Are there any standards or established practices on how to write / represent Right Ascension (RA) and Declination (DEC) values in the sexagesimal format? I have seen all of the following. E.g. for RA:

  • 02 03 04 (assumed to mean 2 hours, 3 hourly minutes and 4 hourly seconds)
  • 2 3 4 (same as above)
  • 2h 3m 4s (same as above)
  • 2d 3' 6" (assumed to mean 2 degrees, 3 degree minutes and 4 degree seconds)
  • 2d 3m 6s (same as above)
  • 15.33 (assumed to mean 15.33 degrees)
  • 9.5H (assumed to mean 9 hours, 30 hourly minutes and zero hourly seconds)
  • 8d 3.5 (assumed to mean 8 degrees, 3 degree minutes and 30 degree seconds)

Likewise for DEC (even though for DEC when the first component does not have a suffix it is assumed by default to be in degrees, not hours). Does any astronomical association publish guidelines for the printing / presentation of such values or on how software should accept user's input of such values?

  • $\begingroup$ Googling "format" "right ascension" "standards" as quoted yields some hits suggesting people are trying to standardize this, but haven't actually done so. $\endgroup$
    – user21
    Aug 2, 2018 at 19:30
  • 1
    $\begingroup$ There is a (slow) general trend towards using decimal degrees for right ascension and declination to have things in consistent units and to avoid difficulties with parsing the sexagesimal values (values within +/- 1 degree of the celestial equator, particularly negative values, are often tricky). But all of the ones you listed above, plus the use (or not) of colon separators, are in use. On the grounds that most software prefers not to have extra letters in between the digits, I tend to prefer putting the units separately as part of the field name/caption, rather than in the value itself. $\endgroup$ Aug 2, 2018 at 20:05

1 Answer 1


There are no standards for representing right ascension and declination apart from the abbreviations RA and Dec, and the symbols $\alpha$ and $\delta$.

The use of decimal degrees, dms, °'", hours and decimals, hms etc. depend on the [author's] style and intent. For example, determining transit times from local sidereal time and right ascension is easier when RA is expressed in hours, whereas doing calculations of angular separation is easier to calculate in software when decimal degrees are used.


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