Centaurus A

Centaurus A or NGC 5128 is a galaxy in the constellation of Centaurus. It was discovered in 1826 by Scottish astronomer James Dunlop from his home in Parramatta, in New South Wales, Australia. There is considerable debate in the literature regarding the galaxy's fundamental properties such as its Hubble type (lenticular galaxy or a giant elliptical galaxy)[9] and distance (10–16 million light-years).[2][3][4][5][6] NGC 5128 is one of the closest radio galaxies to Earth, so its active galactic nucleus has been extensively studied by professional astronomers.[12] The galaxy is also the fifth-brightest in the sky,[12] making it an ideal amateur astronomy target,[13] although the galaxy is only visible from low northern latitudes and the southern hemisphere.

The center of the galaxy contains a supermassive black hole with a mass equivalent to 55 million solar masses,[14] which ejects a relativistic jet that is responsible for emissions in the X-ray and radio wavelengths. By taking radio observations of the jet separated by a decade, astronomers have determined that the inner parts of the jet are moving at about half of the speed of light. X-rays are produced farther out as the jet collides with surrounding gases resulting in the creation of highly energetic particles. The X-ray jets of Centaurus A are thousands of light-years long, while the radio jets are over a million light-years long.[15]

Like other starburst galaxies, a collision is suspected to be responsible for the intense burst of star formation. Models have suggested that Centaurus A was a large elliptical galaxy that collided and merged with a smaller spiral galaxy.[16]

Centaurus A
ESO Centaurus A LABOCA
Centaurus A (NGC 5128)
Observation data (J2000 epoch)
Right ascension13h 25m 27.6s[1]
Declination−43° 01′ 09″[1]
Redshift547 ± 5 km/s[1]
Distance10–16 Mly (3–5 Mpc)[2][3][4][5][6]
Apparent magnitude (V)6.84[7][8]
TypeS0 pec[1] or Ep[9]
Size60,000 ly[10]
Apparent size (V)25′.7 × 20′.0[1]
Notable featuresUnusual dust lane
Other designations
NGC 5128,[1] Arp 153,[1] PGC 46957,[1] 4U 1322–42,[11] Caldwell 77

Observational history

NGC 5128 was discovered on 29 April 1826 by James Dunlop during a survey at the Parramatta Observatory.[17]

In 1847 John Herschel described the galaxy as "two semi-ovals of elliptically formed nebula appearing to be cut asunder and separated by a broad obscure band parallel to the larger axis of the nebula, in the midst of which a faint streak of light parallel to the sides of the cut appears."[18]

In 1949 John Gatenby Bolton, Bruce Slee and Gordon Stanley localized NGC 5128 as one of the first extragalactical radio sources.[19] Five years later, Walter Baade and Rudolph Minkowski suggested that the peculiar structure is the result of a merge event of a giant elliptical galaxy and a small spiral galaxy.[20] The first detection of X-ray emissions, using a sounding rocket, was performed in 1970.[21] In 1975–76 gamma-ray emissions from Centaurus A were observed through the atmospheric Cerenkov technique.[22]

The Einstein Observatory detected an X-ray jet emanating from the nucleus in 1979.[23] Ten years later, young blue stars were found along the central dust band with the Hubble Space Telescope.[24]

The Chandra X-ray Observatory identified in 1999 more than 200 new point sources.[25] Another space telescope, the Spitzer Space Telescope, found a parallelogram-shaped structure of dust in near infrared images of Centaurus A in 2006.[26]

Evidence of gamma emissions with very high energy (more than 100 GeV) was detcted by the H.E.S.S-Observatorium in Namibia in 2009.[27]

The following year, Centaurus A was identified as a source of cosmic rays of highest energies, after years of observations by Pierre Auger Observatory.[28] In 2016 a review of data from Chandra and XMM-Newton, unusual high flares of energy were found in NGC 5128 and the galaxy NGC 4636. Jimmy Erwin of University of Alabama hypothesized the discovery as potentially a black hole in a yet unknown process or an intermediate-mass black hole.[29]


Centaurus A may be described as having a peculiar morphology. As seen from Earth, the galaxy looks like a lenticular or elliptical galaxy with a superimposed dust lane.[30] The peculiarity of this galaxy was first identified in 1847 by John Herschel, and the galaxy was included in Halton Arp's Atlas of Peculiar Galaxies (published in 1966) as one of the best examples of a "disturbed" galaxy with dust absorption.[31] The galaxy's strange morphology is generally recognized as the result of a merger between two smaller galaxies.[32]

Zoom movie of the galaxy Centaurus A, showing different aspects of the galaxy in several wavelengths.
Schematic diagram of the components of the Centaurus A galaxy

The bulge of this galaxy is composed mainly of evolved red stars.[30] The dusty disk, however, has been the site of more recent star formation;[12] over 100 star formation regions have been identified in the disk.[33]


Two supernovae have been detected in Centaurus A.[34] The first supernova, named SN 1986G, was discovered within the dark dust lane of the galaxy by R. Evans in 1986.[35] It was later identified as a Type Ia supernova,[36] which forms when a white dwarf's mass grows large enough to ignite carbon fusion in its center, touching off a runaway thermonuclear reaction, as may happen when a white dwarf in a binary star system strips gas away from the other star. SN 1986G was used to demonstrate that the spectra of type Ia supernovae are not all identical, and that type Ia supernovae may differ in the way that they change in brightness over time.[36]

The second supernova, a type IIb dubbed SN2016adj,[37] was discovered by Backyard Observatory Supernova Search in February 2016.[38]


Distance estimates to Centaurus A established since the 1980s typically range between 3–5 Mpc.[2][3][4][5][6][39] Classical Cepheids discovered in the heavily obscured dust lane of Centaurus A yield a distance between ~3–3.5 Mpc, depending on the nature of the extinction law adopted and other considerations.[4][5] Mira variables[39] and Type II Cepheids[4][5] were also discovered in Centaurus A, the latter being rarely detected beyond the Local Group.[40] The distance to Centaurus A established from several indicators such as Mira variables and planetary nebulae favour a more distant value of ~3.8 Mpc.[9][6]

Nearby galaxies and galaxy group information

Centaurus A is at the center of one of two subgroups within the Centaurus A/M83 Group, a nearby group of galaxies.[41] Messier 83 (the Southern Pinwheel Galaxy) is at the center of the other subgroup. These two groups are sometimes identified as one group[42][43] and sometimes identified as two groups.[44] However, the galaxies around Centaurus A and the galaxies around M83 are physically close to each other, and both subgroups appear not to be moving relative to each other.[45] The Centaurus A/M83 Group is located in the Virgo Supercluster.


Radio waves

Overview over the radio structure of Centaurus A. The whole radio emitting region extends about 1.8 million light years (about 8° degrees in the sky). Through observations with the VLBI technique structures of the jet and the core smaller than a light year could be resolved (corresponding to a resolution of 0.68 x 0.41 milli-arcseconds.)
Overview over the radio structure of Centaurus A. The whole radio emitting region extends about 1.8 million light years (about 8° degrees in the sky). Through observations with the VLBI technique structures of the jet and the core smaller than a light year could be resolved (corresponding to a resolution of 0.68 x 0.41 milli-arcseconds.[46])
CentaurusA VR37 hi
This view of the jets of Centaurus A was created through observations in radio waves with a wavelength of 20 cm with the VLA. The position of the radio jet and the knots within the jets matches very well with the structures seen in the x-ray jet. This region of the jet is named „Inner Lobe“.[47]


Centaurus A (NGC 5128) and its strange globular clusters
Elliptical galaxy Centaurus A and its globular clusters[48]

Centaurus A is located approximately 4° north of Omega Centauri (a globular cluster visible with the naked eye).[13] Because the galaxy has a high surface brightness and relatively large angular size, it is an ideal target for amateur astronomy observations. The bright central bulge and dark dust lane are visible even in finderscopes and large binoculars,[13] and additional structure may be seen in larger telescopes.[13] Centaurus A is visible to the naked eye under exceptionally good conditions.[49]


Centaurus A halo

Centaurus A halo

Radio galaxy Centaurus A by ALMA

The radio galaxy Centaurus A, as seen by ALMA

NGC 5128 galaxy

Image taken by the Wide Field Imager attached to the MPG/ESO 2.2-meter telescope at the La Silla Observatory

Firestorm of Star Birth in Galaxy Centaurus A

"Hubble's panchromatic vision... reveals the vibrant glow of young, blue star clusters..."[50]


A Hubble Space Telescope (HST) image of the dust disk in front of the nucleus of Centaurus A. Credit: HST/NASA/ESA


This image of the central parts of Centaurus A reveals the parallelogram-shaped remains of a smaller galaxy that was absorbed about 200 to 700 million years ago.


The heavily obscured inner (barred?) spiral disk at 24 μm as shown by the Spitzer IR telescope

NGC 5128

Chandra X-ray view of Cen A in X-rays showing one relativistic jet from the central black hole

Centaurus A
Moon centaurus th

A composite image showing the size of the radio glow from the galaxy Centaurus A in comparison to the full Moon

Centauros a-spc

"False-colour image of Centaurus A, showing radio (red), 24-micrometre infrared (green) and 0.5–5 keV X-ray emission (blue)

CentaurusA Center EN

Central part of the galaxy

Video about Centaurus A jets

See also

  • Messier 87 – a giant elliptical galaxy that is also a strong radio source
  • NGC 1316 – a similar lenticular galaxy that is also a strong radio source


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External links

Coordinates: Sky map 13h 25m 27.6s, −43° 01′ 09″


2M1207, 2M1207A or 2MASS J12073346-3932539 is a brown dwarf located in the constellation Centaurus; a companion object, 2M1207b, may be the first extrasolar planetary-mass companion to be directly imaged, and is the first discovered orbiting a brown dwarf.2M1207 was discovered during the course of the 2MASS infrared sky survey: hence the "2M" in its name, followed by its celestial coordinates. With a fairly early (for a brown dwarf) spectral type of M8, it is very young, and probably a member of the TW Hydrae association. Its estimated mass is around 25 Jupiter masses. The companion, 2M1207b, is estimated to have a mass of 3–10 Jupiter masses. Still glowing red hot, it will shrink to a size slightly smaller than Jupiter as it cools over the next few billion years.

An initial photometric estimate for the distance to 2M1207 was 70 parsecs. In December 2005, American astronomer Eric Mamajek reported a more accurate distance (53 ± 6 parsecs) to 2M1207 using the moving cluster method. The new distance gives a fainter luminosity for 2M1207. Recent trigonometric parallax results have confirmed this moving cluster distance, leading to a distance estimate of 53 ± 1 parsec or 172 ± 3 light years.Like classical T Tauri stars, many brown dwarfs are surrounded by disks of gas and dust which accrete onto the brown dwarf. 2M1207 was first suspected to have such a disk because of its broad Hα line. This was later confirmed by ultraviolet spectroscopy. The existence of a dust disk has also been confirmed by infrared observations. In general, accretion from disks is known to produce fast-moving jets, perpendicular to the disk, of ejected material. This has also been observed for 2M1207; an April 2007 paper in the Astrophysical Journal reports that this brown dwarf is spouting jets of material from its poles. The jets, which extend around 109 kilometers into space, were discovered using the Very Large Telescope (VLT) at the European Southern Observatory. Material in the jets streams into space at a few kilometers per second.

Caldwell catalogue

The Caldwell catalogue is an astronomical catalogue of 109 star clusters, nebulae, and galaxies for observation by amateur astronomers. The list was compiled by Patrick Moore as a complement to the Messier catalogue.While the Messier catalogue is used by amateur astronomers as a list of deep-sky objects for observation, Moore noted that Messier's list was not compiled for that purpose and excluded many of the sky's brightest deep-sky objects, such as the Hyades, the Double Cluster (NGC 869 and NGC 884), and the Sculptor Galaxy (NGC 253). The Messier catalogue was actually compiled as a list of known objects that might be confused with comets. Moore also observed that since Messier compiled his list from observations in Paris, it did not include bright deep-sky objects visible in the Southern Hemisphere, such as Omega Centauri, Centaurus A, the Jewel Box, and 47 Tucanae. Moore compiled a list of 109 objects to match the commonly accepted number of Messier objects (he excluded M110), and the list was published in Sky & Telescope in December 1995.Moore used his other surname – Caldwell – to name the list, since the initial of "Moore" is already used for the Messier catalogue. Entries in the catalogue are designated with a "C" and the catalogue number (1 to 109).

Unlike objects in the Messier catalogue, which are listed roughly in the order of discovery by Messier and his colleagues, the Caldwell catalogue is ordered by declination, with C1 being the most northerly and C109 being the most southerly, although two objects (NGC 4244 and the Hyades) are listed out of sequence. Other errors in the original list have since been corrected: it incorrectly identified the S Norma Cluster (NGC 6087) as NGC 6067 and incorrectly labelled the Lambda Centauri Cluster (IC 2944) as the Gamma Centauri Cluster.

Centaurus (disambiguation)

Centaurus is a bright constellation of the southern hemisphere.

Centaurus may also refer to:

Centaurus (Greek mythology), the founder of the Centaur race

Centaurus A, a lenticular galaxy near the constellation

Centaurus Cluster

Centaurus (journal), the journal of the European Society for the History of Science

The Centaurus, a building complex in Islamabad, Pakistan

Bristol Centaurus, an aircraft engine

Centaurus High School, a Colorado school

Centaurus Advisors, a hedge fund run by John D. Arnold

SS Centaurus, a Design 1013 cargo ship

Centaurus A/M83 Group

The Centaurus A/M83 Group is a complex group of galaxies in the constellations Hydra, Centaurus, and Virgo. The group may be roughly divided into two subgroups. The Cen A Subgroup, at a distance of 11.9 Mly (3.66 Mpc), is centered on Centaurus A, a nearby radio galaxy. The M83 Subgroup, at a distance of 14.9 Mly (4.56 Mpc), is centered on the Messier 83 (M83), a face-on spiral galaxy.This group is sometimes identified as one group and sometimes identified as two groups. Hence, some references will refer to two objects named the Centaurus A Group and the M83 Group. However, the galaxies around Centaurus A and the galaxies around M83 are physically close to each other, and both subgroups appear not to be moving relative to each other.The Centaurus A/M83 Group is part of the Virgo Supercluster, the local supercluster of which the Local Group is an outlying member.

Circinus Galaxy

The Circinus Galaxy (ESO 97-G13) is a Seyfert galaxy in the constellation of Circinus. It is located 4 degrees below the Galactic plane, and, at a distance of 4.0 Mpc (13 Mly), and is one of the closest galaxies to the Milky Way. The galaxy is undergoing tumultuous changes, as rings of gas are likely being ejected from the galaxy. Its outermost ring is 1400 light-years across while the inner ring is 260 light-years across. Although the Circinus galaxy can be seen using a small telescope, it was not noticed until 1977 because it lies close to the plane of the Milky Way and is obscured by galactic dust. The Circinus Galaxy is a Type II Seyfert galaxy and is one of the closest known active galaxies to the Milky Way, though it is probably slightly farther away than Centaurus A.

Circinus Galaxy produced supernova SN 1996cr, which was identified over a decade after it exploded. This supernova event was first observed during 2001 as a bright, variable object in a Chandra X-ray Observatory image, but it was not confirmed as a supernova until years later.

The Circinus Galaxy is one of twelve large galaxies in the "Council of Giants" surrounding the Local Group in the Local Sheet.

Dark globular cluster

Dark globular cluster is a proposed type of globular star clusters that has an unusually high mass for the number of stars within it. Proposed in 2015 on the basis of observational data, dark globular clusters are believed to be populated by objects with significant dark matter components, such as central massive black holes.


The HI Parkes All Sky Survey (HIPASS) is a large survey for neutral atomic hydrogen (HI). Most of the data was taken between 1997 and 2002 using CSIRO's 64-m Parkes Telescope. HIPASS covered 71% of the sky and identified more than 5000 galaxies; the major galaxy catalogs are: the "HIPASS Bright Galaxy Catalog" (HIPASS BGC), the southern HIPASS catalog (HICAT), and the northern HIPASS catalog (NHICAT) Discoveries include over 5000 galaxies (incl. several new galaxies), the Leading Arm of the Magellanic Stream and a few gas clouds devoid of stars.

List of galaxy groups and clusters

This page lists some galaxy groups and galaxy clusters.

Defining the limits of galaxy clusters is imprecise as many clusters are still forming. In particular, clusters close to the Milky Way tend to be classified as galaxy clusters even when they are much smaller than more distant clusters.

List of nearest galaxies

This is a list of known galaxies within 3.59 megaparsecs (11.7 million light-years) of the Solar System, in ascending order of distance.

This encompasses all of the about 50 Local Group galaxies, and some that are members of neighboring galaxy groups, the M81 Group and the Centaurus A/M83 Group, and some that are currently not in any defined galaxy group.

The list aims to reflect current knowledge: not all galaxies within the 3.59 Mpc radius have been discovered. Nearby dwarf galaxies are still being discovered, and galaxies located behind the central plane of the Milky Way are extremely difficult to discern. It is possible for any galaxy to mask another located beyond it.

Intergalactic distance measurements are subject to large uncertainties. Figures listed are composites of many measurements, some of which may have had their individual error bars tightened to the point of no longer overlapping with each other.

Local Sheet

The Local Sheet in astronomy is a nearby extragalactic region of space where the Milky Way, the members of the Local Group and other galaxies share a similar peculiar velocity. This region lies within a radius of about 7 Mpc (23 Mly), 0.46 Mpc (1.5 Mly) thick, and galaxies beyond that distance show markedly different velocities. The Local Group has only a relatively small peculiar velocity of 66 km⋅s−1 with respect to the Local Sheet. Typical velocity dispersion of galaxies is only 40 km⋅s−1 in the radial direction. Nearly all nearby bright galaxies belong to the Local Sheet. The Local Sheet is part of the Local Volume and is in the Virgo Supercluster (Local Supercluster). The Local Sheet forms a wall of galaxies delineating one boundary of the Local Void.A significant component of the mean velocity of the galaxies in the Local Sheet appears as the result of the gravitational attraction of the Virgo Cluster of galaxies, resulting in a peculiar motion ~185 km⋅s−1 toward the cluster. A second component is directed away from the center of the Local Void; an expanding region of space spanning an estimated 45 Mpc (150 Mly) that is only sparsely populated with galaxies. This component has a velocity of 259 km⋅s−1. The Local Sheet is inclined 8° from the Local Supercluster (Virgo Supercluster).The so-called Council of Giants is a ring of twelve large galaxies surrounding the Local Group in the Local Sheet, with a radius of 3.75 Mpc (12.2 Mly). Ten of these are spirals, while the remaining two are ellipticals. The two ellipticals (Maffei 1 and Centaurus A) lie on opposite sides of the Local Group, and their formation may have prompted the development of the Local Group. The Local Sheet's own development outlines a concentration of dark matter in a filament.

* The mass is given as the logarithm of the mass in solar masses.

Messier 83

Messier 83 or M83, also known as the Southern Pinwheel Galaxy and NGC 5236, is a barred spiral galaxy approximately 15 million light-years away in the constellation Hydra. Nicolas Louis de Lacaille discovered M83 on February 23, 1752 at the Cape of Good Hope. Charles Messier added it to his catalogue of nebulous objects (now known as the Messier Catalogue) in March 1781. This is one of the closest and brightest barred spiral galaxies in the sky, making it visible with binoculars. Its nickname of the Southern Pinwheel derives from its resemblance to the Pinwheel Galaxy (M101).

This is a massive, grand design spiral galaxy.

The morphological classification of NGC 5236 in the De Vaucouleurs system is SAB(s)c, where the 'SAB' denotes a weak-barred spiral, '(s)' indicates a pure spiral structure with no ring, and 'c' means the spiral arms are loosely wound. The peculiar dwarf galaxy NGC 5253 lies near M83, and the two likely interacted within the last billion years resulting in starburst activity in their central regions.The star formation rate in M83 is higher along the leading edge of the spiral arms, as predicted by density wave theory. NASA's Galaxy Evolution Explorer project reported finding large numbers of new stars in the outer reaches of the galaxy – 20 kpc from the center. It had hitherto been thought that these areas lacked the materials necessary for star formation. Six supernovae have been observed in M83: SN 1923A, SN 1945B, SN 1950B, SN 1957D, SN 1968L and SN 1983N.

M83 is at the center of one of two subgroups within the Centaurus A/M83 Group, a nearby galaxy group. Centaurus A is at the center of the other subgroup. These are sometimes identified as one group and sometimes as two. However, the galaxies around Centaurus A and the galaxies around M83 are physically close to each other, and both subgroups appear not to be moving relative to each other.

NGC 4945

NGC 4945 is a barred spiral galaxy in the constellation Centaurus, visible near the star Xi Centauri. The galaxy was discovered by James Dunlop in 1826 and is thought to be similar to the Milky Way Galaxy, although X-ray observations show that NGC 4945 has an unusual energetic Seyfert 2 nucleus that might house a supermassive black hole. This object has an estimated mass of 1.4+1.4−0.7×106 M☉.

NGC 5102

NGC 5102 is a galaxy in the M83 group of galaxies. It was discovered by John Herschel in 1835.

NGC 5253

NGC 5253 is an irregular galaxy in the constellation Centaurus. It was discovered by John Frederick William Herschel on 15 March 1787.

NGC 5264

NGC 5264, also known as DDO 242, is an irregular galaxy in the constellation Hydra. It is part of the M83 subgroup of the Centaurus A/M83 Group, located some 15 million light years (4.5 megaparsecs) away. The galaxy was discovered on 30 March 1835 by John Herschel, and it was described as "very faint, pretty large, round, very little brighter middle" by John Louis Emil Dreyer, the compiler of the New General Catalogue.NGC 5264 was imaged by the Hubble Space Telescope in 2016. The galaxy is relatively small: it is a dwarf galaxy, a type of galaxy much smaller than normal spiral galaxies and elliptical galaxies. In fact, it is only 11000 light years (3300 parsecs) wide at its widest; our own galaxy, Milky Way, in comparison, is about ten times larger. Dwarf galaxies like these usually have about a billion stars. NGC 5264 also is relatively blue-coloured; this is from it interacting with other galaxies, supplying it with gas for star formation.

RCW 79

RCW 79 is an emission nebula in the constellation Centaurus.

A cosmic bubble of gas and dust, RCW 79 has grown to about 70 light-years in diameter, blown by the winds and radiation from hot young stars. Infrared light from the dust embedded in the nebula is tinted red in this gorgeous false-color view from the Spitzer Space Telescope. A good 17 thousand light-years away in the grand southern constellation Centaurus, the expanding nebula itself has triggered star formation as it plows into the gas and dust surrounding it. In fact, this penetrating infrared picture reveals groups of new stars as yellowish points scattered along the bubble's edge. One remarkable group still lies within its own natal bubble at about 7 o'clock (lower left), while another can be seen near the upper gap at about 3 o'clock (right) from the bubble's center.The nebula is estimate to be 2.0 - 2.5 million years old. It surrounds a central cluster of hot massive stars, with the hottest having spectral types of O4-5.NASA's Spitzer Space Telescope easily detects infrared light from the dust particles in RCW 79. The young stars within RCW79 radiate ultraviolet light that excites molecules of dust within the bubble. This causes the dust grains to emit infrared light that is detected by Spitzer and seen here as the extended red features.

Radio galaxy

Radio galaxies and their relatives, radio-loud quasars and blazars, are types of active galaxy nuclei that are very luminous at radio wavelengths, with luminosities up to 1039 W between 10 MHz and 100 GHz. The radio emission is due to the synchrotron process. The observed structure in radio emission is determined by the interaction between twin jets and the external medium, modified by the effects of relativistic beaming. The host galaxies are almost exclusively large elliptical galaxies. Radio-loud active galaxies can be detected at large distances, making them valuable tools for observational cosmology. Recently, much work has been done on the effects of these objects on the intergalactic medium, particularly in galaxy groups and clusters.

SN 1986G

SN 1986G was a supernova that was observed on May 3, 1986 by Robert Evans. Its host galaxy, Centaurus A, is about 15 million light-years away in the constellation Centaurus. Since Centaurus A is about 15 million light-years away from us, this supernova happened 15 million years ago.

SN 1986G was a bright blue-green star in the middle of the left part of the dust belt of Centaurus A. The blue-green color occurs because David Malin could take the red plate used in this composite image only one year after the supernova occurred, and it had faded away at that time.

Sea interferometry

Sea interferometry, also known as Sea-cliff interferometry, is a form of radio astronomy that uses radio waves reflected off the sea to produce an interference pattern. It is the radio wave analogue to Lloyd's mirror. The technique was invented and exploited in Australia between 1945 and 1948.A radio detecting aerial is placed on top of a cliff, which detects electromagnetic waves coming directly from the source and waves reflected off the water surface. The two sets of waves are then combined to form an interference pattern such as that produced by two separate aerials. The reflected wavefront travels an additional distance 2h sin(i) before reaching the detector where h and i are the height of the cliff and the inclination of the incoming wavefront respectively. It acts as a second aerial twice the height of the cliff below the first.Sea interferometers are drift instruments, that is, they are fixed and their pointing direction changes with the rotation of the Earth.

The interference patterns for a sea interferometer commence sharply as soon as the source rises above the horizon, instead of fading in gradually as for a normal interferometer. Since it consists of just one detector, there is no need for connecting cables or for preamplifiers. A sea interferometer also has double the sensitivity of a pair of detectors set up to the same separation. Sea interferometry greatly increases the resolving power of the instrument.The quality of data obtained by a sea interferometer is affected by a number of factors. Waves on the water surface and variable refraction adversely affect the signal, and the curvature of the Earth's surface must be taken into account. These difficulties can be overcome by observing for extended periods, and calibrating the instrument on sources of known position.Among the discoveries made using sea interferometry are that sunspots emit strong radio waves

and that the source of radio wave emission from Cygnus A is small (less than 8 arcminutes in diameter). The technique also discovered six new sources including Centaurus A.

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