This section is mainly based on the ‘Units home page’ of the National
Institute of Standards and Technology (see the NIST website),
which generally agrees with
IAU recommendations
For interested users we recommend to read Thompson and Taylor (2008).
In summary, The Gaia collaboration uses ‘acceptable SI units’ (as defined below)
as default, plus a few extensions also described below.
Following Thompson and Taylor (2008), we interpret
the term ‘SI units’ as the seven SI base units (kg, m, s, mol, A, K,
cd; see Section 4.1 in Thompson and Taylor (2008)), plus the 20 SI derived units (N, V, Hz, Gy, W,
etc.; see Section 4.2 in Thompson and Taylor (2008)), plus the two SI supplementary units
(rad and sr; Section 4.2.2 in Thompson and Taylor (2008)), including multiples and sub-multiples of these units
formed by using SI prefixes ($\mathrm{M}={10}^{6}$, $\mathrm{k}={10}^{3}$, $\mu ={10}^{-6}$, etc.; Table 5 in Thompson and Taylor (2008)). The term ‘acceptable SI units’ is
taken to denote the ‘SI units’ (as defined above), plus those units
accepted by the CIPM (Comité International des Poids et Mesures) for
use with the SI (notably angular degree, arcminute, arcsecond, minute,
hour, and day; Tables 6 and 7 in Thompson and Taylor (2008)), plus those units temporarily
accepted for use with the SI (Table 9 and Section 5.2 in
Thompson and Taylor (2008)), including multiples and sub-multiples of these units.
We also follow Thompson and Taylor (2008) and the IAU by recognizing that the use of time
intervals expressed in units of Julian years
(year), distances in units of parsecs (pc) or astronomical units
(au), and source brightness/luminosity in units of magnitudes (mag) is
allowed. The use of the non-SI unit Å is ‘temporarily accepted’ by
Thompson and Taylor (2008) and ‘deprecated’ by the IAU; we propose that this unit is not used.