturn Occultation.gif
Occultation of the planet Saturn by the Moon on 3 November 2001.
Jupiter (the bright object in the upper right) a few minutes before being occulted by the Moon on 16 June 2005 OccultJupiter 6-7-2005 6-07PM Nelson NZ.jpg
Jupiter (the bright object in the upper right) a few minutes before being occulted by the Moon on 16 June 2005

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. Some angular diameters of stars have been measured by timing of lunar occultations, which is useful for determining effective temperatures 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 with the optical quasar and its jet, [3] and a fundamental prerequisite for Maarten Schmidt's discovery of the cosmological nature of quasars.

Several times during the year the Moon can be seen occulting a planet. [4] 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

A grazing occultation of Rhea by Dione, two moons of Saturn, imaged by Cassini-Huygens Dione Rhea Occultation.jpg
A grazing occultation of Rhea by Dione, two moons of Saturn, imaged by Cassini–Huygens

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. [2] 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 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. [5] 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, [6] 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. 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. [7]

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. [8] 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 2002 TX300 , Ixion 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 curves of small bodis 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) can often be derived.

Examples

Notable occultations of asteroids
NameChordsMeasured
profile (km)
704 Interamnia 35350×304
39 Laetitia ~16219×142
94 Aurora 9225×173
375 Ursula 6216±10
444 Gyptis 6179×150
48 Doris 4278×142

Asteroids

  • On 29 May 1983, 2 Pallas occulted the naked-eye bright spectroscopic binary star 1 Vulpeculae 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 by Galileo in October 1991). [9]
  • 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. [10]
  • 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 being determined. [11] The star Regulus was occulted by the asteroid 163 Erigone in the early morning of 20 March 2014. [12] This was the brightest occultation of an asteroid ever predicted to occur over a populated area. As the main belt asteroid passed in front of the star its 100 km (60 mi) 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. [12] [13] Bad weather obscured the occultation. [14]
This animation shows the path of the shadow of the dwarf planet Makemake during an occultation of a faint star in April 2011. Note: the actual shape of the shadow on Earth will not be exactly round as shown here. This video is to illustrate the phenomenon.

Distant objects

  • Preliminary results of a 6 November 2010 occultation by the dwarf planet Eris of a magnitude 17 star in the constellation of Cetus placed an upper limit on Eris's diameter of 2320 km, making it almost the same size as Pluto. [15] 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 was observed in a stellar transit on 21 January 2017, identifying a ring. [16]
  • On 3 June 2017, a star was occulted by the Kuiper belt object 486958 Arrokoth, the first such occultation detected. [17] 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. [18]

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 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.

Artificial occultations

The Big Occulting Steerable Satellite (BOSS) was 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. [19]

The proposed satellite would have a dimension of 70 by 70 metres (230 ft × 230 ft), 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. [20]

See also

Related Research Articles

Eclipse Astronomical event where one body is hidden by another

An eclipse is an astronomical event that occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three celestial objects is known as a syzygy. Apart from syzygy, the term eclipse is also used when a spacecraft reaches a position where it can observe two celestial bodies so aligned. An eclipse is the result of either an occultation or a transit.

Planet Astronomical object

A planet is a large astronomical body that is neither a star nor a stellar remnant. There are competing scientific definitions of a 'planet'. In the dynamicist definition adopted by the International Astronomical Union (IAU), a planet is a non-stellar body that is massive enough to be rounded by its own gravity, that directly orbits a star, and that has cleared its orbital zone of competing objects. The IAU has also declared that there are eight planets in the Solar System, independently of the formal definition. In the geological definition used by most planetologists, a planet is a rounded sub-stellar body, possibly a satellite. In addition to the eight Solar planets accepted by the IAU, these include dwarf planets such as Eris and Pluto and planetary-mass moons. Bodies meeting the geological definition are sometimes called "planetary-mass objects" or "planemos" for short.

Conjunction (astronomy) When two astronomical objects have the same right ascension or the same ecliptic longitude

In astronomy, a conjunction occurs when two astronomical objects or spacecraft have either the same right ascension or the same ecliptic longitude, usually as observed from Earth. The astronomical symbol for conjunction is ☌ and handwritten . The conjunction symbol is not used in modern astronomy. It continues to be used in astrology.

Timeline of Solar System astronomy Timeline of the history of Solar System astronomy

The following is a timeline of Solar System astronomy.

Deimos (moon) Smaller, outer moon of Mars

Deimos is the smaller and outermost of the two natural satellites of Mars, the other being Phobos. Deimos has a mean radius of 6.2 km (3.9 mi) and takes 30.3 hours to orbit Mars. Deimos is 23,460 km (14,580 mi) from Mars, much farther than Mars's other moon, Phobos. It is named after Deimos, the Ancient Greek god and personification of dread and terror, and who is also a son of Ares and Aphrodite and the twin brother of Phobos.

Regulus Star in the constellation Leo

Regulus, designated α Leonis, is the brightest object in the constellation Leo and one of the brightest stars in the night sky, lying approximately 79 light years from the Sun. Regulus appears singular, but is actually a quadruple star system composed of four stars that are organized into two pairs. The spectroscopic binary Regulus A consists of a blue-white main-sequence star and its companion, which has not yet been directly observed, but is probably a white dwarf. HD 87884 is separated from Regulus by 176″ and is itself a close pair. Regulus, along with five slightly dimmer stars have collectively been called 'the Sickle', which is an asterism that marks the head of Leo.

Minor-planet moon Natural satellite of a minor planet

A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. As of January 2022, there are 457 minor planets known or suspected to have moons. Discoveries of minor-planet moons are important because the determination of their orbits provides estimates on the mass and density of the primary, allowing insights of their physical properties that is generally not otherwise possible.

Transit (astronomy) Term in astronomy

In astronomy, a 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 the larger body, covering a small portion of it.

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 this common center of mass is not located within the interior of either object, in order to exclude the typical planet–satellite systems and planetary systems.

Extraterrestrial sky Extraterrestrial view of outer space

In astronomy, an extraterrestrial sky is a view of outer space from the surface of an astronomical body other than Earth.

A transit of a minor planet takes place when a minor planet passes directly between an observer and another heavenly body, obscuring a small part of that body's disc. From the perspective of observers on Earth, transits of the Sun and Moon by minor planets are very rare, as the minor planets orbiting between the Earth and those bodies are few and very small. Transits of the Sun would be more visible from the outer planets.

Moons of Mars Natural satellites orbiting Mars

The two moons of Mars are Phobos and Deimos. They are irregular in shape. Both were discovered by American astronomer Asaph Hall in August 1877 and are named after the Greek mythological twin characters Phobos (fear) and Deimos who accompanied their father Ares into battle. Ares, god of war, was known to the Romans as Mars.

Astronomy on Mars

In many cases astronomical phenomena viewed from the planet Mars are the same or similar to those seen from Earth but sometimes they can be quite different. For example, because the atmosphere of Mars does not contain an ozone layer, it is also possible to make UV observations from the surface of Mars.

Exomoon Moon beyond the Solar System

An exomoon or extrasolar moon is a natural satellite that orbits an exoplanet or other non-stellar extrasolar body.

Solar eclipses on Jupiter When moons of Jupiter pass before the Sun

Solar eclipses on Jupiter occur when any of the natural satellites of Jupiter pass in front of the Sun as seen from the planet Jupiter.

Lunar observation Methods and instruments used to observe the moon

The Moon is the largest natural satellite of and the closest major astronomical object to Earth. The Moon may be observed by using a variety of optical instruments, ranging from the naked eye to large telescopes. The Moon is the only celestial body upon which surface features can be discerned with the unaided eyes of most people.

Syzygy (astronomy) Configuration of celestial bodies

In astronomy, a syzygy is a roughly straight-line configuration of three or more celestial bodies in a gravitational system.

This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy features an extensive vocabulary and a significant amount of jargon.

The word "transit" refers to cases where the nearer object appears smaller than the more distant object. Cases where the nearer object appears larger and completely hides the more distant object are known as occultations.

References

  1. ↑ Jean Meeus, "La périodicité des occultations", Ciel et Terre, 87 (1971), 240–252 ADS link – English translation in: Jean Meeus, "Series of occultations", in: Mathematical Astronomy Morsels (Richmond: Willmann-Bell, 1997), pp. 113–124.
  2. 1 2 "Occultations of bright stars by planets between 0 and 4000" . Retrieved 16 June 2005.
  3. ↑ Hazard, C.; Mackey, M. B.; Shimmins, A. J. (1963). "Investigation of the Radio Source 3C273 by the method of Lunar Occultations". Nature. 197 (4872): 1037. Bibcode:1963Natur.197.1037H. doi:10.1038/1971037a0. S2CID   4270661.
  4. ↑ The International Occultation Timing Association (IOTA). "Bright Planet & Asteroid Occultations by the Moon for 2021" . Retrieved 10 November 2021.
  5. ↑ Albers, Steven, Mutual Occultations of Planets: 1557-2230, Sky and Telescope, March 1979
  6. ↑ "Archived copy". Archived from the original on 13 August 2012. Retrieved 9 August 2012.{{cite web}}: CS1 maint: archived copy as title (link)
  7. ↑ Bob King (20 December 2020). "Jupiter and Saturn Embrace in Solstice Conjunction". Sky & Telescope.
  8. ↑ "Stellar Occultations". MIT Planetary Astronomy Lab. 20 December 2007. Retrieved 26 October 2009.
  9. ↑ Dunham, David W.; (45 more authors) (1990). "The size and shape of (2) Pallas from the 1983 occultation of 1 Vulpecuale". Astronomical Journal . 99 (5): 1636. Bibcode:1990AJ.....99.1636D. doi:10.1086/115446.
  10. ↑ Preston, Steve. "Asteroid Occultation Updates" . Retrieved 25 February 2009.
  11. ↑ "1998 European Asteroidal Occultation Results". euraster.net (a website for Asteroidal Occultation Observers in Europe). 21 March 1998. Retrieved 1 December 2008. (Chords) Archived 23 July 2011 at the Wayback Machine
  12. 1 2 Dunham, David (2006). "The International Occultation Timing Association 24th Annual Meeting at Mt. Cuba Observatory, Greenville, Delaware". International Occultation Timing Association. Archived from the original on 18 July 2011. Retrieved 13 February 2011.
  13. ↑ Vitagliano, Aldo (2010). "The Solex Page". Università degli Studi di Napoli Federico II. Archived from the original on 18 September 2015. Retrieved 13 February 2011.
  14. ↑ "Volunteer observers invited to time the March 20, 2014 Occultation of Regulus".
  15. ↑ Brown, Mike (2010). "The shadowy hand of Eris". Mike Brown's Planets. Retrieved 7 November 2010.
  16. ↑ Ortiz, J.L.; (91 more authors) (12 October 2017). "The size, shape, density and ring of the dwarf planet Haumea from a stellar occultation". Nature. 550 (7675): 219–223. arXiv: 2006.03113 . Bibcode:2017Natur.550..219O. doi:10.1038/nature24051. hdl: 10045/70230 . PMID   29022593. S2CID   205260767.
  17. ↑ "New Mysteries Surround New Horizons' Next Flyby Target: NASA's New Horizons spacecraft doesn't zoom past its next science target until New Year's Day 2019, but the Kuiper Belt object, known as 2014 MU69, is already revealing surprises". NASA. 5 July 2017.
  18. ↑ "NASA's New Horizons Team Strikes Gold in Argentina". New Horizons: NASA's Mission to Pluto. 19 July 2017.
  19. ↑ Copi, C. J.; Starkman, G. D. (2000). "The Big Occulting Steerable Satellite (BOSS)". The Astrophysical Journal. 532 (1): 581–592. arXiv: astro-ph/9904413 . Bibcode:2000ApJ...532..581C. doi:10.1086/308525. S2CID   18790887.
  20. ↑ "The X-ray Occulting Steerable Satellite (XOSS)". CASE. Archived from the original on 19 July 2011. Retrieved 9 February 2007.

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