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Fifty-seven navigational
star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s and additionally the star
Polaris Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris ( Latinized to ''Alpha Ursae Minoris'') and is commonly called the North Star or Pole Star. With an apparent magnitude th ...
are given a special status in the field of
celestial navigation Celestial navigation, also known as astronavigation, is the practice of position fixing using stars and other celestial bodies that enables a navigator to accurately determine their actual current physical position in space (or on the surface o ...
. Of the approximately 6,000 stars visible to the
naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying, light-collecting optical instrument, such as a telescope or microscope, or eye protection. Vision corrected to norma ...
under optimal conditions, these selected stars are among the brightest and span 38
constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the earliest constellation ...
s of the
celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphe ...
from the
declination In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of th ...
of −70° to +89°. Many of the selected stars were named in
antiquity Antiquity or Antiquities may refer to: Historical objects or periods Artifacts *Antiquities, objects or artifacts surviving from ancient cultures Eras Any period before the European Middle Ages (5th to 15th centuries) but still within the histo ...
by the
Babylonia Babylonia (; Akkadian: , ''māt Akkadī'') was an ancient Akkadian-speaking state and cultural area based in the city of Babylon in central-southern Mesopotamia (present-day Iraq and parts of Syria). It emerged as an Amorite-ruled state c ...
ns, Greeks, Romans, and Arabs. The star
Polaris Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris ( Latinized to ''Alpha Ursae Minoris'') and is commonly called the North Star or Pole Star. With an apparent magnitude th ...
, often called either the "Pole Star" or the "North Star", is treated specially due to its proximity to the north
celestial pole The north and south celestial poles are the two points in the sky where Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The north and south celestial poles appear permanently directly overhead to observers a ...
. When navigating in the
Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...
, a simple and quick technique can be used with Polaris to determine the observers
latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north ...
or, for larger maritime vessels can be used to calculate any
gyrocompass A gyrocompass is a type of non-magnetic compass which is based on a fast-spinning disc and the rotation of the Earth (or another planetary body if used elsewhere in the universe) to find geographical direction automatically. The use of a gyroc ...
error that may exist. The other 57 selected stars have daily positions given in
nautical almanac A nautical almanac is a publication describing the positions of a selection of celestial bodies for the purpose of enabling navigators to use celestial navigation to determine the position of their ship while at sea. The Almanac specifies for eac ...
s, aiding the
navigator A navigator is the person on board a ship or aircraft responsible for its navigation.Grierson, MikeAviation History—Demise of the Flight Navigator FrancoFlyers.org website, October 14, 2008. Retrieved August 31, 2014. The navigator's prima ...
in efficiently performing observations on them. A second group of 115 "tabulated stars" can also be used for celestial navigation, but are often less familiar to the navigator and require extra calculations. Although
Polaris Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris ( Latinized to ''Alpha Ursae Minoris'') and is commonly called the North Star or Pole Star. With an apparent magnitude th ...
can quickly and simply give a solution for latitude in the northern hemisphere, it can not participate in giving a position fix including longitude - it is for this reason it is excluded from the list of 57 primary navigational stars, each of which can be used to produce (in conjunction with each other, known time in relation to the prime meridian and a set of sight reduction tables) an actual latitudinal and longitudinal positional fix. For purposes of identification, the positions of navigational stars — expressed as declination and sidereal hour angle — are often rounded to the nearest degree. In addition to tables, star charts provide an aid to the navigator in identifying the navigational stars, showing
constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the earliest constellation ...
s, relative positions, and brightness. In practical use for sight reductions whilst at sea, tables can further assist a navigator by giving approximate altitudes (angles above the horizon) and azimuths (degrees as read from the compass) from an assumed or estimated position, usually helping to quickly determine the location and then quickly identify a particular navigational star that may be useful for a sight reduction.


Background

Under optimal conditions, approximately 6,000 stars are visible to the naked eye of an observer on Earth.Bowditch, 2002, p. 249. Of these, 58 stars are known in the field of navigational astronomy as "selected stars", including 19 stars of the first magnitude, 38 stars of the second magnitude, and
Polaris Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris ( Latinized to ''Alpha Ursae Minoris'') and is commonly called the North Star or Pole Star. With an apparent magnitude th ...
. The selection of the stars is made by
Her Majesty's Nautical Almanac Office His Majesty's Nautical Almanac Office (HMNAO), now part of the United Kingdom Hydrographic Office, was established in 1832 on the site of the Royal Observatory, Greenwich (ROG), where ''The Nautical Almanac'' had been published since 1767. HMNAO ...
and the
US Naval Observatory United States Naval Observatory (USNO) is a scientific and military facility that produces geopositioning, navigation and timekeeping data for the United States Navy and the United States Department of Defense. Established in 1830 as the Depo ...
, in the production of the yearly ''
Nautical Almanac A nautical almanac is a publication describing the positions of a selection of celestial bodies for the purpose of enabling navigators to use celestial navigation to determine the position of their ship while at sea. The Almanac specifies for eac ...
'' which the two organizations have published jointly since 1958. Criteria in the choice of stars includes their distribution across the celestial sphere, brightness, and ease of identification. Information for another 115 stars, known as "tabulated stars", is also available to the navigator. This list provides information on the name, approximate position in the celestial sphere, and
apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's ...
of the 58 selected stars in tabular form and by star charts. These stars are typically used in two ways by the navigator. The first is to obtain a
line of position A position line or line of position (LOP) is a line (or, on the surface of the earth, a curve) that can be both identified on a chart (nautical chart or aeronautical chart) and translated to the surface of the earth. The intersection of a minimum o ...
by use of a
sextant A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects. The primary use of a sextant is to measure the angle between an astronomical object and the horizon for the purposes of ce ...
observation and the techniques of celestial navigation.Bowditch, 2002, pp. 301–303. Multiple lines of position can be intersected to obtain a position known as a celestial fix. The second typical use of the navigational stars is to determine gyrocompass error by computing the
azimuth An azimuth (; from ar, اَلسُّمُوت, as-sumūt, the directions) is an angular measurement in a spherical coordinate system. More specifically, it is the horizontal angle from a cardinal direction, most commonly north. Mathematical ...
of a star and comparing it to an azimuth measured using the ship's gyrocompass.Bowditch, 2002, pp. 271–274. Numerous other applications also exist. Navigators typically refer to stars using one of two naming systems for stars: common names and Bayer's designations. All of the selected stars have had a common name since 1953, and many were named in antiquity by the Arabs, Greeks, Romans, and Babylonians. Bayer's naming convention has been in use since 1603, and consists of a Greek letter combined with the possessive form of the star's constellation. Both names are shown for each star in the tables and charts below. Each star's approximate position on the celestial sphere is given using the
equatorial coordinate system The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects. It may be implemented in spherical or rectangular coordinates, both defined by an origin at the centre of Earth, a fu ...
. The celestial sphere is an imaginary globe of infinite size with the Earth at its center.Bowditch, 2002, p. 234. Positions on the celestial sphere are often expressed using two coordinates:
declination In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of th ...
and sidereal hour angle, which are similar to latitude and
longitude Longitude (, ) is a geographic coordinate that specifies the east– west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek let ...
on the surface of the Earth. To define declination, the Earth's
equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can also ...
is projected out to the celestial sphere to construct the
celestial equator The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. This plane of reference bases the equatorial coordinate system. In other words, the celestial equator is an abstract proj ...
, and declination is measured in degrees north or south of this celestial equator. Sidereal hour angle is a measurement between 0° and 360°, indicating how far west a body is from an arbitrarily chosen point on the celestial sphere called the '' First Point of Aries''. Note that right ascension, as used by astronomers, is 360° minus the sidereal hour angle. The final characteristic provided in the tables and star charts is the star's brightness, expressed in terms of apparent magnitude. Magnitude is a logarithmic scale of brightness, designed so that a body of one magnitude is approximately 2.512 times brighter than a body of the next magnitude.The value is actually the fifth root of 100, an
irrational number In mathematics, the irrational numbers (from in- prefix assimilated to ir- (negative prefix, privative) + rational) are all the real numbers that are not rational numbers. That is, irrational numbers cannot be expressed as the ratio of two inte ...
known as Pogson's Ratio. See
Bowditch, 2002, p. 219. Thus, a body of magnitude 1 is 2.5125, or 100 times brighter than a body of magnitude 6. The dimmest stars that can be seen through a 200-inch terrestrial telescope are of the 20th magnitude, and very bright objects like the Sun and a full Moon have magnitudes of −26.7 and −12.6 respectively.


Table

The table of navigational stars provides several types of information. In the first column is the identifying index number, followed by the common name, the Bayer designation, and the etymology of the common name. Then the star's approximate position, suitable for identification purposes, is given in terms of declination and sidereal hour angle, followed by the star's magnitude. The final column presents citations to the sources of the data, ''The American Practical Navigator'' and the star's entry at the
SIMBAD SIMBAD (the Set of Identifications, Measurements and Bibliography for Astronomical Data) is an astronomical database of objects beyond the Solar System. It is maintained by the Centre de données astronomiques de Strasbourg (CDS), France. SIM ...
database, a project of the Strasbourg Astronomical Data Center or CDS.


Star charts

Navigators often use star charts to identify a star by its position relative to other stars. References like the ''Nautical Almanac'' and ''The American Practical Navigator'' provide four star charts, covering different portions of the celestial sphere. Two of these charts are
azimuthal equidistant projection The azimuthal equidistant projection is an azimuthal map projection. It has the useful properties that all points on the map are at proportionally correct distances from the center point, and that all points on the map are at the correct azimut ...
s of the north and south poles. The other two cover the equatorial region of the celestial sphere, from the declination of 30° south to 30° north. The two equatorial charts are
mercator projection The Mercator projection () is a cylindrical map projection presented by Flemish geographer and cartographer Gerardus Mercator in 1569. It became the standard map projection for navigation because it is unique in representing north as up and s ...
s, one for the eastern hemisphere of the celestial sphere and one for the western hemisphere. Note that unlike familiar maps, east is shown to the left and west is shown to the right. With this orientation, the navigator can hold the star chart overhead, and the arrangement of the stars on the chart will resemble the stars in the sky. In the star charts, constellations are labelled with capital letters and indicated by dotted lines collecting their stars. The 58 selected stars for navigation are shown in blue and labelled with their common name, star number, and a Greek letter to indicate their Bayer designation. The additional 115 tabulated stars that can also be used for navigation are shown in red and labelled with a Greek letter to indicate their Bayer designation. Some additional stars not suitable for navigation are also included on the charts to indicate constellations, they are presented as unlabelled small red dots.


Equatorial stars

; Equatorial stars of the eastern hemisphere The equatorial region of the celestial sphere's eastern hemisphere includes 17 navigational stars from Alpheratz in the constellation Andromeda to Denebola in Leo. It also includes stars from the constellations Cetus, Aries, Taurus, Orion, Canis Major and Minor, Gemini, and Hydra. Of particular note among these stars are "the dog star" Sirius, the brightest star in the sky, and four stars of the easily identified constellation Orion. Image:Bowditch-equatorial-stars-180-360.svg, center, 600px, alt=Equatorial stars with SHA from 180 to 360, rect 767 236 803 264 Diphda (#4) SHA 349 Dec. S 18 rect 672 37 708 71 Hamal (#6) SHA 328 Dec. N 23 rect 616 134 657 178 Menkar (#8) SHA 315 Dec. N 04 rect 513 72 544 106 Aldebaran (#10) SHA 291 Dec. N 16 rect 474 192 508 225 Rigel (#11) SHA 282 Dec. S 08 rect 466 124 491 154 Bellatrix (#13) SHA 279 Dec. N 06 rect 460 18 486 45 Elnath (#14) SHA 279 Dec. N 29 rect 454 162 472 184 Alnilam (#15) SHA 276 Dec. S 01 rect 416 108 461 159 Betelgeuse (#16) SHA 271 Dec. N 07 rect 375 228 407 266 Sirius (#18) SHA 259 Dec. S 17 rect 363 293 397 320 Adhara (#19) SHA 256 Dec. S 29 rect 313 130 352 165 Procyon (#20) SHA 245 Dec. N 05 rect 304 21 347 54 Pollux (#21) SHA 244 Dec. N 28 rect 199 195 232 232 Alphard (#25) SHA 218 Dec. S 09 rect 803 13 840 46 Alpheratz (#1) SHA 358 Dec. N 29 rect 154 95 192 135 Regulus (#26) SHA 208 Dec. N 12 rect 46 81 77 121 Denebola (#28) SHA 183 Dec. N 15 desc top-right ; Equatorial stars of the western hemisphere The equatorial region of the celestial sphere's western hemisphere includes 13 navigational stars from Gienah in the constellation Corvus to Markab in Pegasus. It also includes stars from the constellations Virgo, Bootes, Libra, Corona Borealis, Scorpio, Ophiuchus, Sagittarius, and Aquila. The
variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as e ...
Arcturus is the brightest star in this group. Image:Bowditch-equatorial-stars-0-180.svg, center, 600px, alt=Equatorial stars with SHA from 0 to 180, rect 812 240 829 265 Geinah (#29) SHA 165 Dec. S 19 rect 731 206 760 236 Spica (#33) SHA 159 Dec. S 11 rect 673 63 707 105 Arcturus (#37) SHA 146 Dec. N 19 rect 634 228 665 262 Zubenelgenubi (#39) SHA 138 Dec. S 16 rect 587 27 627 62 Alphecca (#41) SHA 127 Dec. N 27 rect 525 279 558 309 Antares (#42) SHA 113 Dec. S 26 rect 477 228 513 259 Sabik (#44) SHA 103 Dec. S 16 rect 456 92 489 124 Rasalhague (#46) SHA 096 Dec. N 13 rect 364 277 400 309 Nunki (#50) SHA 076 Dec. S 26 rect 306 106 335 149 Altair (#51) SHA 063 Dec. N 19 rect 186 111 229 147 Enif (#54) SHA 034 Dec. N 10 rect 103 293 139 325 Fomalhaut (#56) SHA 016 Dec. S 30 rect 99 80 132 117 Markab (#57) SHA 014 Dec. N 15 desc top-right


Northern stars

Image:Navigational-stars-above-30-degrees-north.svg, 400px, alt=Northern navigational stars, rect 93 183 121 212 Schedar (#3) SHA 350 Dec. N 56 rect 119 104 150 135 Mirfac (#9) SHA 309 Dec. N 50 rect 172 63 211 103 Capella (#12) SHA 281 Dec. N 46 rect 282 177 315 220 Dubhe (#27) SHA 194 Dec. N 62 rect 306 230 337 259 Alioth (#32) SHA 167 Dec. N 56 rect 315 267 345 297 Alkaid (#34) SHA 153 Dec. N 49 rect 240 243 269 267 Kochab (#40) SHA 137 Dec. N 74 rect 202 332 233 364 Eltanin (#47) SHA 091 Dec. N 51 rect 179 366 203 398 Vega (#49) SHA 081 Dec. N 39 rect 110 314 136 349 Deneb (#53) SHA 050 Dec. N 45 rect 198 199 230 235 Polaris SHA 319 Dec. N 89 desc top-right The 11 northern stars are those with a declination between 30° north and 90° north. They are listed in order of decreasing sidereal hour angle, or from the vernal equinox westward across the sky. Starting with Schedar in the constellation Cassiopeia, the list includes stars from the constellations Auriga, the Great and Little Bears, Draco, Lyra and Cygnus. The two brightest northern stars are Vega and Capella. In the star chart to the right, declination is shown by the radial coordinate, starting at 90° north in the center and decreasing to 30° north at the outer edge. Sidereal hour angle is shown as the angular coordinate, starting at 0° at the left of the chart, and increasing counter-clockwise.


Southern stars

Image:Navigational-stars-below-30-degrees-south.svg, 400px, alt=Southern navigational stars, rect 351 191 379 218 Ankaa (#2) SHA 354 Dec. S 42 rect 297 158 322 192 Achernar (#5) SHA 336 Dec. S 57 rect 311 94 341 121 Acamar (#7) SHA 316 Dec. S 40 rect 191 85 223 119 Canopus (#17) SHA 264 Dec. S 53 rect 146 129 171 156 Avior (#22) SHA 234 Dec. S 59 rect 92 105 123 135 Suhail (#23) SHA 223 Dec. S 43 rect 155 166 189 193 Miaplacidus (#24) SHA 222 Dec. S 70 rect 123 222 145 251 Acrux (#30) SHA 174 Dec. S 63 rect 102 227 120 247 Gacrux (#31) SHA 172 Dec. S 57 rect 122 262 146 280 Hadar (#35) SHA 149 Dec. S 60 rect 60 302 83 330 Menkent (#36) SHA 149 Dec. S 60 rect 135 278 158 299 Rigel Kentaurus (#38) SHA 140 Dec. S 61 rect 183 272 206 304 Atria (#43) SHA 108 Dec. S 69 rect 194 381 215 405 Shaula (#45) SHA 097 Dec. S 34 rect 224 394 249 416 Kaus Australis (#48) SHA 084 Dec. S 34 rect 268 299 292 321 Peacock (#52) SHA 054 Dec. S 57 rect 322 278 347 304 Al Na'ir (#55) SHA 028 Dec S 47 rect 385 262 414 286 Fomalhaut (#56) SHA 016 Dec. S 30 desc top-right The 18 southern stars are those with a declination between 30° south and 90° south. They are listed in order of decreasing sidereal hour angle, or from the vernal equinox westward across the sky. Starting with Ankaa in the constellation Phoenix, the list includes stars from the constellations Eridanus, Carina, Crux, Centaurus, Libra, Triangulum Australe, Scorpio, Sagittarius, Pavo, and Grus. Canopus, Rigil Kentaurus, Achernar, and Hadar are the brightest stars in the southern sky. In the star chart to the right, declination is shown by the radial coordinate, starting at 90° south in the center and decreasing to 30° south at the outer edge. Sidereal hour angle is shown as the angular coordinate, starting at 0° at the right of the chart, and increasing clockwise.


Footnotes

;; Notes ;; Citations


References

* * * * {{DEFAULTSORT:Navigational Stars
Stars 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 ...
Navigation Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation ...
Navigation