Units and Quantities (astropy.units)

Introduction

astropy.units handles defining, converting between, and performing arithmetic with physical quantities, such as meters, seconds, Hz, etc. It also handles logarithmic units such as magnitude and decibel.

astropy.units does not know spherical geometry or sexagesimal (hours, min, sec): if you want to deal with celestial coordinates, see the astropy.coordinates package.

Getting Started

Most users of the astropy.units package will work with “quantities”: the combination of a value and a unit. The easiest way to create a Quantity is to simply multiply or divide a value by one of the built-in units. It works with scalars, sequences and Numpy arrays:

>>> from astropy import units as u
>>> 42.0 * u.meter
<Quantity 42.0 m>
>>> [1., 2., 3.] * u.m
<Quantity [ 1., 2., 3.] m>
>>> import numpy as np
>>> np.array([1., 2., 3.]) * u.m
<Quantity [ 1., 2., 3.] m>

You can get the unit and value from a Quantity using the unit and value members:

>>> q = 42.0 * u.meter
>>> q.value
42.0
>>> q.unit
Unit("m")

From this simple building block, it’s easy to start combining quantities with different units:

>>> 15.1 * u.meter / (32.0 * u.second)  
<Quantity 0.471875 m / s>
>>> 3.0 * u.kilometer / (130.51 * u.meter / u.second)  
<Quantity 0.022986744310780783 km s / m>
>>> (3.0 * u.kilometer / (130.51 * u.meter / u.second)).decompose()  
<Quantity 22.986744310780782 s>

Unit conversion is done using the to() method, which returns a new Quantity in the given unit:

>>> x = 1.0 * u.parsec
>>> x.to(u.km)  
<Quantity 30856775814671.914 km>

It is also possible to work directly with units at a lower level, for example, to create custom units:

>>> from astropy.units import imperial

>>> cms = u.cm / u.s
>>> # ...and then use some imperial units
>>> mph = imperial.mile / u.hour

>>> # And do some conversions
>>> q = 42.0 * cms
>>> q.to(mph)  
<Quantity 0.939513242662849 mi / h>

Units that “cancel out” become a special unit called the “dimensionless unit”:

>>> u.m / u.m
Unit(dimensionless)

astropy.units is able to match compound units against the units it already knows about:

>>> (u.s ** -1).compose()  
[Unit("Bq"), Unit("Hz"), Unit("3.7e+10 Ci")]

And it can convert between unit systems, such as SI or CGS:

>>> (1.0 * u.Pa).cgs
<Quantity 10.0 Ba>

The units mag, dex and dB are special, being logarithmic units, for which a value is the logarithm of a physical quantity in a given unit. These can be used with a physical unit in parentheses to create a corresponding logarithmic quantity:

>>> -2.5 * u.mag(u.ct / u.s)
<Magnitude -2.5 mag(ct / s)>
>>> from astropy import constants as c
>>> u.Dex((c.G * u.M_sun / u.R_sun**2).cgs)  
<Dex 4.43842814841305 dex(cm / s2)>

astropy.units also handles equivalencies, such as that between wavelength and frequency. To use that feature, equivalence objects are passed to the to() conversion method. For instance, a conversion from wavelength to frequency doesn’t normally work:

>>> (1000 * u.nm).to(u.Hz)  
Traceback (most recent call last):
  ...
UnitConversionError: 'nm' (length) and 'Hz' (frequency) are not convertible

but by passing an equivalency list, in this case spectral(), it does:

>>> (1000 * u.nm).to(u.Hz, equivalencies=u.spectral())
<Quantity 299792457999999.94 Hz>

Quantities and units can be printed nicely to strings using the Format String Syntax, the preferred string formatting syntax in recent versions of python. Format specifiers (like 0.03f) in new-style format strings will used to format the quantity value:

>>> q = 15.1 * u.meter / (32.0 * u.second)
>>> q  
<Quantity 0.471875 m / s>
>>> "{0:0.03f}".format(q)
'0.472 m / s'

The value and unit can also be formatted separately. Format specifiers used on units can be used to choose the unit formatter:

>>> q = 15.1 * u.meter / (32.0 * u.second)
>>> q  
<Quantity 0.471875 m / s>
>>> "{0.value:0.03f} {0.unit:FITS}".format(q)
'0.472 m s-1'

Using astropy.units

• Quantity
  • Creating Quantity instances
  • Converting to different units
  • Plotting quantities
  • Arithmetic
  • Numpy functions
  • Dimensionless quantities
  • Converting to plain Python scalars
  • Functions Accepting Quantities
  • Known issues with conversion to numpy arrays
  • Subclassing Quantity
• Standard units
  • The dimensionless unit
  • Enabling other units
• Combining and defining units
• Decomposing and composing units
  • Reducing a unit to its irreducible parts
  • Automatically composing a unit into more complex units
  • Converting between systems
• Magnitudes and other Logarithmic Units
  • Creating Logarithmic Quantities
  • Converting to different units
  • Arithmetic
  • Numpy functions
  • Dimensionless logarithmic quantities
• String representations of units
  • Converting units to string representations
  • Creating units from strings
  • Built-in formats
  • Unrecognized Units
• Equivalencies
  • Built-in equivalencies
  • Writing new equivalencies
  • Displaying available equivalencies
  • Using equivalencies in larger pieces of code
• Low-level unit conversion
  • Direct Conversion
  • Incompatible Conversions

See Also

• FITS Standard for units in FITS.
• The Units in the VO 1.0 Standard for representing units in the VO.
• OGIP Units: A standard for storing units in OGIP FITS files.
• Standards for astronomical catalogues units.
• IAU Style Manual.
• A table of astronomical unit equivalencies

Reference/API

astropy.units.quantity Module

This module defines the Quantity object, which represents a number with some associated units. Quantity objects support operations like ordinary numbers, but will deal with unit conversions internally.

Classes

Quantity

A Quantity represents a number with some associated unit.

Class Inheritance Diagram

astropy.units Package

This subpackage contains classes and functions for defining and converting between different physical units.

This code is adapted from the pynbody units module written by Andrew Pontzen, who has granted the Astropy project permission to use the code under a BSD license.

Functions

add_enabled_equivalencies(equivalencies)	Adds to the equivalencies enabled in the unit registry.
add_enabled_units(units)	Adds to the set of units enabled in the unit registry.
brightness_temperature(beam_area, disp)	Defines the conversion between Jy/beam and “brightness temperature”, \(T_B\), in Kelvins.
def_physical_type(unit, name)	Adds a new physical unit mapping.
def_unit(s[, represents, doc, format, ...])	Factory function for defining new units.
dimensionless_angles()	Allow angles to be equivalent to dimensionless (with 1 rad = 1 m/m = 1).
doppler_optical(rest)	Return the equivalency pairs for the optical convention for velocity.
doppler_radio(rest)	Return the equivalency pairs for the radio convention for velocity.
doppler_relativistic(rest)	Return the equivalency pairs for the relativistic convention for velocity.
get_current_unit_registry()
get_physical_type(unit)	Given a unit, returns the name of the physical quantity it represents.
logarithmic()	Allow logarithmic units to be converted to dimensionless fractions
mass_energy()	Returns a list of equivalence pairs that handle the conversion between mass and energy.
parallax()	Returns a list of equivalence pairs that handle the conversion between parallax angle and distance.
quantity_input	A decorator for validating the units of arguments to functions.
set_enabled_equivalencies(equivalencies)	Sets the equivalencies enabled in the unit registry.
set_enabled_units(units)	Sets the units enabled in the unit registry.
spectral()	Returns a list of equivalence pairs that handle spectral wavelength, wave number, frequency, and energy equivalences.
spectral_density(wav[, factor])	Returns a list of equivalence pairs that handle spectral density with regard to wavelength and frequency.
temperature()	Convert between Kelvin, Celsius, and Fahrenheit here because Unit and CompositeUnit cannot do addition or subtraction properly.
temperature_energy()	Convert between Kelvin and keV(eV) to an equivalent amount.

Classes

CompositeUnit	Create a composite unit using expressions of previously defined units.
Decibel
DecibelUnit([physical_unit, function_unit])	Logarithmic physical units expressed in dB
Dex
DexUnit([physical_unit, function_unit])	Logarithmic physical units expressed in magnitudes
FunctionQuantity	A representation of a (scaled) function of a number with a unit.
FunctionUnitBase([physical_unit, function_unit])	Abstract base class for function units.
IrreducibleUnit	Irreducible units are the units that all other units are defined in terms of.
LogQuantity	A representation of a (scaled) logarithm of a number with a unit
LogUnit([physical_unit, function_unit])	Logarithmic unit containing a physical one
MagUnit([physical_unit, function_unit])	Logarithmic physical units expressed in magnitudes
Magnitude
NamedUnit	The base class of units that have a name.
PrefixUnit	A unit that is simply a SI-prefixed version of another unit.
Quantity	A Quantity represents a number with some associated unit.
Unit	The main unit class.
UnitBase	Abstract base class for units.
UnitConversionError	Used specifically for errors related to converting between units or interpreting units in terms of other units.
UnitsError	The base class for unit-specific exceptions.
UnitsWarning	The base class for unit-specific exceptions.
UnrecognizedUnit	A unit that did not parse correctly.

Class Inheritance Diagram

astropy.units.format Package

A collection of different unit formats.

Functions

get_format([format])

Get a formatter by name.

Classes

Base	The abstract base class of all unit formats.
Generic	A “generic” format.
CDS	Support the Centre de Données astronomiques de Strasbourg Standards for Astronomical Catalogues 2.0 format, and the complete set of supported units.
Console	Output-only format for to display pretty formatting at the console.
Fits	The FITS standard unit format.
Latex	Output LaTeX to display the unit based on IAU style guidelines.
LatexInline	Output LaTeX to display the unit based on IAU style guidelines with negative powers.
OGIP	Support the units in Office of Guest Investigator Programs (OGIP) FITS files.
Unicode	Output-only format to display pretty formatting at the console using Unicode characters.
Unscaled	A format that doesn’t display the scale part of the unit, other than that, it is identical to the Generic format.
VOUnit	The IVOA standard for units used by the VO.

Class Inheritance Diagram

astropy.units.si Module

This package defines the SI units. They are also available in the astropy.units namespace.

Available Units

Unit	Description	Represents	Aliases	SI Prefixes
A	ampere: base unit of electric current in SI		ampere, amp	T
a	annum (a)	\(\mathrm{365.25\,d}\)	annum	T
Angstrom	ångström: 10 ** -10 m	\(\mathrm{0.1\,nm}\)	AA, angstrom	F
arcmin	arc minute: angular measurement	\(\mathrm{0.016666667\,{}^{\circ}}\)	arcminute	T
arcsec	arc second: angular measurement	\(\mathrm{0.00027777778\,{}^{\circ}}\)	arcsecond	T
bar	bar: pressure	\(\mathrm{100000\,Pa}\)		F
Bq	becquerel: unit of radioactivity	\(\mathrm{Hz}\)	becquerel	F
C	coulomb: electric charge	\(\mathrm{A\,s}\)	coulomb	T
cd	candela: base unit of luminous intensity in SI		candela	T
Ci	curie: unit of radioactivity	\(\mathrm{2.7027027 \times 10^{-11}\,Bq}\)	curie	F
d	day (d)	\(\mathrm{24\,h}\)	day	T
deg	degree: angular measurement 1/360 of full rotation	\(\mathrm{0.017453293\,rad}\)	degree	T
deg_C	Degrees Celsius		Celsius	F
eV	Electron Volt	\(\mathrm{1.6021766 \times 10^{-19}\,J}\)	electronvolt	T
F	Farad: electrical capacitance	\(\mathrm{\frac{C}{V}}\)	Farad, farad	T
fortnight	fortnight	\(\mathrm{2\,wk}\)		F
g	gram (g)	\(\mathrm{0.001\,kg}\)	gram	T
H	Henry: inductance	\(\mathrm{\frac{Wb}{A}}\)	Henry, henry	T
h	hour (h)	\(\mathrm{3600\,s}\)	hour, hr	T
hourangle	hour angle: angular measurement with 24 in a full circle	\(\mathrm{15\,{}^{\circ}}\)		F
Hz	Frequency	\(\mathrm{\frac{1}{s}}\)	Hertz, hertz	T
J	Joule: energy	\(\mathrm{N\,m}\)	Joule, joule	T
K	Kelvin: temperature with a null point at absolute zero.		Kelvin	T
kg	kilogram: base unit of mass in SI.		kilogram	F
l	liter: metric unit of volume	\(\mathrm{1000\,cm^{3}}\)	L, liter	T
lm	lumen: luminous flux	\(\mathrm{cd\,sr}\)	lumen	T
lx	lux: luminous emittence	\(\mathrm{\frac{lm}{m^{2}}}\)	lux	T
m	meter: base unit of length in SI		meter	T
mas	arc second: angular measurement	\(\mathrm{0.001\,{}^{\prime\prime}}\)		F
micron	micron: alias for micrometer (um)	\(\mathrm{\mu m}\)		F
min	minute (min)	\(\mathrm{60\,s}\)	minute	T
mol	mole: amount of a chemical substance in SI.		mole	T
N	Newton: force	\(\mathrm{\frac{kg\,m}{s^{2}}}\)	Newton, newton	T
Ohm	Ohm: electrical resistance	\(\mathrm{\frac{V}{A}}\)	ohm, Ohm	T
Pa	Pascal: pressure	\(\mathrm{\frac{J}{m^{3}}}\)	Pascal, pascal	T
%	percent: one hundredth of unity, factor 0.01	\(\mathrm{0.01\,}\)	pct	F
rad	radian: angular measurement of the ratio between the length on an arc and its radius		radian	T
S	Siemens: electrical conductance	\(\mathrm{\frac{A}{V}}\)	Siemens, siemens	T
s	second: base unit of time in SI.		second	T
sday	Sidereal day (sday) is the time of one rotation of the Earth.	\(\mathrm{86164.091\,s}\)		F
sr	steradian: unit of solid angle in SI	\(\mathrm{rad^{2}}\)	steradian	T
t	Metric tonne	\(\mathrm{1000\,kg}\)	tonne	F
T	Tesla: magnetic flux density	\(\mathrm{\frac{Wb}{m^{2}}}\)	Tesla, tesla	T
uas	arc second: angular measurement	\(\mathrm{1 \times 10^{-6}\,{}^{\prime\prime}}\)		F
V	Volt: electric potential or electromotive force	\(\mathrm{\frac{J}{C}}\)	Volt, volt	T
W	Watt: power	\(\mathrm{\frac{J}{s}}\)	Watt, watt	T
Wb	Weber: magnetic flux	\(\mathrm{V\,s}\)	Weber, weber	T
wk	week (wk)	\(\mathrm{7\,d}\)	week	F
yr	year (yr)	\(\mathrm{365.25\,d}\)	year	T

astropy.units.cgs Module

This package defines the CGS units. They are also available in the top-level astropy.units namespace.

Available Units

Unit	Description	Represents	Aliases	SI Prefixes
abC	abcoulomb: CGS (EMU) of charge	\(\mathrm{Bi\,s}\)	abcoulomb	F
Ba	Barye: CGS unit of pressure	\(\mathrm{\frac{g}{cm\,s^{2}}}\)	Barye, barye	T
Bi	Biot: CGS (EMU) unit of current	\(\mathrm{\frac{cm^{1/2}\,g^{1/2}}{s}}\)	Biot, abA, abampere, emu	F
C	coulomb: electric charge	\(\mathrm{A\,s}\)	coulomb	F
cd	candela: base unit of luminous intensity in SI		candela	F
cm	centimeter (cm)	\(\mathrm{cm}\)	centimeter	F
D	Debye: CGS unit of electric dipole moment	\(\mathrm{3.3333333 \times 10^{-30}\,C\,m}\)	Debye, debye	T
deg_C	Degrees Celsius		Celsius	F
dyn	dyne: CGS unit of force	\(\mathrm{\frac{cm\,g}{s^{2}}}\)	dyne	T
erg	erg: CGS unit of energy	\(\mathrm{\frac{cm^{2}\,g}{s^{2}}}\)		T
Fr	Franklin: CGS (ESU) unit of charge	\(\mathrm{\frac{cm^{3/2}\,g^{1/2}}{s}}\)	Franklin, statcoulomb, statC, esu	F
G	Gauss: CGS unit for magnetic field	\(\mathrm{0.0001\,T}\)	Gauss, gauss	T
g	gram (g)	\(\mathrm{0.001\,kg}\)	gram	F
Gal	Gal: CGS unit of acceleration	\(\mathrm{\frac{cm}{s^{2}}}\)	gal	T
K	Kelvin: temperature with a null point at absolute zero.		Kelvin	F
k	kayser: CGS unit of wavenumber	\(\mathrm{\frac{1}{cm}}\)	Kayser, kayser	T
mol	mole: amount of a chemical substance in SI.		mole	F
P	poise: CGS unit of dynamic viscosity	\(\mathrm{\frac{g}{cm\,s}}\)	poise	T
rad	radian: angular measurement of the ratio between the length on an arc and its radius		radian	F
s	second: base unit of time in SI.		second	F
sr	steradian: unit of solid angle in SI	\(\mathrm{rad^{2}}\)	steradian	F
St	stokes: CGS unit of kinematic viscosity	\(\mathrm{\frac{cm^{2}}{s}}\)	stokes	T
statA	statampere: CGS (ESU) unit of current	\(\mathrm{\frac{Fr}{s}}\)	statampere	F

astropy.units.astrophys Module

This package defines the astrophysics-specific units. They are also available in the astropy.units namespace.

Available Units

Unit	Description	Represents	Aliases	SI Prefixes
adu	adu			T
AU	astronomical unit: approximately the mean Earth–Sun distance.	\(\mathrm{1.4959787 \times 10^{11}\,m}\)	au, astronomical_unit	T
barn	barn: unit of area used in HEP	\(\mathrm{1 \times 10^{-28}\,m^{2}}\)	barn	T
beam	beam			T
bin	bin			T
bit	b (bit)		b, bit	T
byte	B (byte)	\(\mathrm{8\,bit}\)	B, byte	T
chan	chan			T
ct	count (ct)		count, count	T
cycle	cycle: angular measurement, a full turn or rotation	\(\mathrm{6.2831853\,rad}\)	cy	F
earthMass	Earth mass	\(\mathrm{5.9742 \times 10^{24}\,kg}\)	M_earth, Mearth	T
electron	Number of electrons			F
jupiterMass	Jupiter mass	\(\mathrm{1.8987 \times 10^{27}\,kg}\)	M_jup, Mjup, M_jupiter, Mjupiter	T
Jy	Jansky: spectral flux density	\(\mathrm{1 \times 10^{-26}\,\frac{W}{Hz\,m^{2}}}\)	Jansky, jansky	T
lyr	Light year	\(\mathrm{9.4607305 \times 10^{15}\,m}\)	lightyear	T
M_e	Electron mass	\(\mathrm{9.1093829 \times 10^{-31}\,kg}\)		F
M_p	Proton mass	\(\mathrm{1.6726218 \times 10^{-27}\,kg}\)		F
pc	parsec: approximately 3.26 light-years.	\(\mathrm{3.0856776 \times 10^{16}\,m}\)	parsec	T
ph	photon (ph)		photon, photon	T
pix	pixel (pix)		pixel, pixel	T
R	Rayleigh: photon flux	\(\mathrm{7.9577472 \times 10^{8}\,\frac{ph}{s\,sr\,m^{2}}}\)	Rayleigh, rayleigh	T
Ry	Rydberg: Energy of a photon whose wavenumber is the Rydberg constant	\(\mathrm{13.605693\,eV}\)	rydberg	T
solLum	Solar luminance	\(\mathrm{3.846 \times 10^{26}\,W}\)	L_sun, Lsun	T
solMass	Solar mass	\(\mathrm{1.9891 \times 10^{30}\,kg}\)	M_sun, Msun	T
solRad	Solar radius	\(\mathrm{6.95508 \times 10^{8}\,m}\)	R_sun, Rsun	T
Sun	Sun			F
u	Unified atomic mass unit	\(\mathrm{1.6605389 \times 10^{-27}\,kg}\)	Da, Dalton	T
vox	voxel (vox)		voxel, voxel	T

astropy.units.function.units Module

This package defines units that can also be used as functions of other units. If called, their arguments are used to initialize the corresponding function unit (e.g., u.mag(u.ct/u.s)). Note that the prefixed versions cannot be called, as it would be unclear what, e.g., u.mmag(u.ct/u.s) would mean.

Available Units

Unit	Description	Represents	Aliases	SI Prefixes
dB	Decibel: ten per base 10 logarithmic unit	\(\mathrm{0.1\,dex}\)	decibel	F
dex	Dex: Base 10 logarithmic unit			F
mag	Astronomical magnitude: -2.5 per base 10 logarithmic unit	\(\mathrm{-0.4\,dex}\)		T

astropy.units.imperial Module

This package defines colloquially used Imperial units. By default, they are not enabled. To enable them, do:

>>> from astropy.units import imperial
>>> imperial.enable()  

Available Units

Unit	Description	Represents	Aliases	SI Prefixes
ac	International acre	\(\mathrm{43560\,ft^{2}}\)	acre	F
BTU	British thermal unit	\(\mathrm{1.0550559\,kJ}\)	btu	F
cal	Thermochemical calorie: pre-SI metric unit of energy	\(\mathrm{4.184\,J}\)	calorie	F
cup	U.S.	\(\mathrm{0.5\,pint}\)		F
deg_F	Degrees Fahrenheit		Fahrenheit	F
foz	U.S.	\(\mathrm{0.125\,cup}\)	fluid_oz, fluid_ounce	F
ft	International foot	\(\mathrm{12\,inch}\)	foot	F
fur	Furlong	\(\mathrm{660\,ft}\)	furlong	F
gallon	U.S.	\(\mathrm{3.7854118\,\mathcal{l}}\)		F
hp	Electrical horsepower	\(\mathrm{745.69987\,W}\)	horsepower	F
inch	International inch	\(\mathrm{2.54\,cm}\)		F
kcal	Calorie: colloquial definition of Calorie	\(\mathrm{1000\,cal}\)	Cal, Calorie, kilocal, kilocalorie	F
kip	Kilopound: force	\(\mathrm{1000\,lbf}\)	kilopound	F
kn	nautical unit of speed: 1 nmi per hour	\(\mathrm{\frac{nmi}{h}}\)	kt, knot, NMPH	F
lb	International avoirdupois pound: mass	\(\mathrm{16\,oz}\)	lbm, pound	F
lbf	Pound: force	\(\mathrm{\frac{ft\,slug}{s^{2}}}\)		F
mi	International mile	\(\mathrm{5280\,ft}\)	mile	F
mil	Thousandth of an inch	\(\mathrm{0.001\,inch}\)	thou	F
nmi	Nautical mile	\(\mathrm{1852\,m}\)	nauticalmile, NM	F
oz	International avoirdupois ounce: mass	\(\mathrm{28.349523\,g}\)	ounce	F
pint	U.S.	\(\mathrm{0.5\,quart}\)		F
psi	Pound per square inch: pressure	\(\mathrm{\frac{lbf}{inch^{2}}}\)		F
quart	U.S.	\(\mathrm{0.25\,gallon}\)		F
slug	slug: mass	\(\mathrm{32.174049\,lb}\)		F
st	International avoirdupois stone: mass	\(\mathrm{14\,lb}\)	stone	F
tbsp	U.S.	\(\mathrm{0.5\,foz}\)	tablespoon	F
ton	International avoirdupois ton: mass	\(\mathrm{2000\,lb}\)		F
tsp	U.S.	\(\mathrm{0.33333333\,tbsp}\)	teaspoon	F
yd	International yard	\(\mathrm{3\,ft}\)	yard	F

Functions

enable()

Enable Imperial units so they appear in results of find_equivalent_units and compose.

astropy.units.cds Module

This package defines units used in the CDS format.

Contains the units defined in Centre de Données astronomiques de Strasbourg Standards for Astronomical Catalogues 2.0 format, and the complete set of supported units. This format is used by VOTable up to version 1.2.

To include them in compose and the results of find_equivalent_units, do:

>>> from astropy.units import cds
>>> cds.enable()  

Available Units

Unit	Description	Represents	Aliases	SI Prefixes
%	percent	\(\mathrm{\%}\)		F
---	dimensionless and unscaled	\(\mathrm{}\)		F
\h	Planck constant	\(\mathrm{6.6260696 \times 10^{-34}\,J\,s}\)		T
A	Ampere	\(\mathrm{A}\)		T
a	year	\(\mathrm{a}\)		T
a0	Bohr radius	\(\mathrm{5.2917721 \times 10^{-11}\,m}\)		T
AA	Angstrom	\(\mathrm{\mathring{A}}\)	Å, Angstrom, Angstroem	T
al	Light year	\(\mathrm{lyr}\)		T
alpha	Fine structure constant	\(\mathrm{0.0072973526\,}\)		T
arcm	minute of arc	\(\mathrm{{}^{\prime}}\)	arcmin	T
arcs	second of arc	\(\mathrm{{}^{\prime\prime}}\)	arcsec	T
atm	atmosphere	\(\mathrm{101325\,Pa}\)		T
AU	astronomical unit	\(\mathrm{AU}\)	au	T
bar	bar	\(\mathrm{bar}\)		T
barn	barn	\(\mathrm{barn}\)		T
bit	bit	\(\mathrm{bit}\)		T
byte	byte	\(\mathrm{byte}\)		T
C	Coulomb	\(\mathrm{C}\)		T
c	speed of light	\(\mathrm{2.9979246 \times 10^{8}\,\frac{m}{s}}\)		T
cal	calorie	\(\mathrm{4.1854\,J}\)		T
cd	candela	\(\mathrm{cd}\)		T
Crab	Crab (X-ray) flux			T
ct	count	\(\mathrm{ct}\)		T
D	Debye (dipole)	\(\mathrm{D}\)		T
d	Julian day	\(\mathrm{d}\)		T
deg	degree	\(\mathrm{{}^{\circ}}\)	°, degree	T
dyn	dyne	\(\mathrm{dyn}\)		T
e	electron charge	\(\mathrm{1.6021766 \times 10^{-19}\,C}\)		T
eps0	electric constant	\(\mathrm{8.8541878 \times 10^{-12}\,\frac{F}{m}}\)		T
erg	erg	\(\mathrm{erg}\)		T
eV	electron volt	\(\mathrm{eV}\)		T
F	Farad	\(\mathrm{F}\)		T
G	Gravitation constant	\(\mathrm{6.67384 \times 10^{-11}\,\frac{m^{3}}{kg\,s^{2}}}\)		T
g	gram	\(\mathrm{g}\)		T
gauss	Gauss	\(\mathrm{G}\)		T
geoMass	Earth mass	\(\mathrm{M_{\oplus}}\)	Mgeo	T
H	Henry	\(\mathrm{H}\)		T
h	hour	\(\mathrm{h}\)		T
hr	hour	\(\mathrm{h}\)		T
Hz	Hertz	\(\mathrm{Hz}\)		T
inch	inch	\(\mathrm{0.0254\,m}\)		T
J	Joule	\(\mathrm{J}\)		T
JD	Julian day	\(\mathrm{d}\)		T
jovMass	Jupiter mass	\(\mathrm{M_{\rm J}}\)	Mjup	T
Jy	Jansky	\(\mathrm{Jy}\)		T
k	Boltzmann	\(\mathrm{1.3806488 \times 10^{-23}\,\frac{J}{K}}\)		T
K	Kelvin	\(\mathrm{K}\)		T
l	litre	\(\mathrm{\mathcal{l}}\)		T
lm	lumen	\(\mathrm{lm}\)		T
Lsun	solar luminosity	\(\mathrm{L_{\odot}}\)	solLum	T
lx	lux	\(\mathrm{lx}\)		T
lyr	Light year	\(\mathrm{lyr}\)		T
m	meter	\(\mathrm{m}\)		T
mag	magnitude	\(\mathrm{mag}\)		T
mas	millisecond of arc	\(\mathrm{marcsec}\)		F
me	electron mass	\(\mathrm{9.1093829 \times 10^{-31}\,kg}\)		T
min	minute	\(\mathrm{min}\)		T
MJD	Julian day	\(\mathrm{d}\)		T
mmHg	millimeter of mercury	\(\mathrm{133.32239\,Pa}\)		T
mol	mole	\(\mathrm{mol}\)		T
mp	proton mass	\(\mathrm{1.6726218 \times 10^{-27}\,kg}\)		T
Msun	solar mass	\(\mathrm{M_{\odot}}\)	solMass	T
mu0	magnetic constant	\(\mathrm{1.2566371 \times 10^{-6}\,\frac{N}{A^{2}}}\)	µ0	T
muB	Bohr magneton	\(\mathrm{9.2740097 \times 10^{-24}\,\frac{J}{T}}\)		T
N	Newton	\(\mathrm{N}\)		T
Ohm	Ohm	\(\mathrm{\Omega}\)		T
Pa	Pascal	\(\mathrm{Pa}\)		T
pc	parsec	\(\mathrm{pc}\)		T
ph	photon	\(\mathrm{ph}\)		T
pi	π	\(\mathrm{3.1415927\,}\)		T
pix	pixel	\(\mathrm{pix}\)		T
ppm	parts per million	\(\mathrm{1 \times 10^{-6}\,}\)		T
R	gas constant	\(\mathrm{8.3144621\,\frac{J}{K\,mol}}\)		T
rad	radian	\(\mathrm{rad}\)		T
Rgeo	Earth equatorial radius	\(\mathrm{6378136\,m}\)		T
Rjup	Jupiter equatorial radius	\(\mathrm{71492000\,m}\)		T
Rsun	solar radius	\(\mathrm{R_{\odot}}\)	solRad	T
Ry	Rydberg	\(\mathrm{R_{\infty}}\)		T
s	second	\(\mathrm{s}\)	sec	T
S	Siemens	\(\mathrm{S}\)		T
sr	steradian	\(\mathrm{sr}\)		T
Sun	solar unit	\(\mathrm{Sun}\)		T
T	Tesla	\(\mathrm{T}\)		T
t	metric tonne	\(\mathrm{1000\,kg}\)		T
u	atomic mass	\(\mathrm{1.6605389 \times 10^{-27}\,kg}\)		T
V	Volt	\(\mathrm{V}\)		T
W	Watt	\(\mathrm{W}\)		T
Wb	Weber	\(\mathrm{Wb}\)		T
yr	year	\(\mathrm{a}\)		T
µas	microsecond of arc	\(\mathrm{\mu arcsec}\)		F

Functions

enable()

Enable CDS units so they appear in results of find_equivalent_units and compose.

astropy.units.equivalencies Module

A set of standard astronomical equivalencies.

Functions

parallax()	Returns a list of equivalence pairs that handle the conversion between parallax angle and distance.
spectral()	Returns a list of equivalence pairs that handle spectral wavelength, wave number, frequency, and energy equivalences.
spectral_density(wav[, factor])	Returns a list of equivalence pairs that handle spectral density with regard to wavelength and frequency.
doppler_radio(rest)	Return the equivalency pairs for the radio convention for velocity.
doppler_optical(rest)	Return the equivalency pairs for the optical convention for velocity.
doppler_relativistic(rest)	Return the equivalency pairs for the relativistic convention for velocity.
mass_energy()	Returns a list of equivalence pairs that handle the conversion between mass and energy.
brightness_temperature(beam_area, disp)	Defines the conversion between Jy/beam and “brightness temperature”, \(T_B\), in Kelvins.
dimensionless_angles()	Allow angles to be equivalent to dimensionless (with 1 rad = 1 m/m = 1).
logarithmic()	Allow logarithmic units to be converted to dimensionless fractions
temperature()	Convert between Kelvin, Celsius, and Fahrenheit here because Unit and CompositeUnit cannot do addition or subtraction properly.
temperature_energy()	Convert between Kelvin and keV(eV) to an equivalent amount.

astropy.units.function Package

This subpackage contains classes and functions for defining and converting between different function units and quantities, i.e., using units which are some function of a physical unit, such as magnitudes and decibels.

Classes

Decibel
DecibelUnit([physical_unit, function_unit])	Logarithmic physical units expressed in dB
Dex
DexUnit([physical_unit, function_unit])	Logarithmic physical units expressed in magnitudes
FunctionQuantity	A representation of a (scaled) function of a number with a unit.
FunctionUnitBase([physical_unit, function_unit])	Abstract base class for function units.
LogQuantity	A representation of a (scaled) logarithm of a number with a unit
LogUnit([physical_unit, function_unit])	Logarithmic unit containing a physical one
MagUnit([physical_unit, function_unit])	Logarithmic physical units expressed in magnitudes
Magnitude

Class Inheritance Diagram

Acknowledgments

This code is adapted from the pynbody units module written by Andrew Pontzen, who has granted the Astropy project permission to use the code under a BSD license.