<p>Right ascension is usually given in hours, minutes and seconds, but declination is usually given in degrees, arcminutes (') and arcseconds (&quot;), 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<span class="math-container">$^{\circ}$</span>08&quot;55.6', in accordance with <a href="https://classic.sdss.org/dr3/coverage/IAU.html" rel="nofollow noreferrer">the SDSS source naming system</a>. Surveys or naming conventions that refer to sources by coordinates usually use something similar, <a href="http://cdsweb.u-strasbg.fr/Dic/iau-spec.html#S3.2.1" rel="nofollow noreferrer">per IAU standards</a>, which, as ProfRob notes, allows for truncation but not rounding..</p>
<p>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 <a href="https://en.wikipedia.org/wiki/PSR_J0737%E2%88%923039" rel="nofollow noreferrer">PSR J0737-3039</a> has a right ascension 07h37m51.248s and a declination of -30<span class="math-container">$^{\circ}$</span>39'40.83&quot;; hence its name. However, the coordinates of a source depend on the reference <a href="https://en.wikipedia.org/wiki/Epoch_(astronomy)" rel="nofollow noreferrer">epoch</a>. 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 &quot;0737&quot; (although everyone really just says &quot;the double pulsar&quot;, since it's the only double pulsar known!). The &quot;J&quot; in the SDSS name of your source is used to make it clear that the coordinates are in the J2000 epoch.</p>

CC BY-SA 4.0
2021-07-21T05:50:15.620Z
I believe that the coordinates are truncated in the name as apposed to rounded. Worth noting perhaps.
ProfRob
CC BY-SA 4.0
2021-07-21T13:07:32.700Z
@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.
HDE 226868
CC BY-SA 4.0
2021-07-21T14:51:51.253Z
http://cdsweb.u-strasbg.fr/Dic/iau-spec.html#S3.2.1
ProfRob
CC BY-SA 4.0
2021-07-21T15:04:17.580Z
@ProfRob Ah, you meant in general. Thanks.
HDE 226868
CC BY-SA 4.0
2021-07-20T22:51:28.153Z
2021-07-21T15:06:28.297Z
2021-07-21T15:06:28.297Z
HDE 226868
<p>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 <em>SDSSJ170013.70+400855.6</em>; 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?</p>

<observational-astronomy><coordinate><data-analysis>
Conventional units for astronomical object coordinates
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# Conventional units for astronomical object coordinates

(Asked by: username_0 on 2021-07-20)

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?

**Tags:** observational-astronomy, coordinate, data-analysis


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## 1 answers

### Answer by username_1 on 2021-07-20. Score: 7 | Accepted answer

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](https://classic.sdss.org/dr3/coverage/IAU.html). Surveys or naming conventions that refer to sources by coordinates usually use something similar, [per IAU standards](http://cdsweb.u-strasbg.fr/Dic/iau-spec.html#S3.2.1), which, as username_2 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](https://en.wikipedia.org/wiki/PSR_J0737%E2%88%923039) 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](https://en.wikipedia.org/wiki/Epoch_(astronomy)). 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.

> **Comments:**
> **username_2**: I believe that the coordinates are truncated in the name as apposed to rounded. Worth noting perhaps.  
> **username_1**: @username_2 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.  
> **username_2**: http://cdsweb.u-strasbg.fr/Dic/iau-spec.html#S3.2.1  
> **username_1**: @username_2 Ah, you meant in general. Thanks.  

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