As it is relatively easy to find an official value for a large number of physical constants, (thanks to CODATA), it is not so easy for some units widely used in astronomy (Wikipedia, Google and IAU give different values). So in 2012, what are the "official normalized values" (with the largest precision) for :

  • Year (the one used for light-years) (in $\mathrm{s}$)?
  • Solar mass (in $\mathrm{kg}$)?
  • Astronomical unit (in $\mathrm{m}$)?
  • Light-year (in $\mathrm{m}$)?
  • Parsec (in $\mathrm{m}$)?
  • 2
    $\begingroup$ Not that this is a bad question as is, but it would make it a whole lot better if you include the conflicting values you've gotten from different sources for these units. $\endgroup$
    – David Z
    Sep 2, 2012 at 3:23

1 Answer 1


The IAU General Assembly 2012 finished a few days ago. Assuming resolution B2 (PDF) was passed, the astronomical unit has been frozen and the following values are exact by definition $$ 1\mathrm{a} = 365.25\mathrm{d} = 365.25 \cdot 86\,400\mathrm{s} = 31\,557\,600\mathrm{s} \\ 1\mathrm{ly} = 299\,792\,458 \mathrm{\frac ms} \cdot 1\mathrm{a} = 9\,460\,730\,472\,580\,800\mathrm{m} \\ 1\mathrm{au} = 149\,597\,870\,700 \mathrm{m} $$

This also gives the exact value $$ 1\mathrm{pc} = 1\mathrm{au} \cdot \cot 1'' = 1\mathrm{au} \cdot \cot \frac{\pi}{648\,000} \approx 30\,856\,775\,814\,671\,916\mathrm{m} $$

Keep in mind that this makes both year and astronomical unit well-defined units which only approximate the (time-dependant) physical quantities they where derived from originally.

The heliocentric gravitational constant $GM_S$ (and thus the solar mass $M_S$) still needs to be determined observationally.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.