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J designator

(designator using equatorial coordinates at year 2000)

I'm using the term J designator for identifiers or descriptions of astronomical objects consisting of a J followed by numbers, which are commonly used to designate astronomical objects. It describes the object's directional position using equatorial coordinates. Example number:


The meaning of the numbers are as follows:

(I presume the right ascension and declination are also measured from the plane of Earth equator at time J2000.0 epoch.) Thus J162702.56+432833.9 means: "At noon on January 1, 2000 GMT, the object was at 16 hours 27 minutes and 2.56 seconds right ascension and +43 degrees 28 arcminutes 33.9 arcseconds declination (according to equatorial coordinate axes of that same time)." Some of the simplified formats showing less precision:


Note that minutes and/or arcminutes can be specified with decimal fractions. The "HH" and "DD" fields can be reduced to a single digit, i.e., they do not need to be zero-filled, so an odd number of digits so "J110+3344", for example, means "1 hour 10 minutes, 33 degrees 44 arcminutes". Without the initial "J", the epoch is not specified. A "B" in place of the "J" (e.g., "B110+3344") indicates epoch B1950.0 instead of J2000.0. Often, the object is abbreviated further, e.g., after the first mention within a paper or lecture. For example, in a lengthy discussion of J162702.56+432833.9, further mentions might be abbreviated as J1627. For much-discussed objects, sometimes such a short abbreviation is all that is mentioned, but generally the type of object under discussion is a further clue to exactly which object is being discussed.

Often an object is specified by a survey or project that discovered it along with the right ascension and declination, with or without the J, e.g., "SDSS J1517+3353" for the Sloan Digital Sky Survey.

Further reading:
BB110+3344epoch B1950.0
JJ162702.56+432833.9epoch J2000.0

Referenced by pages:
equatorial coordinate system
epoch J2000.0
phone number
submillimeter galaxy designator
survey-based designator