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25-602: The observatory and its former annex, the § Leiden Southern Station , are known for the discovery of 6,000 double stars and for Proxima Centauri , made by astronomer Robert Innes . At the observatory, 578 identifications of minor planet were made, a record number at the time. The Minor Planet Center credits the observatory as the site where 147 minor planets were discovered by astronomers Harry Wood , Cyril Jackson , Hendrik van Gent , Ernest Johnson , Ejnar Hertzsprung , Jacobus Bruwer and Joseph Churms (see § List of discovered minor planets ) . The origins of

50-513: A 9-inch Grubb refractor was now used but repolished in 1908. Mr J. Franklin-Adams gifted the observatory a 10-inch triple O.G. Cooke Star-Camera in 1909. J.B. Rissik, Minister for Lands, permitted the purchase of a 26-inch refracting telescope from the Grubb Telescope Company in 1909. With the formation of the Union of South Africa in 1910, South Africa's two colonial observatories came under

75-464: A component which is being measured relative to another component, A in this case. Discoverer designations are also listed; however, traditional discoverer abbreviations such as Δ and Σ have been encoded into a string of uppercase Roman letters, so that, for example, Δ65 has become DUN  65 and Σ2451 has become STF 2451. Further examples of this are shown in the adjacent table. Hartbeespoort Too Many Requests If you report this error to

100-416: A few thousand of the over 100,000 known visual double stars. Confirmation of a visual double star as a binary star can be achieved by observing the relative motion of the components. If the motion is part of an orbit , or if the stars have similar radial velocities or the difference in their proper motions is small compared to their common proper motion, the pair is probably physical. When observed over

125-418: A multiple-star system), but it is not known for certain whether Mizar and Alcor are gravitationally bound. Since the beginning of the 1780s, both professional and amateur double star observers have telescopically measured the distances and angles between double stars to determine the relative motions of the pairs. If the relative motion of a pair determines a curved arc of an orbit , or if the relative motion

150-588: A new Meteorological Department was temporarily established in Johannesburg . A location was sought for the new observatory and the Johannesburg Town Council made a decision on 12 May 1903, located within the municipal boundaries. The land of eight acres, on a ridge west of the Indian War Memorial, was on the northern boundary of the farm Doornfontein , presently part of the suburb Observatory and

175-451: A short period of time, the components of both optical doubles and long-period visual binaries will appear to be moving in straight lines; for this reason, it can be difficult to distinguish between these two possibilities. Some bright visual double stars have a Bayer designation . In this case, the components may be denoted by superscripts. An example of this is α Crucis (Acrux), whose components are α Crucis and α Crucis. Since α Crucis

200-404: Is a spectroscopic binary , this is actually a multiple star. Superscripts are also used to distinguish more distant, physically unrelated, pairs of stars with the same Bayer designation, such as α Capricorni , ξ Centauri , and ξ Sagittarii . These optical pairs are resolvable by the naked eye. Apart from these pairs, the components of a double star are generally denoted by the letters A (for

225-418: Is an optical double , a chance line-of-sight alignment of two stars at different distances from the observer. Binary stars are important to stellar astronomers as knowledge of their motions allows direct calculation of stellar mass and other stellar parameters. The only (possible) case of "binary star" whose two components are separately visible to the naked eye is the case of Mizar and Alcor (though actually

250-446: Is designated RHD 1 . Other examples include Δ65, the 65th double discovered by James Dunlop , and Σ2451, discovered by F. G. W. Struve . The Washington Double Star Catalog , a large database of double and multiple stars, contains over 100,000 entries, each of which gives measures for the separation of two components. Each double star forms one entry in the catalog; multiple stars with n components will be represented by entries in

275-526: Is only the inability to telescopically observe two separate stars that distinguishes non-visual and visual binaries. Mizar , in Ursa Major , was observed to be double by Benedetto Castelli and Galileo . The identification of other doubles soon followed: Robert Hooke discovered one of the first double-star systems, Gamma Arietis , in 1664, while the bright southern star Acrux , in the Southern Cross ,

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300-493: Is small compared to the common proper motion of both stars, it may be concluded that the pair is in mutual orbit as a binary star. Otherwise, the pair is optical. Multiple stars are also studied in this way, although the dynamics of multiple stellar systems are more complex than those of binary stars. The following are three types of paired stars: Improvements in telescopes can shift previously non-visual binaries into visual binaries, as happened with Polaris A in 2006. It

325-406: The separation , or angular distance, between the two component stars in the sky and the position angle . The position angle specifies the direction in which the stars are separated and is defined as the bearing from the brighter component to the fainter, where north is 0°. These measurements are called measures . In the measures of a visual binary, the position angle will change progressively and

350-745: The South African government decided to amalgamate all astronomical research into one body, which later became known as the South African Astronomical Observatory (SAAO); it has its headquarters in Cape Town and has Sutherland as its outstation. The main Cape telescopes were moved to Sutherland, and the Radcliffe Observatory at Pretoria was also dismantled. The main-belt asteroid 1585 Union , discovered by Ernest Johnson in 1947,

375-451: The brighter, primary , star) and B (for the fainter, secondary , star) appended to the designation, of whatever sort, of the double star. For example, the components of α Canis Majoris (Sirius) are α Canis Majoris A and α Canis Majoris B (Sirius A and Sirius B); the components of 44 Boötis are 44 Boötis A and 44 Boötis B; the components of ADS 16402 are ADS 16402A and ADS 16402B; and so on. The letters AB may be used together to designate

400-446: The catalog for n −1 pairs, each giving the separation of one component of the multiple star from another. Codes such as AC are used to denote which components are being measured—in this case, component C relative to component A. This may be altered to a form such as AB-D to indicate the separation of a component from a close pair of components (in this case, component D relative to the pair AB.) Codes such as Aa may also be used to denote

425-774: The control of the Minister of the Interior. With the removal of the meteorological functions, the observatory was renamed Observatory of the Union of South Africa (Union Observatory) on 1 April 1912. It became the Republic Observatory in 1961. Well remembered for the quality of its Directors, work done on minor planets and the discovery of Proxima Centauri , growing light pollution problems in Johannesburg led to its closure in 1971–1972. The Observatory's buildings at 18a Gill Street, Observatory, Johannesburg, still exist. At that time

450-631: The observatory began when Theodore Reunert of the South African Association for the Advancement of Science petitioned Alfred Milner Governor of the Transvaal Colony on 29 October 1902 for the establishment of a meteorological and astronomical observatory in Johannesburg. Assistant Colonial Secretary W.H. Moor agreed to the project on 17 December 1902 with the budget increased from £1,350 to £5,629 due to equipment changes. On 1 April 1903,

475-412: The pair. In the case of multiple stars, the letters C, D, and so on may be used to denote additional components, often in order of increasing separation from the brightest star, A. Visual doubles are also designated by an abbreviation for the name of their discoverer followed by a catalogue number unique to that observer. For example, the pair α Centauri AB was discovered by Father Richaud in 1689, and so

500-420: The separation between the two stars will oscillate between maximum and minimum values. Plotting the measures in the plane will produce an ellipse. This is the apparent orbit , the projection of the orbit of the two stars onto the celestial sphere; the true orbit can be computed from it. Although it is expected that the majority of catalogued visual doubles are visual binaries, orbits have been computed for only

525-455: The site of 147 minor planet discoveries , made by the following list of astronomers: Double star In observational astronomy , a double star or visual double is a pair of stars that appear close to each other as viewed from Earth , especially with the aid of optical telescopes . This occurs because the pair either forms a binary star (i.e. a binary system of stars in mutual orbit , gravitationally bound to each other) or

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550-516: Was discovered to be double by Fontenay in 1685. Since that time, the search has been carried out thoroughly and the entire sky has been examined for double stars down to a limiting apparent magnitude of about 9.0. At least 1 in 18 stars brighter than 9.0 magnitude in the northern half of the sky are known to be double stars visible with a 36-inch (910 mm) telescope . The unrelated categories of optical doubles and true binaries are lumped together for historical and practical reasons. When Mizar

575-421: Was found to be a binary, it was quite difficult to determine whether a double star was a binary system or only an optical double. Improved telescopes, spectroscopy, and photography are the basic tools used to make the distinction. After it was determined to be a visual binary, Mizar's components were found to be spectroscopic binaries themselves. Observation of visual double stars by visual measurement will yield

600-539: Was given as a gift by the Bezuidenhout family, with a further two acres sold for £500. The land was given only for the use of science and a road, later called Observatory Avenue, was also to be built close to the site. The observatory building was built and the formal opening took place on 17 January 1905 by Governor Milner, but had no astronomical equipment. In 1906 it was lent a Hamberg universal instrument (2 5/8-inch object glass) by Dr Oskar Backlund . In September 1907

625-603: Was named for the Union observatory. Union Observatory went through a number of name changes: Its directors were: The Leiden Southern Station ( 081 ) was a collaboration between the Dutch Leiden Observatory and Union Observatory. From 1938 to 1954 it was an annex to the Union Observatory, and was moved to Hartbeespoort in 1954 due to light pollution. It operated until 1978. The Minor Planet Center credits Union Observatory ("Johannesburg"), as

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