Fifty-seven navigational

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

s and additionally the star

Polaris Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris (Latinisation of names, Latinized to ''Alpha Ursae Minoris'') and is commonly called the North Star or Pole Star. With an ...

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 ...

. Of the approximately six thousand 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 magnification, magnifying, Optical telescope#Light-gathering power, light-collecting optical instrument, such as a telescope or microsc ...

under optimal conditions, these selected stars are among the brightest and span thirty-eight

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

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 sphere, ...

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. The declination angle is measured north (positive) or ...

of −70° to +89°. Many of the selected stars were named in antiquity by the

Babylonia Babylonia (; , ) was an Ancient history, ancient Akkadian language, Akkadian-speaking state and cultural area based in the city of Babylon in central-southern Mesopotamia (present-day Iraq and parts of Kuwait, Syria and Iran). It emerged as a ...

ns,

Greeks Greeks or Hellenes (; , ) are an ethnic group and nation native to Greece, Greek Cypriots, Cyprus, Greeks in Albania, southern Albania, Greeks in Turkey#History, Anatolia, parts of Greeks in Italy, Italy and Egyptian Greeks, Egypt, and to a l ...

, Romans, and

Arabs Arabs (, , ; , , ) are an ethnic group mainly inhabiting the Arab world in West Asia and North Africa. A significant Arab diaspora is present in various parts of the world. Arabs have been in the Fertile Crescent for thousands of yea ...

. The star

, 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 at ...

. 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 by humans as being in the same celestial sphere, celestial hemisphere relative to the invariable plane of the Solar ...

, a simple and quick technique can be used with Polaris to determine the observers

latitude In geography, latitude is a geographic coordinate system, geographic 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 t ...

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 (geometry), direction automaticall ...

error that may exist. The other fifty-seven selected stars have daily positions given in nautical almanacs, 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 prim ...

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

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 chart A star chart is a celestial map of the night sky with astronomical objects laid out on a grid system. They are used to identify and locate constellations, stars, nebulae, galaxies, and planets. They have been used for human navigation since tim ...

s provide an aid to the navigator in identifying the navigational stars, showing

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 six thousand stars are visible to the naked eye of an observer on Earth. Of these, fifty-eight stars are known in the field of navigational astronomy as "selected stars", including nineteen stars of the first magnitude, thirty-eight stars of the second

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

, and

. The selection of the stars is made by His Majesty's Nautical Almanac Office and the US Naval Observatory, in the production of the yearly '' Nautical Almanac'' 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 Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

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 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 cel ...

observation and the techniques of celestial navigation. 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 ) is the horizontal angle from a cardinal direction, most commonly north, in a local or observer-centric spherical coordinate system. Mathematically, the relative position vector from an observer ( origin) to a point ...

of a star and comparing it to an azimuth measured using the ship's gyrocompass. 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 astronomical object, celestial objects. It may be implemented in spherical coordinate system, spherical or Cartesian coordinate system, rect ...

. The celestial sphere is an imaginary globe of infinite size with the Earth at its center. Positions on the celestial sphere are often expressed using two coordinates:

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 lett ...

on the surface of the Earth. To define declination, the Earth's

equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

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. By extension, it is also a plane of reference in the equatorial coordinate system. Due to Earth's axial tilt, the celestial ...

, 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 are all the real numbers that are not rational numbers. That is, irrational numbers cannot be expressed as the ratio of two integers. When the ratio of lengths of two line segments is an irrational number, ...

known as Pogson's Ratio. See Thus, a body of magnitude 1 is 2.512⁵ (~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 astronomy, astronomical database of objects beyond the Solar System. It is maintained by the Centre de données astronomiques de Strasbourg (CDS), Fr ...

database, a project of the Strasbourg Astronomical Data Center or CDS.

Star charts

Navigators often use

s 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 azimuth ...

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 projections, 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 systematically with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are ...

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 a luminous spheroid of plasma held together by self-gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night; their immense distances from Earth make them appear as fixed points of ...

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

Navigation