167:
809:
1299:
862:
1323:
946:
1275:
185:
1311:
1287:
744:. Coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch. Right ascension for "fixed stars" on the equator increases by about 3.1 seconds per year or 5.1 minutes per century, but for fixed stars away from the equator the rate of change can be anything from negative infinity to positive infinity. (To this must be added the
928:. As the equatorial mount became widely adopted for observation, the equatorial coordinate system, which includes right ascension, was adopted at the same time for simplicity. Equatorial mounts could then be accurately pointed at objects with known right ascension and declination by the use of
407:, is similar to right ascension but increases westward rather than eastward. Usually measured in degrees (°), it is the complement of right ascension with respect to 24. It is important not to confuse sidereal hour angle with the astronomical concept of
202:
lies toward the grey arrow, the star marked by a green arrow will appear to rise somewhere in the east about midnight (the Earth drawn from "above" turns anticlockwise). After the observer reaches the green arrow, dawn will over-power (see blue sky
268:(apart from the sun) do so at the September equinox. On those dates at midnight, such objects will reach ("culminate" at) their highest point (their meridian). How high depends on their declination; if 0° declination (i.e. on the
920:, it became possible for astronomers to observe celestial objects in greater detail, provided that the telescope could be kept pointed at the object for a period of time. The easiest way to do that is to use an
20:
924:, which allows the telescope to be aligned with one of its two pivots parallel to the Earth's axis. A motorized clock drive often is used with an equatorial mount to cancel out the
207:) the star's light for about six hours, before it sets on the western horizon. The Right ascension of the star is about 18. 18 means it is a March early-hours star and in
287:. Astronomers have chosen this unit to measure right ascension because they measure a star's location by timing its passage through the highest point in the sky as the
257:. Right ascension is measured continuously in a full circle from that alignment of Earth and Sun in space, that equinox, the measurement increasing towards the east.
740:(including right ascension) are inherently relative to the year of their observation, and astronomers specify them with reference to a particular year, known as an
965:(green)) and increases eastward (towards the left). The lines of right ascension (blue) from pole to pole divide the sky into 24 hours, each equivalent to 15°.
1177:
U.S. Naval
Observatory Nautical Almanac Office; U.K. Hydrographic Office; H.M. Nautical Almanac Office (2008). "Time Scales and Coordinate Systems, 2010".
283:
could have been chosen for right ascension, but it is customarily measured in hours (), minutes (), and seconds (), with 24 being equivalent to a
1353:
760:
is increasing quickly—in AD 2000 it was 2.5h, but when it gets closest to the north celestial pole in 2100 its right ascension will be 6h. The
752:
increase in right ascension by 24h, or about 5.6' per century, whereas stars within 23.5° of an ecliptic pole undergo a net change of
1343:
295:, is the projection of a longitude line onto the celestial sphere. Since a complete circle contains 24 of right ascension or 360° (
1176:
1087:
Solis, stellæ, aut alterius cujusdam signi, est gradus æquatorus cum quo simul exoritur in sphæra recta"; roughly translated, "
1016:
1056:
848:
826:
905:
who measured stars in equatorial coordinates in the 2nd century BC. But
Hipparchus and his successors made their
234:
76:
35:
830:
736:, causes the coordinates of stationary celestial objects to change continuously, if rather slowly. Therefore,
938:
46:(blue). Right ascension is measured eastward up to 24 along the celestial equator from the primary direction.
1006:
996:
737:
220:
96:
1091:
of the Sun, stars, or any other sign, is the degree of the equator that rises together in a right sphere"
976:
88:
1265:
910:
388:), they can be used to time the positions of objects in the sky. For example, if a star with RA =
792:
728:
The Earth's axis traces a small circle (relative to its celestial equator) slowly westward about the
215:
the March equinox. If 6 RA the star would be a March late-hours star, at its high (meridian) at dusk.
229:. Both right ascension and longitude measure an angle from a primary direction (a zero point) on an
1236:
869:
got its name. Ancient astronomy was very concerned with the rise and set of celestial objects. The
1132:
819:
254:
765:
208:
146:, the point on the celestial equator that rises with any celestial object as seen from most
1108:
412:
404:
292:
238:
212:
8:
1327:
925:
769:
761:
748:
of a star.) Over a precession cycle of 26,000 years, "fixed stars" that are far from the
385:
288:
204:
1126:
1076:
1315:
1303:
1122:
1001:
284:
1224:
1052:
1047:
U.S. Naval
Observatory Nautical Almanac Office (1992). Seidelmann, P. Kenneth (ed.).
962:
897:
or parallel sphere). An object's right ascension was its ascension on a right sphere.
874:
741:
269:
246:
68:
43:
264:
are longest visible (appear throughout the night) at the March equinox; those with 0
1279:
1210:
921:
878:
784:
733:
723:
242:
178:
119:
92:
64:
31:
1216:
1102:
355:(also written as 15″). A full circle, measured in right-ascension units, contains
19:
1021:
929:
166:
1243:
1348:
981:
933:
729:
296:
107:
1204:
1046:
1337:
1026:
749:
745:
684:
560:
473:
397:
155:
131:
1291:
1072:
906:
882:
788:
411:, which measures the angular distance of an object westward from the local
280:
1255:
1232:
986:
435:
291:. The line which passes through the highest point in the sky, called the
174:
139:
84:
80:
27:
1011:
902:
833: in this section. Unsourced material may be challenged and removed.
732:, completing one cycle in about 26,000 years. This movement, known as
408:
379:
189:
901:
The concept of right ascension has been known at least as far back as
893:
or oblique sphere). At the poles, objects did not rise or set (right,
211:
in the morning. If 12 RA, the star would be a March all-night star as
1247:
917:
877:(red) which rose or set at the same time as an object (green) on the
352:
332:
226:
932:. The first star catalog to use right ascension and declination was
889:
or right sphere). From almost anywhere else, they were not (center,
808:
396:
will be on the/at its meridian (at its apparent highest point) 18.5
184:
991:
958:
147:
39:
260:
As seen from Earth (except at the poles), objects noted to have 12
954:
861:
780:
757:
250:
230:
193:
151:
135:
127:
945:
773:
442:
273:
123:
83:
of the) point in question above the Earth. When paired with
118:, or the point on the celestial equator that rises with any
225:
Right ascension is the celestial equivalent of terrestrial
1286:
1051:. University Science Books, Mill Valley, CA. p. 735.
315:
of a circle is measured as 1 of right ascension, or 15°;
272:) then at Earth's equator they are directly overhead (at
199:
72:
1205:
MEASURING THE SKY A Quick Guide to the
Celestial Sphere
198:
Assuming the day of the year is the March equinox: the
1263:
1107:. Wells and Lilly and J.W. Burditt, Boston. pp.
776:
are always at right ascension 18 and 6 respectively.
150:
on Earth, where the celestial equator intersects the
1104:
A Compendious
Treatise on the Use of Globes and Maps
791:. Prior to J2000.0, astronomers used the successive
331:
of a circle is measured as 1 of right ascension, or
384:Because right ascensions are measured in hours (of
1049:Explanatory Supplement to the Astronomical Almanac
913:, and the use of RA was limited to special cases.
233:. Right ascension is measured from the Sun at the
67:of a particular point measured eastward along the
1335:
392:is at its meridian, then a star with RA =
351:of a circle contains 1 of right ascension, or
265:
261:
34:. The primary direction of the system is the
1017:Right ascension of the ascending node (RAAN)
418:
1225:"(α,δ) – Right Ascension & Declination"
881:. As seen from the equator, both were on a
192:are depicted here. The symbol ♈︎ marks the
1179:The Astronomical Almanac for the Year 2010
1217:Celestial Equatorial Coordinate Explorers
1181:. U.S. Govt. Printing Office. p. B2.
949:The entire sky, divided into two halves.
849:Learn how and when to remove this message
944:
860:
787:. The prefix "J" indicates that it is a
717:
183:
165:
18:
1121:
1100:
1033:
91:specify the location of a point on the
1354:Technical factors of Western astrology
1336:
1222:
1211:Celestial Equatorial Coordinate System
1207:James B. Kaler, University of Illinois
957:(at right, at the intersection of the
1071:
779:The currently used standard epoch is
1131:. Macmillan Co., New York. pp.
831:adding citations to reliable sources
802:
783:, which is January 1, 2000 at 12:00
16:Astronomical equivalent of longitude
13:
1081:. Apud Johannem Blaeu. p. 65.
14:
1365:
1198:
249:from south to north at the March
177:(green) as seen from outside the
1321:
1309:
1297:
1285:
1273:
807:
253:and is currently located in the
1344:Astronomical coordinate systems
818:needs additional citations for
795:B1875.0, B1900.0, and B1950.0.
1219:University of Nebraska-Lincoln
1213:University of Nebraska-Lincoln
1185:
1169:
1160:
1148:
1139:
1115:
1094:
1065:
1040:
161:
130:, where the celestial equator
1:
939:Historia Coelestis Britannica
403:Sidereal hour angle, used in
30:as seen on the inside of the
1128:An Introduction to Astronomy
1007:Geographic coordinate system
997:Equatorial coordinate system
649:
541:
469:
241:, which is the place on the
221:Equatorial coordinate system
97:equatorial coordinate system
38:, the ascending node of the
7:
977:Celestial coordinate system
969:
756:0h. The right ascension of
688:
564:
477:
335:(also written as 15′); and
10:
1370:
1166:Moulton (1916), pp. 92–95.
916:With the invention of the
798:
721:
377:
245:where the Sun crosses the
218:
885:from pole to pole (left,
440:
434:
431:
428:
425:
419:Symbols and abbreviations
1237:University of Nottingham
89:astronomical coordinates
1191:Blaeu (1668), p. 40–41.
1145:Moulton (1916), p. 126.
1155:Explanatory Supplement
1101:Lathrop, John (1821).
1078:Institutio Astronomica
966:
898:
738:equatorial coordinates
216:
181:
111:
47:
1223:Merrifield, Michael.
953:(blue) begins at the
948:
873:was the point on the
864:
718:Effects of precession
386:rotation of the Earth
187:
169:
22:
1034:Notes and references
911:ecliptic coordinates
827:improve this article
405:celestial navigation
255:constellation Pisces
239:First Point of Aries
142:. It contrasts with
1175:see, for instance,
1123:Moulton, Forest Ray
770:South Ecliptic Pole
762:North Ecliptic Pole
205:Rayleigh scattering
1002:Equinoctial colure
967:
899:
217:
182:
48:
1250:) – to determine
895:sphaera parallela
875:celestial equator
859:
858:
851:
715:
714:
353:15 seconds of arc
333:15 minutes of arc
270:celestial equator
247:celestial equator
144:oblique ascension
69:celestial equator
44:celestial equator
1361:
1326:
1325:
1324:
1314:
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1312:
1302:
1301:
1300:
1290:
1289:
1278:
1277:
1276:
1269:
1244:Sidereal pointer
1240:
1192:
1189:
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1173:
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1112:
1098:
1092:
1082:
1073:Blaeu, Guilielmi
1069:
1063:
1062:
1044:
926:Earth's rotation
922:equatorial mount
879:celestial sphere
854:
847:
843:
840:
834:
811:
803:
793:Besselian epochs
755:
724:Axial precession
710:
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267:
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243:celestial sphere
179:celestial sphere
120:celestial object
114:) refers to the
93:celestial sphere
65:angular distance
61:
32:celestial sphere
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1120:
1116:
1099:
1095:
1089:Right ascension
1070:
1066:
1059:
1045:
1041:
1036:
1031:
1022:Setting circles
972:
951:Right ascension
930:setting circles
891:sphaera obliqua
867:right ascension
855:
844:
838:
835:
824:
812:
801:
753:
730:celestial poles
726:
720:
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370:
368:
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360:
358:
357:24 × 60 × 60 =
356:
345:
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339:
338:
336:
325:
322:
319:
318:
316:
309:
306:
303:
302:
300:
223:
197:
171:Right ascension
164:
104:right ascension
59:
51:Right ascension
24:Right ascension
17:
12:
11:
5:
1367:
1357:
1356:
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1318:
1306:
1294:
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1241:
1220:
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1208:
1200:
1199:External links
1197:
1194:
1193:
1184:
1168:
1159:
1157:(1992), p. 11.
1147:
1138:
1114:
1093:
1085:Ascensio recta
1064:
1057:
1038:
1037:
1035:
1032:
1030:
1029:
1024:
1019:
1014:
1009:
1004:
999:
994:
989:
984:
982:Celestial pole
979:
973:
971:
968:
961:(red) and the
942:(1712, 1725).
934:John Flamsteed
857:
856:
815:
813:
806:
800:
797:
750:ecliptic poles
722:Main article:
719:
716:
713:
712:
687:
650:
648:
588:
584:
583:
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542:
540:
501:
497:
496:
476:
470:
468:
450:
446:
445:
439:
433:
430:
427:
420:
417:
398:sidereal hours
297:degrees of arc
219:Main article:
163:
160:
112:ascensio recta
15:
9:
6:
4:
3:
2:
1366:
1355:
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1259:
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1245:
1242:
1238:
1234:
1230:
1229:Sixty Symbols
1226:
1221:
1218:
1215:
1212:
1209:
1206:
1203:
1202:
1188:
1180:
1172:
1163:
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1118:
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1106:
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1097:
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1079:
1074:
1068:
1060:
1058:0-935702-68-7
1054:
1050:
1043:
1039:
1028:
1027:Sidereal time
1025:
1023:
1020:
1018:
1015:
1013:
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1008:
1005:
1003:
1000:
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995:
993:
990:
988:
985:
983:
980:
978:
975:
974:
964:
960:
956:
955:March equinox
952:
947:
943:
941:
940:
935:
931:
927:
923:
919:
914:
912:
908:
907:star catalogs
904:
896:
892:
888:
887:sphaera recta
884:
880:
876:
872:
868:
863:
853:
850:
842:
832:
828:
822:
821:
816:This section
814:
810:
805:
804:
796:
794:
790:
786:
782:
777:
775:
771:
767:
763:
759:
751:
747:
746:proper motion
743:
739:
735:
731:
725:
686:
651:
589:
586:
585:
562:
543:
502:
499:
498:
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471:
451:
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416:
414:
410:
406:
401:
399:
387:
381:
376:
354:
334:
298:
294:
290:
289:Earth rotates
286:
282:
277:
275:
271:
258:
256:
252:
248:
244:
240:
236:
235:March equinox
232:
228:
222:
214:
210:
206:
201:
195:
194:March equinox
191:
186:
180:
176:
172:
168:
159:
157:
156:oblique angle
153:
149:
145:
141:
137:
133:
129:
125:
122:as seen from
121:
117:
113:
109:
105:
102:An old term,
100:
98:
94:
90:
86:
82:
78:
77:March equinox
74:
70:
66:
62:
56:
53:(abbreviated
52:
45:
42:(red) on the
41:
37:
36:March equinox
33:
29:
25:
21:
1328:Solar System
1256:
1251:
1228:
1187:
1178:
1171:
1162:
1154:
1150:
1141:
1127:
1117:
1103:
1096:
1088:
1084:
1077:
1067:
1048:
1042:
950:
937:
915:
900:
894:
890:
886:
883:great circle
870:
866:
845:
836:
825:Please help
820:verification
817:
789:Julian epoch
778:
727:
402:
383:
281:angular unit
278:
259:
224:
170:
143:
115:
103:
101:
58:
54:
50:
49:
23:
1316:Outer space
1304:Spaceflight
1233:Brady Haran
987:Declination
436:Sexagesimal
285:full circle
196:direction.
190:hour angles
175:declination
173:(blue) and
162:Explanation
140:right angle
85:declination
81:hour circle
28:declination
1338:Categories
1012:Hour angle
903:Hipparchus
734:precession
409:hour angle
380:Hour angle
378:See also:
367:24 × 60 =
132:intersects
1280:Astronomy
1248:Torquetum
918:telescope
871:ascension
375:, or 24.
237:i.e. the
227:longitude
148:latitudes
116:ascension
71:from the
63:) is the
57:; symbol
1235:for the
1125:(1916).
1075:(1668).
992:Ecliptic
970:See also
959:ecliptic
839:May 2012
768:and the
605:minute,
413:meridian
394:20 00 00
293:meridian
213:opposite
209:blue sky
188:Various
87:, these
79:to the (
40:ecliptic
1266:Portals
963:equator
799:History
781:J2000.0
758:Polaris
709:
690:
681:
669:
665:
653:
645:
628:
624:
607:
603:
591:
587:Second
580:
566:
557:
545:
537:
520:
516:
504:
500:Minute
493:
479:
465:
453:
443:radians
400:later.
390:1 30 00
349:
337:
329:
317:
313:
301:
251:equinox
231:equator
152:horizon
136:horizon
128:equator
95:in the
75:at the
1055:
774:Dorado
754:
647:circle
626:hour,
539:circle
518:hour,
467:circle
438:system
432:Symbol
274:zenith
154:at an
1349:Angle
1292:Stars
1135:–126.
1111:, 39.
766:Draco
742:epoch
683:′, 15
559:°, 15
449:Hour
429:Value
365:, or
346:86400
138:at a
124:Earth
108:Latin
1053:ISBN
865:How
711:rad
582:rad
495:rad
426:Unit
326:1440
279:Any
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936:'s
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276:).
200:Sun
126:'s
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1252:RA
1231:.
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483:π
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