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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 Coordinate System. 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 minutes 33.9 seconds declination.

Simpler formats with less precision:


(Note that fractional minutes can be specified.) The "HH" and "DD" fields can be reduced to a single digit, i.e., they do not need to be zero-filled.

Without the initial "J", the Epoch is not specified. Sometimes 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.


Referenced by:
Equatorial Coordinate System
Phone Number
Submillimeter Galaxy Designator
Survey-based Designator