Fifty-seven navigational stars and additionally the star Polaris 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, these 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 either the "Pole Star" or the "North Star", is treated specially due to its proximity to the north celestial pole. When navigating in the Northern Hemisphere, a simple and quick technique can be used with Polaris to determine the observers latitude or, for larger maritime vessels can be used to calculate any gyrocompass error that may exist. 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.
Although Polaris 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 constellations, 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.
Under optimal conditions, approximately 6,000 stars are visible to the naked eye of an observer on Earth.[1] 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.[1] 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.[2] Criteria in the choice of stars includes their distribution across the celestial sphere, brightness, and ease of identification.[3] Information for another 115 stars, known as "tabulated stars", is also available to the navigator.[1] 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.[4] 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.[5] Numerous other applications also exist.
Navigators typically refer to stars using one of two naming systems for stars: common names and Bayer's designations.[1] 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.[1] 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.[1] 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.[6] 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.[6] 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.[7] [8] Thus, a body of magnitude 1 is 2.5125 (~100) times brighter than a body of magnitude 6.[8] 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.[8]
Key to the table | ||
---|---|---|
Column title | Description | |
No. | The number used to identify stars in navigation publications and star charts.[9] | |
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 common name | Etymology of the common name.[10] | |
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. | |
App. magnitude | Apparent magnitude, an indicator of the star's brightness. |
No. | Common name | Bayer designation | Etymology of common name | SHA | Declination | App. magnitude | References | |
---|---|---|---|---|---|---|---|---|
1 | Alpheratz | Andromedae | αthe horse's navel | 358 | N 29° | 2.06 | [11] | |
2 | Ankaa | Phoenicis | αcoined name, "phoenix bird" in Arabic | 354 | S 42° | 2.37 | [12] | |
3 | Schedar | Cassiopeiae | αthe breast (of Cassiopeia) | 350 | N 56° | 2.25 | [13] | |
4 | Diphda | Ceti | βthe second frog (Fomalhaut was once the first) | 349 | S 18° | 2.04 | [14] | |
5 | Achernar | Eridani | αend of the river (Eridanus) | 336 | S 57° | 0.50 | [15] | |
6 | Hamal | Arietis | αfull-grown lamb | 328 | N 23° | 2.00 | [16] | |
7 | Acamar | Eridani | θanother form of Achernar | 316 | S 40° | 3.2 | [17] | |
8 | Menkar | Ceti | αnose (of the whale) | 315 | N 04° | 2.5 | [18] | |
9 | Mirfak | Persei | αelbow of the Pleiades | 309 | N 50° | 1.82 | [19] | |
10 | Aldebaran | Tauri | αfollower (of the Pleiades) | 291 | N 16° | 0.85 var | [20] | |
11 | Rigel | Orionis | βfoot (left foot of Orion) | 282 | S 08° | 0.12 | [21] | |
12 | Capella | Aurigae | αlittle she-goat | 281 | N 46° | 0.71 | [22] | |
13 | Bellatrix | Orionis | γfemale warrior | 279 | N 06° | 1.64 | [23] | |
14 | Elnath | Tauri | βone butting with the horns | 279 | N 29° | 1.68 | [24] | |
15 | Alnilam | Orionis | εstring of pearls | 276 | S 01° | 1.70 | [25] | |
16 | Betelgeuse | Orionis | αthe hand of al-Jauzā (i.e. Orion) | 271 | N 07° | 0.58 var | [26] | |
17 | Canopus | Carinae | αcity of ancient Egypt | 264 | S 53° | -0.72 | [27] | |
18 | Sirius | Canis Majoris | αthe scorching one (popularly, the dog star) | 259 | S 17° | -1.47 | [28] | |
19 | Adhara | Canis Majoris | εthe virgin(s) | 256 | S 29° | 1.51 | [29] | |
20 | Procyon | Canis Minoris | αbefore the dog (rising before the dog star, Sirius) | 245 | N 05° | 0.34 | [30] | |
21 | Pollux | Geminorum | βZeus' other twin son (Castor, α Gem, is the first twin) | 244 | N 28° | 1.15 | [31] | |
22 | Avior | Carinae | ε1coined name | 234 | S 59° | 2.4 | [32] | |
23 | Suhail | Velorum | λshortened form of Al Suhail, one Arabic name for Canopus | 223 | S 43° | 2.23 | [33] | |
24 | Miaplacidus | Carinae | βquiet or still waters | 222 | S 70° | 1.70 | [34] | |
25 | Alphard | Hydrae | αsolitary star of the serpent | 218 | S 09° | 2.00 | [35] | |
26 | Regulus | Leonis | αthe prince | 208 | N 12° | 1.35 | [36] | |
27 | Dubhe | Ursae Majoris | α1the bear's back | 194 | N 62° | 1.87 | [37] | |
28 | Denebola | Leonis | βtail of the lion | 183 | N 15° | 2.14 | [38] | |
29 | Gienah | Corvi | γright wing of the raven | 176 | S 17° | 2.80 | [39] | |
30 | Acrux | Crucis | α1coined from Bayer name | 174 | S 63° | 1.40 | [40] | |
31 | Gacrux | Crucis | γcoined from Bayer name | 172 | S 57° | 1.63 | [41] | |
32 | Alioth | Ursae Majoris | εanother form of Capella | 167 | N 56° | 1.76 | [42] | |
33 | Spica | Virginis | αthe ear of corn | 159 | S 11° | 1.04 | [43] | |
34 | Alkaid | Ursae Majoris | ηleader of the daughters of the bier | 153 | N 49° | 1.85 | [44] | |
35 | Hadar | Centauri | βleg of the centaur | 149 | S 60° | 0.60 | [45] | |
36 | Menkent | Centauri | θshoulder of the centaur | 149 | S 36° | 2.06 | [46] | |
38 | Rigil Kentaurus | Centauri | α1foot of the centaur | 140 | S 61° | -0.01 | [47] | |
37 | Arcturus | Bootis | αthe bear's guard | 146 | N 19° | -0.04 var | [48] | |
39 | Zubenelgenubi | Librae | αsouthern claw (of the scorpion) | 138 | S 16° | 3.28 | [49] | |
40 | Kochab | Ursae Minoris | βshortened form of "north star" (named when it was that,[50] ca. 1500 BC – AD 300). | 137 | N 74° | 2.08 | [51] | |
41 | Alphecca | Corona Borealis | αfeeble one (in the crown) | 127 | N 27° | 2.24 | [52] | |
42 | Antares | Scorpii | αrival of Mars (in color) | 113 | S 26° | 1.09 | [53] | |
43 | Atria | Trianguli Australis | αcoined from Bayer name | 108 | S 69° | 1.92 | [54] | |
44 | Sabik | Ophiuchi | ηsecond winner or conqueror | 103 | S 16° | 2.43 | [55] | |
45 | Shaula | Scorpii | λcocked-up part of the scorpion's tail | 097 | S 37° | 1.62 | [56] | |
46 | Rasalhague | Ophiuchi | αhead of the serpent charmer | 096 | N 13° | 2.10 | [57] | |
47 | Eltanin | Draconis | γhead of the dragon | 091 | N 51° | 2.23 | [58] | |
48 | Kaus Australis | Sagittarii | εsouthern part of the bow (of Sagittarius) | 084 | S 34° | 1.80 | [59] | |
49 | Vega | Lyrae | αthe falling eagle or vulture | 081 | N 39° | 0.03 | [60] | |
50 | Nunki | Sagittarii | σconstellation of the holy city (Eridu) | 076 | S 26° | 2.06 | [61] | |
51 | Altair | Aquilae | αflying eagle or vulture | 063 | N 09° | 0.77 | [62] | |
52 | Peacock | Pavonis | αCoined from the English name of the constellation | 054 | S 57° | 1.91 | [63] | |
53 | Deneb | Cygni | αtail of the hen | 050 | N 45° | 1.25 | [64] | |
54 | Enif | Pegasi | εnose of the horse | 034 | N 10° | 2.40 | [65] | |
55 | Al Na'ir | Gruis | αbright one (of the southern fish's tail) | 028 | S 47° | 1.74 | [66] | |
56 | Fomalhaut | Piscis Austrini | αmouth of the southern fish | 016 | S 30° | 1.16 | [67] | |
57 | Markab | Pegasi | αsaddle (of Pegasus) | 014 | N 15° | 2.49 | [68] | |
Polaris | Ursae Minoris | αthe pole (star) | 319 | N 89° | 2.01 var[69] | [70] |
Key to the Star charts | ||
---|---|---|
Item | Description | |
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. |
rect 767 236 803 264 Diphda (#4) SHA 349 Dec. S 18rect 672 37 708 71 Hamal (#6) SHA 328 Dec. N 23rect 616 134 657 178 Menkar (#8) SHA 315 Dec. N 04rect 513 72 544 106 Aldebaran (#10) SHA 291 Dec. N 16rect 474 192 508 225 Rigel (#11) SHA 282 Dec. S 08rect 466 124 491 154 Bellatrix (#13) SHA 279 Dec. N 06rect 460 18 486 45 Elnath (#14) SHA 279 Dec. N 29rect 454 162 472 184 Alnilam (#15) SHA 276 Dec. S 01rect 416 108 461 159 Betelgeuse (#16) SHA 271 Dec. N 07rect 375 228 407 266 Sirius (#18) SHA 259 Dec. S 17rect 363 293 397 320 Adhara (#19) SHA 256 Dec. S 29rect 313 130 352 165 Procyon (#20) SHA 245 Dec. N 05rect 304 21 347 54 Pollux (#21) SHA 244 Dec. N 28rect 199 195 232 232 Alphard (#25) SHA 218 Dec. S 09rect 803 13 840 46 Alpheratz (#1) SHA 358 Dec. N 29rect 154 95 192 135 Regulus (#26) SHA 208 Dec. N 12rect 46 81 77 121 Denebola (#28) SHA 183 Dec. N 15
desc top-right
rect 812 240 829 265 Geinah (#29) SHA 165 Dec. S 19rect 731 206 760 236 Spica (#33) SHA 159 Dec. S 11rect 673 63 707 105 Arcturus (#37) SHA 146 Dec. N 19rect 634 228 665 262 Zubenelgenubi (#39) SHA 138 Dec. S 16rect 587 27 627 62 Alphecca (#41) SHA 127 Dec. N 27rect 525 279 558 309 Antares (#42) SHA 113 Dec. S 26rect 477 228 513 259 Sabik (#44) SHA 103 Dec. S 16rect 456 92 489 124 Rasalhague (#46) SHA 096 Dec. N 13rect 364 277 400 309 Nunki (#50) SHA 076 Dec. S 26rect 306 106 335 149 Altair (#51) SHA 063 Dec. N 19rect 186 111 229 147 Enif (#54) SHA 034 Dec. N 10rect 103 293 139 325 Fomalhaut (#56) SHA 016 Dec. S 30rect 99 80 132 117 Markab (#57) SHA 014 Dec. N 15
desc top-right
rect 93 183 121 212 Schedar (#3) SHA 350 Dec. N 56rect 119 104 150 135 Mirfac (#9) SHA 309 Dec. N 50rect 172 63 211 103 Capella (#12) SHA 281 Dec. N 46rect 282 177 315 220 Dubhe (#27) SHA 194 Dec. N 62rect 306 230 337 259 Alioth (#32) SHA 167 Dec. N 56rect 315 267 345 297 Alkaid (#34) SHA 153 Dec. N 49rect 240 243 269 267 Kochab (#40) SHA 137 Dec. N 74rect 202 332 233 364 Eltanin (#47) SHA 091 Dec. N 51rect 179 366 203 398 Vega (#49) SHA 081 Dec. N 39rect 110 314 136 349 Deneb (#53) SHA 050 Dec. N 45rect 198 199 230 235 Polaris SHA 319 Dec. N 89
desc top-rightThe 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.
rect 351 191 379 218 Ankaa (#2) SHA 354 Dec. S 42rect 297 158 322 192 Achernar (#5) SHA 336 Dec. S 57rect 311 94 341 121 Acamar (#7) SHA 316 Dec. S 40rect 191 85 223 119 Canopus (#17) SHA 264 Dec. S 53rect 146 129 171 156 Avior (#22) SHA 234 Dec. S 59rect 92 105 123 135 Suhail (#23) SHA 223 Dec. S 43rect 155 166 189 193 Miaplacidus (#24) SHA 222 Dec. S 70rect 123 222 145 251 Acrux (#30) SHA 174 Dec. S 63rect 102 227 120 247 Gacrux (#31) SHA 172 Dec. S 57rect 122 262 146 280 Hadar (#35) SHA 149 Dec. S 60rect 60 302 83 330 Menkent (#36) SHA 149 Dec. S 60rect 135 278 158 299 Rigel Kentaurus (#38) SHA 140 Dec. S 61rect 183 272 206 304 Atria (#43) SHA 108 Dec. S 69rect 194 381 215 405 Shaula (#45) SHA 097 Dec. S 34rect 224 394 249 416 Kaus Australis (#48) SHA 084 Dec. S 34rect 268 299 292 321 Peacock (#52) SHA 054 Dec. S 57rect 322 278 347 304 Al Na'ir (#55) SHA 028 Dec S 47rect 385 262 414 286 Fomalhaut (#56) SHA 016 Dec. S 30
desc top-rightThe 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.