Astrophysics (index)about

J Designator

(designator using ecliptic 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 Ecliptic Coordinate System, i.e., in relation to the Ecliptic (the plane of Earth's orbit around the Sun). Example number:


The meaning of the numbers are as follows:
  • The letter J - meaning as per J2000.0 Equinox and Epoch J2000.0.
  • HH - hours Right Ascension (1/24 of a circle)
  • MM - minutes right ascension (1/60 of an hour)
  • SS - seconds right ascension (1/60 of a minute)
  • ss - tenths/hundredths of a second right ascension
  • +/- - Declination above or below (north or south)
  • DD - degrees declination (1/360 of a circle)
  • MM - minutes declination (1/60 of a degree)
  • SS - seconds declination (1/60 of a minute)
  • s - tenths of a second declination

I presume the right ascension and declination are also measured from the plane of Earth's orbit at time J20000 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:


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:
Phone Number
Submillimeter Galaxy Designator
Survey-based Designator