Fifty-eight selected navigational stars are given a special status in the field of celestial navigation. Of the approximately 6,000 stars visible to the naked eye under optimal conditions, the selected stars are among the brightest and span 38 constellations of the celestial sphere from the declination of −70° to +89°. Many of the selected stars were named in antiquity by the Babylonians, Greeks, Romans, and Arabs. The star Polaris, often called the "North Star", is treated specially due to its proximity to the north celestial pole. When navigating in the Northern Hemisphere, special techniques can be used with Polaris to determine latitude or gyrocompass error. The other 57 selected stars have daily positions given in nautical almanacs, aiding the navigator 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. 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 constellations, relative positions, and brightness.
Under optimal conditions, approximately 6,000 stars are visible to the naked eye of an observer on Earth. 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. The selection of the stars is made by Her 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 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 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 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 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: declination and sidereal hour angle, which are similar to latitude and longitude on the surface of the Earth. To define declination, the Earth's equator is projected out to the celestial sphere to construct the celestial equator, 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. 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.
|Key to the table|
|No.||The number used to identify stars in navigation publications and star charts.|
|Common name||The name of the star commonly used navigation publications and star charts.|
|Bayer designation||Another name of the star which combines a Greek letter with the possessive form of its constellation's Latin name.|
|Etymology of the common name.|
|SHA||Sidereal hour angle (SHA), the angular distance west of the vernal equinox.|
|Dec.||Declination, the angular distance north or south of the celestial equator.|
|Apparent magnitude, an indicator of the star's brightness.|
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 database, a project of the Strasbourg Astronomical Data Center or CDS.
|Etymology of common name||SHA||Declination||App.
|1||Alpheratz||Andromedae αα Andromedae||the horse's navel||358||29N 29°||2.06|||
|2||Ankaa||Phoenicis αα Phoenicis||coined name, "phoenix bird" in Arabic||354||-42S 42°||2.37|||
|3||Schedar||Cassiopeiae αα Cassiopeiae||the breast (of Cassiopeia)||350||56N 56°||2.25|||
|4||Diphda||Ceti ββ Ceti||the second frog (Fomalhaut was once the first)||349||-18S 18°||2.04|||
|5||Achernar||Eridani αα Eridani||end of the river (Eridanus)||336||-57S 57°||0.50|||
|6||Hamal||Arietis αα Arietis||full-grown lamb||328||23N 23°||2.00|||
|7||Acamar||Eridani θθ Eridani||another form of Achernar||316||-40S 40°||3.2|||
|8||Menkar||Ceti α α Ceti||nose (of the whale)||315||4N 04°||2.5|||
|9||Mirfak||Persei α α Persei||elbow of the Pleiades||309||50N 50°||1.82|||
|10||Aldebaran||Tauri αα Tauri||follower (of the Pleiades)||291||16N 16°||0.85 var|||
|11||Rigel||Orionis β β Orionis||foot (left foot of Orion)||282||-8S 08°||0.12|||
|12||Capella||Aurigae α α Aurigae||little she-goat||281||46N 46°||0.71|||
|13||Bellatrix||Orionis γγ Orionis||female warrior||279||6N 06°||1.64|||
|14||Elnath||Tauri ββ Tauri||one butting with the horns||279||29N 29°||1.68|||
|15||Alnilam||Orionis εε Orionis||string of pearls||276||-1S 01°||1.70|||
|16||Betelgeuse||Orionis αα Orionis||the arm pit (of Orion)||271||7N 07°||0.58 var|||
|17||Canopus||Carinae αα Carinae||city of ancient Egypt||264||-53S 53°||−0.72|||
|18||Sirius||Canis Majoris αα Canis Majoris||the scorching one (popularly, the dog star)||259||-17S 17°||−1.47|||
|19||Adhara||Canis Majoris εε Canis Majoris||the virgin(s)||256||-29S 29°||1.51|||
|20||Procyon||Canis Minoris αα Canis Minoris||before the dog (rising before the dog star, Sirius)||245||5N 05°||0.34|||
|21||Pollux||Geminorum β β Geminorum||Zeus' other twin son (Castor, α Gem, is the first twin)||244||28N 28°||1.15|||
|22||Avior||Carinae εε1 Carinae||coined name||234||-59S 59°||2.4|||
|23||Suhail||Velorum λλ Velorum||shortened form of Al Suhail, one Arabic name for Canopus||223||-43S 43°||2.23|||
|24||Miaplacidus||Carinae ββ Carinae||quiet or still waters||222||-70S 70°||1.70|||
|25||Alphard||Hydrae α α Hydrae||solitary star of the serpent||218||-9S 09°||2.00|||
|26||Regulus||Leonis α α Leonis||the prince||208||12N 12°||1.35|||
|27||Dubhe||Ursae Majoris αα1 Ursae Majoris||the bear's back||194||62N 62°||1.87|||
|28||Denebola||Leonis β β Leonis||tail of the lion||183||15N 15°||2.14|||
|29||Gienah||Corvi γ γ Corvi||right wing of the raven||176||-17S 17°||2.80|||
|30||Acrux||Crucis αα1 Crucis||coined from Bayer name||174||-63S 63°||1.40|||
|31||Gacrux||Crucis γγ Crucis||coined from Bayer name||172||-57S 57°||1.63|||
|32||Alioth||Ursae Majoris ε ε Ursae Majoris||another form of Capella||167||56N 56°||1.76|||
|33||Spica||Virginis α α Virginis||the ear of corn||159||-11S 11°||1.04|||
|34||Alkaid||Ursae Majoris η η Ursae Majoris||leader of the daughters of the bier||153||49N 49°||1.85|||
|35||Hadar||Centauri β β Centauri||leg of the centaur||149||-60S 60°||0.60|||
|36||Menkent||Centauri θ θ Centauri||shoulder of the centaur||149||-36S 36°||2.06|||
|38||Rigil Kentaurus||Centauri αα1 Centauri||foot of the centaur||140||-61S 61°||−0.01|||
|37||Arcturus||Bootis αα Bootis||the bear's guard||146||19N 19°||−0.04 var|||
|39||Zubenelgenubi||Librae α α Librae||southern claw (of the scorpion)||138||-16S 16°||3.28|||
|40||Kochab||Ursae Minoris β β Ursae Minoris||shortened form of "north star" (named when it was that, ca. 1500 BC – AD 300).||137||74N 74°||2.08|||
|41||Alphecca||Corona Borealis α α Corona Borealis||feeble one (in the crown)||127||27N 27°||2.24|||
|42||Antares||Scorpii αα Scorpii||rival of Mars (in color)||113||-26S 26°||1.09|||
|43||Atria||Trianguli Australis αα Trianguli Australis||coined from Bayer name||108||-69S 69°||1.92|||
|44||Sabik||Ophiuchi η η Ophiuchi||second winner or conqueror||103||-16S 16°||2.43|||
|45||Shaula||Scorpii λ λ Scorpii||cocked-up part of the scorpion's tail||097||-37S 37°||1.62|||
|46||Rasalhague||Ophiuchi α α Ophiuchi||head of the serpent charmer||096||13N 13°||2.10|||
|47||Eltanin||Draconis γ γ Draconis||head of the dragon||091||51 N 51°||2.23|||
|48||Kaus Australis||Sagittarii ε ε Sagittarii||southern part of the bow (of Sagittarius)||084||-34S 34°||1.80|||
|49||Vega||Lyrae α α Lyrae||the falling eagle or vulture||081||39N 39°||0.03|||
|50||Nunki||Sagittarii σ σ Sagittarii||constellation of the holy city (Eridu)||076||-26S 26°||2.06|||
|51||Altair||Aquilae α α Aquilae||flying eagle or vulture||063||9N 09°||0.77|||
|52||Peacock||Pavonis α α Pavonis||Coined from the English name of the constellation||054||-57S 57°||1.91|||
|53||Deneb||Cygni αα Cygni||tail of the hen||050||45N 45°||1.25|||
|54||Enif||Pegasi εε Pegasi||nose of the horse||034||10N 10°||2.40|||
|55||Al Na'ir||Gruis αα Gruis||bright one (of the southern fish's tail)||028||-47S 47°||1.74|||
|56||Fomalhaut||Piscis Austrini α α Piscis Austrini||mouth of the southern fish||016||-30S 30°||1.16|||
|57||Markab||Pegasi α α Pegasi||saddle (of Pegasus)||014||15N 15°||2.49|||
|99* ||Polaris||Ursae Minoris αα Ursae Minoris||the pole (star)||319||89N 89°||2.01 var|||
|Key to the Star charts|
|UPPERCASE TEXT||Constellation names are indicated in uppercase text.|
||Selected star of magnitude 1.5 and brighter. Labeled with common name, star number, and Greek letter to indicate Bayer designation.|
||Selected star of magnitude 1.6 and fainter. Labeled with common name, star number, and Greek letter to indicate Bayer designation.|
||Tabulated star of magnitude 2.5 and brighter. Labeled with Greek letter to indicate Bayer designation.|
||Tabulated star of magnitude 2.6 and fainter. Labeled with Greek letter to indicate Bayer designation.|
||Untabulated star. Not labeled.|
|Dotted line||Constellation outline.|
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 projections 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.
The equatorial region of the celestial sphere's eastern hemisphere includes 16 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.
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 Arcturus is the brightest star in this group.
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.
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.