Radio Galaxy Zoo Talk

What does J mean?

  • Martin_Sexton by Martin_Sexton

    In discussions I keep seeing J values quoted for FIRST, SDSS etc. These J values look like image identifiers but they don't bear any obvious relation to the Ra/Dec values.

    How can I use the J values to search FIRST, SDSS etc to see if I agree with other people's identifications of possible sources etc?


  • JeanTate by JeanTate in response to Martin Sexton's comment.

    Welcome to RGZ, Martin Sexton!

    I think the "J" stands for "J2000.0", which is the 'zero point' for the celestial coordinates quoted (celestial coordinates are Right Ascension (RA) - sorta equivalent to longitude - and Declination (Dec) - latitude). As the zero point for the celestial coordinate system tied to the Earth's rotational axis is not fixed (due to precession, nutation, etc), astronomers tie it a "Julian year", and the start of 2000 (hence 2000.0) is the most commonly used one today.

    The numbers after a J - "091322.4+261753", say, or "091319.57+261814.7" - are the RA and Dec of the object, expressed in hoursminutesseconds+/-degreesminutesseconds (RA is given either in "hours:minutes:seconds" (hours go from 00 to 24) or degrees (0 to 360)), so the first object has a position of 9h 13m 22.4s in RA, and +26d 17m 53s in Dec.

    I don't know when astronomers starting using the position of an object as its name (or ID), but it sure is handy (except for the fact that hh:mm:ss.s isn't as easy to work with as plain degrees).

    Happy hunting! 😃


  • DocR by DocR scientist

    J is for Julian (the calendar). Check out the wikipedia article: