(February 2000)

The unit in which physical parameters are expressed is a fundamental parameter which becomes especially important when data are to be compared, or used outside a specialized field of science — most physicists have never heard of astronomy-specific units.

Special care has been taken to try to use the standard SI units,
and to convert if necessary the unit to such standards:
for instance, we use the string
`0.1nm` to express Angströms (Å*=10 ^{–10}m*),
since the Angström is a non-standard unit.
Another example is

Only simple power functions of physical units are accepted, which means
that e.g. `solMass`* ^{3/2}* (solar mass at a

The standard adopted here differs from the OGIP ones *(
OGIP memo 93-001 about ``Specification of Physical Units within OGIP FITS files"
by Ian M. George & Lorella Angelini, August 1993)* for
the syntax of composite units (operator symbols),
and in the usage of math functions (only the square bracketted units,
representing the *log* function is accepted here) and of obsolete CGS units
which are not avoided here;
the basic symbols however agree.

**Rule****2**: the only allowed numerical factor is at the
very beginning of the Unit string. The
structure of the unit is therefore the concatenation of
*factor* and *unit_expression*
and we will write the ``number of pixels per Å" as
`10pix/nm`, and neither `pix/0.1nm` nor `pix/(0.1nm)`.

The numerical *factor* may include the letter `x`
for the multiplication, as `1.5x10+11` to express the number
*1.5×10 ^{11}*

**Rule****3**: The operators to express a compound unit are

**Rule****4**: a simple (non-compound) unit is made of a
*basic unit* symbol, eventually preceded
by a *multiple prefix*.

Among several possible expressions of a unit, it is
preferable to choose the shortest one; this leads also to prefer
the division (**/**) to the multiplication of the inverse,
e.g. prefer `km/s` to `km.s-1`.

Symbol Explanation Definition

(c)—Unitless value(c)%Unitless value, in percent10^{–2}(a)ayear (alsoyr)365.25d = 31.5576×10^{6}s(b)AAmpere(a)AUastronomical unit1.49598×10^{11}m(a)arcminminute of arc1/60°(a)arcsecsecond of arc1/60arcmin(e)barnbarn (cross-section)10m^{–28}^{2}(c)bitbinary information unit (computer storage)(c)bytebyte (computer storage) 8 bitCCoulomb (electric charge) A⋅s(b)cdCandela (luminous intensity)(c)ctCount (events)DDebye(1/3)×10C^{–29}⋅m(a)dday24h = 86.4×10^{3}s(a)degdegree of arc (°)π/180rad(e)eVelectron-Volt1.602177×10J^{–19}FFarad (electric capacitance) C/V(b)ggram10kg^{–3}(a)hhour of time (sideral if appropriate) 3600sHHenry (inductance) Wb/AHzHertz (frequency) s^{–1}JJoule (energy) N⋅m(a)JyJansky10W/m^{–26}/Hz^{2}(b)KKelvinlmlumen (luminous flux) cd⋅srlxlux (illuminance) lm/m^{2}(b)mmetre(a)magmagnitudes(a)masmillisecond of arc(π/6.48)×10rad^{–8}(a)minminute of time (sideral if appropriate)(b)molmoleNNewton (force) kg⋅m/s^{2}Ohm(Ω) Ohm (electric resistance) V/APaPascal (pressure) N/m^{2}(a)pcparsec3.0857 ×10^{16}m(c)pixpixel (image element)(b)radradian (angle)(e)RyRydberg (energy)(1/2) (2πe= 13.60583 eV^{2}/hc)^{2}m_{e}c^{2}(b)ssecond of timeSSiemens (electric conductance) A/V(c)solLumSolar luminosity3.826×10W^{26}(c)solMassSolar mass1.989×10kg^{30}(c)solRadSolar radius6.9599×10m^{8}(c)SunUnit referring to the Sun (e.g. abundances)srsteradian (solid angle)TTesla (magnetic field intensity) Wb/m^{2}VVolts (electric potential) W/AWWatt (power) J/sWbWeber (magnetic flux) V⋅s(c)yryear (alsoa)365.25d = 31.5576×10^{6}s

Symbol Explanation Value

ddeci10^{–1}ccenti10^{–2}mmilli10^{–3}umicro (µ)10^{–6}nnano10^{–9}ppico10^{–12}ffemto10^{–15}aatto10^{–18}zzepto10^{–21}yyocto10^{–24}

Symbol Explanation Value

dadeca10hhecto10^{2}kkilo10^{3}Mmega10^{6}Ggiga10^{9}Ttera10^{12}Ppeta10^{15}Eexa10^{18}Zzetta10^{21}Yyotta10^{24}

See the *Unit Calculator*

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