Flamsteed designations for stars are similar to Bayer designations, except that they use numbers instead of Greek letters. Each star is assigned a number and the Latin genitive of the constellation it lies in (see List of constellations for a list of constellations and the genitive forms of their names). Flamsteed designation contained 2554 stars.
The numbers were originally assigned in order of increasing right ascension within each constellation, but due to the effects of precession they are now slightly out of order in some places. This method of designating stars first appeared in a preliminary version of John Flamsteed's Historia Coelestis Britannica which was published by Edmond Halley and Isaac Newton in 1712 without Flamsteed's approval. The final version of Flamsteed's catalogue published in 1725 after his death omitted any number designations altogether.
The designations gained popularity throughout the eighteenth century, and are now commonly used when no Bayer designation exists; however, where a Bayer designation does exist for a star it is used almost exclusively and the Flamsteed designation is almost never used. Examples of well-known stars which are usually referred to by their Flamsteed numbers include 51 Pegasi (see Extrasolar planet), and 61 Cygni (see Parallax). Flamsteed designations do, however, tend to trump the Bayer designation if the latter contains an extra attached number, so "55 Cancri" is more common than "Rho-1 Cancri".
There are examples of stars bearing Flamsteed designations for constellations in which they do not lie, just as there are for Bayer designations, because of the compromises that had to be made when the modern constellation boundaries were drawn up.
It should also be noted that Flamsteed's catalogue covered only the stars visible from Great Britain, and therefore stars of the far southern constellations have no Flamsteed numbers. Some stars, such as the nearby star 82 Eridani, were named in a major southern-hemisphere catalog called Uranometria Argentina, by Benjamin Gould, and are not true Flamsteed numbers, and should properly contain a G, as in 82 G. Eridani. Apart from a handful of cases, these designations are not currently in use.
Some entries in Flamsteed's catalog are errors: for instance, Flamsteed observed Uranus in 1690 but did not recognize it as a planet and entered it into his catalog as a star called "34 Tauri".
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