# Conventional units for astronomical object coordinates

I would like to know what the conventional units for identifying a celestial object (J2000.0) are. For instance, I am reading a paper in which they refer to object SDSSJ170013.70+400855.6; I am guessing that 170013.70 is equivalent to 17h 00m 13.70s but unsure about the second number as the leading number is greater than 24. Does this change based on the specific sky survey or is there a typical one that most surveys use?

Right ascension is usually given in hours, minutes and seconds, but declination is usually given in degrees, arcminutes (') and arcseconds ("), with one arcminute being 1/60th of a degree and one arcsecond being 1/60th of an arcminute. This means that the declination of the source you list should be 40$$^{\circ}$$08"55.6', in accordance with the SDSS source naming system. Surveys or naming conventions that refer to sources by coordinates usually use something similar, per IAU standards, which, as ProfRob notes, allows for truncation but not rounding..

As an example, modern pulsar naming uses the right ascension in hours and minutes (neglecting seconds) and the declination in degrees (and sometimes arcminutes, but neglecting arcseconds). The notable double pulsar system PSR J0737-3039 has a right ascension 07h37m51.248s and a declination of -30$$^{\circ}$$39'40.83"; hence its name. However, the coordinates of a source depend on the reference epoch. These coordinates are for the J2000 epoch; older pulsars are usually named by their coordinates in the B1950 epoch, though they may since have been assigned a J2000 name as well - which can be a little confusing, particularly if I was to informally just refer to the double pulsar as "0737" (although everyone really just says "the double pulsar", since it's the only double pulsar known!). The "J" in the SDSS name of your source is used to make it clear that the coordinates are in the J2000 epoch.

• I believe that the coordinates are truncated in the name as apposed to rounded. Worth noting perhaps. Jul 21 at 5:50
• @ProfRob I had thought about that, but the listed declination on Simbad uses 56.23 arcseconds, which certainly differs from 55.6 regardless of truncation or rounding. Looks like it might just be an issue of listing the 2MASS measurement, as far as I can tell. Ah, unless you mean SDSS truncating their measurement, which is also possible. Jul 21 at 13:07
• cdsweb.u-strasbg.fr/Dic/iau-spec.html#S3.2.1 Jul 21 at 14:51
• @ProfRob Ah, you meant in general. Thanks. Jul 21 at 15:04