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 4.438067627303133 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'

See Also

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.
SpecificTypeQuantity Superclass for Quantities of specific physical type.
QuantityInfoBase([bound])
QuantityInfo([bound]) Container for meta information like name, description, format.

Class Inheritance Diagram

Inheritance diagram of astropy.units.quantity.Quantity, astropy.units.quantity.SpecificTypeQuantity, astropy.units.quantity.QuantityInfoBase, astropy.units.quantity.QuantityInfo

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.
molar_mass_amu() Returns the equivalence between amu and molar mass.
parallax() Returns a list of equivalence pairs that handle the conversion between parallax angle and distance.
pixel_scale(pixscale) Convert between pixel distances (in units of pix) and angular units, given a particular pixscale.
plate_scale(platescale) Convert between lengths (to be interpreted as lengths in the focal plane) and angular units with a specified platescale.
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(scale, bases, powers[, …]) 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(st[, doc, format, namespace]) 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(*args, **kwargs) Logarithmic physical units expressed in magnitudes
Magnitude
NamedUnit(st[, doc, format, namespace]) The base class of units that have a name.
PrefixUnit(st[, represents, doc, format, …]) A unit that is simply a SI-prefixed version of another unit.
Quantity A Quantity represents a number with some associated unit.
QuantityInfo([bound]) Container for meta information like name, description, format.
QuantityInfoBase([bound])
SpecificTypeQuantity Superclass for Quantities of specific physical type.
Unit(st[, represents, doc, format, namespace]) 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.
UnitTypeError Used specifically for errors in setting to units not allowed by a class.
UnitsError The base class for unit-specific exceptions.
UnitsWarning The base class for unit-specific warnings.
UnrecognizedUnit(st[, doc, format, namespace]) A unit that did not parse correctly.

Class Inheritance Diagram

Inheritance diagram of astropy.units.core.CompositeUnit, astropy.units.function.logarithmic.Decibel, astropy.units.function.logarithmic.DecibelUnit, astropy.units.function.logarithmic.Dex, astropy.units.function.logarithmic.DexUnit, astropy.units.function.core.FunctionQuantity, astropy.units.function.core.FunctionUnitBase, astropy.units.core.IrreducibleUnit, astropy.units.function.logarithmic.LogQuantity, astropy.units.function.logarithmic.LogUnit, astropy.units.function.logarithmic.MagUnit, astropy.units.function.logarithmic.Magnitude, astropy.units.core.NamedUnit, astropy.units.core.PrefixUnit, astropy.units.quantity.Quantity, astropy.units.quantity.QuantityInfo, astropy.units.quantity.QuantityInfoBase, astropy.units.quantity.SpecificTypeQuantity, astropy.units.core.Unit, astropy.units.core.UnitBase, astropy.units.core.UnitConversionError, astropy.units.core.UnitTypeError, astropy.units.core.UnitsError, astropy.units.core.UnitsWarning, astropy.units.core.UnrecognizedUnit

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

Inheritance diagram of astropy.units.format.base.Base, astropy.units.format.generic.Generic, astropy.units.format.cds.CDS, astropy.units.format.console.Console, astropy.units.format.fits.Fits, astropy.units.format.latex.Latex, astropy.units.format.latex.LatexInline, astropy.units.format.ogip.OGIP, astropy.units.format.unicode_format.Unicode, astropy.units.format.generic.Unscaled, astropy.units.format.vounit.VOUnit

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 annum (a) \(\mathrm{365.25\,d}\) annum Yes
A ampere: base unit of electric current in SI   ampere, amp Yes
Angstrom ångström: 10 ** -10 m \(\mathrm{0.1\,nm}\) AA, angstrom No
arcmin arc minute: angular measurement \(\mathrm{0.016666667\,{}^{\circ}}\) arcminute Yes
arcsec arc second: angular measurement \(\mathrm{0.00027777778\,{}^{\circ}}\) arcsecond Yes
bar bar: pressure \(\mathrm{100000\,Pa}\)   No
Bq becquerel: unit of radioactivity \(\mathrm{Hz}\) becquerel No
C coulomb: electric charge \(\mathrm{A\,s}\) coulomb Yes
cd candela: base unit of luminous intensity in SI   candela Yes
Ci curie: unit of radioactivity \(\mathrm{3.7 \times 10^{10}\,Bq}\) curie No
d day (d) \(\mathrm{24\,h}\) day Yes
deg degree: angular measurement 1/360 of full rotation \(\mathrm{0.017453293\,rad}\) degree Yes
deg_C Degrees Celsius   Celsius No
eV Electron Volt \(\mathrm{1.6021766 \times 10^{-19}\,J}\) electronvolt Yes
F Farad: electrical capacitance \(\mathrm{\frac{C}{V}}\) Farad, farad Yes
fortnight fortnight \(\mathrm{2\,wk}\)   No
g gram (g) \(\mathrm{0.001\,kg}\) gram Yes
h hour (h) \(\mathrm{3600\,s}\) hour, hr Yes
H Henry: inductance \(\mathrm{\frac{Wb}{A}}\) Henry, henry Yes
hourangle hour angle: angular measurement with 24 in a full circle \(\mathrm{15\,{}^{\circ}}\)   No
Hz Frequency \(\mathrm{\frac{1}{s}}\) Hertz, hertz Yes
J Joule: energy \(\mathrm{N\,m}\) Joule, joule Yes
K Kelvin: temperature with a null point at absolute zero.   Kelvin Yes
kg kilogram: base unit of mass in SI.   kilogram No
l liter: metric unit of volume \(\mathrm{1000\,cm^{3}}\) L, liter Yes
lm lumen: luminous flux \(\mathrm{cd\,sr}\) lumen Yes
lx lux: luminous emittence \(\mathrm{\frac{lm}{m^{2}}}\) lux Yes
m meter: base unit of length in SI   meter Yes
mas milli arc second: angular measurement \(\mathrm{0.001\,{}^{\prime\prime}}\)   No
micron micron: alias for micrometer (um) \(\mathrm{\mu m}\)   No
min minute (min) \(\mathrm{60\,s}\) minute Yes
mol mole: amount of a chemical substance in SI.   mole Yes
N Newton: force \(\mathrm{\frac{kg\,m}{s^{2}}}\) Newton, newton Yes
Ohm Ohm: electrical resistance \(\mathrm{\frac{V}{A}}\) ohm Yes
Pa Pascal: pressure \(\mathrm{\frac{J}{m^{3}}}\) Pascal, pascal Yes
% percent: one hundredth of unity, factor 0.01 \(\mathrm{0.01\,}\) pct No
rad radian: angular measurement of the ratio between the length on an arc and its radius   radian Yes
s second: base unit of time in SI.   second Yes
S Siemens: electrical conductance \(\mathrm{\frac{A}{V}}\) Siemens, siemens Yes
sday Sidereal day (sday) is the time of one rotation of the Earth. \(\mathrm{86164.091\,s}\)   No
sr steradian: unit of solid angle in SI \(\mathrm{rad^{2}}\) steradian Yes
t Metric tonne \(\mathrm{1000\,kg}\) tonne No
T Tesla: magnetic flux density \(\mathrm{\frac{Wb}{m^{2}}}\) Tesla, tesla Yes
uas micro arc second: angular measurement \(\mathrm{1 \times 10^{-6}\,{}^{\prime\prime}}\)   No
V Volt: electric potential or electromotive force \(\mathrm{\frac{J}{C}}\) Volt, volt Yes
W Watt: power \(\mathrm{\frac{J}{s}}\) Watt, watt Yes
Wb Weber: magnetic flux \(\mathrm{V\,s}\) Weber, weber Yes
wk week (wk) \(\mathrm{7\,d}\) week No
yr year (yr) \(\mathrm{365.25\,d}\) year Yes

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 No
Ba Barye: CGS unit of pressure \(\mathrm{\frac{g}{cm\,s^{2}}}\) Barye, barye Yes
Bi Biot: CGS (EMU) unit of current \(\mathrm{\frac{cm^{1/2}\,g^{1/2}}{s}}\) Biot, abA, abampere No
C coulomb: electric charge \(\mathrm{A\,s}\) coulomb No
cd candela: base unit of luminous intensity in SI   candela No
cm centimeter (cm) \(\mathrm{cm}\) centimeter No
D Debye: CGS unit of electric dipole moment \(\mathrm{3.3333333 \times 10^{-30}\,C\,m}\) Debye, debye Yes
deg_C Degrees Celsius   Celsius No
dyn dyne: CGS unit of force \(\mathrm{\frac{cm\,g}{s^{2}}}\) dyne Yes
erg erg: CGS unit of energy \(\mathrm{\frac{cm^{2}\,g}{s^{2}}}\)   Yes
Fr Franklin: CGS (ESU) unit of charge \(\mathrm{\frac{cm^{3/2}\,g^{1/2}}{s}}\) Franklin, statcoulomb, statC, esu No
g gram (g) \(\mathrm{0.001\,kg}\) gram No
G Gauss: CGS unit for magnetic field \(\mathrm{0.0001\,T}\) Gauss, gauss Yes
Gal Gal: CGS unit of acceleration \(\mathrm{\frac{cm}{s^{2}}}\) gal Yes
K Kelvin: temperature with a null point at absolute zero.   Kelvin No
k kayser: CGS unit of wavenumber \(\mathrm{\frac{1}{cm}}\) Kayser, kayser Yes
mol mole: amount of a chemical substance in SI.   mole No
P poise: CGS unit of dynamic viscosity \(\mathrm{\frac{g}{cm\,s}}\) poise Yes
rad radian: angular measurement of the ratio between the length on an arc and its radius   radian No
s second: base unit of time in SI.   second No
sr steradian: unit of solid angle in SI \(\mathrm{rad^{2}}\) steradian No
St stokes: CGS unit of kinematic viscosity \(\mathrm{\frac{cm^{2}}{s}}\) stokes Yes
statA statampere: CGS (ESU) unit of current \(\mathrm{\frac{Fr}{s}}\) statampere No

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     Yes
AU astronomical unit: approximately the mean Earth–Sun distance. \(\mathrm{1.4959787 \times 10^{11}\,m}\) au, astronomical_unit Yes
barn barn: unit of area used in HEP \(\mathrm{1 \times 10^{-28}\,m^{2}}\) barn Yes
beam beam     Yes
bin bin     Yes
bit b (bit)   b Yes
byte B (byte) \(\mathrm{8\,bit}\) B Yes
chan chan     Yes
ct count (ct)   count Yes
cycle cycle: angular measurement, a full turn or rotation \(\mathrm{6.2831853\,rad}\) cy No
earthMass Earth mass \(\mathrm{5.9723647 \times 10^{24}\,kg}\) M_earth, Mearth No
earthRad Earth radius \(\mathrm{6356800\,m}\) R_earth, Rearth No
electron Number of electrons     No
jupiterMass Jupiter mass \(\mathrm{1.8981872 \times 10^{27}\,kg}\) M_jup, Mjup, M_jupiter, Mjupiter No
jupiterRad Jupiter radius \(\mathrm{71492000\,m}\) R_jup, Rjup, R_jupiter, Rjupiter No
Jy Jansky: spectral flux density \(\mathrm{1 \times 10^{-26}\,\frac{W}{Hz\,m^{2}}}\) Jansky, jansky Yes
lyr Light year \(\mathrm{9.4607305 \times 10^{15}\,m}\) lightyear Yes
M_e Electron mass \(\mathrm{9.1093836 \times 10^{-31}\,kg}\)   No
M_p Proton mass \(\mathrm{1.6726219 \times 10^{-27}\,kg}\)   No
pc parsec: approximately 3.26 light-years. \(\mathrm{3.0856776 \times 10^{16}\,m}\) parsec Yes
ph photon (ph)   photon Yes
pix pixel (pix)   pixel Yes
R Rayleigh: photon flux \(\mathrm{7.9577472 \times 10^{8}\,\frac{ph}{s\,sr\,m^{2}}}\) Rayleigh, rayleigh Yes
Ry Rydberg: Energy of a photon whose wavenumber is the Rydberg constant \(\mathrm{13.605693\,eV}\) rydberg Yes
solLum Solar luminance \(\mathrm{3.828 \times 10^{26}\,W}\) L_sun, Lsun No
solMass Solar mass \(\mathrm{1.9884754 \times 10^{30}\,kg}\) M_sun, Msun No
solRad Solar radius \(\mathrm{6.957 \times 10^{8}\,m}\) R_sun, Rsun No
Sun Sun     No
u Unified atomic mass unit \(\mathrm{1.660539 \times 10^{-27}\,kg}\) Da, Dalton Yes
vox voxel (vox)   voxel Yes

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 No
dex Dex: Base 10 logarithmic unit     No
mag Astronomical magnitude: -2.5 per base 10 logarithmic unit \(\mathrm{-0.4\,dex}\)   Yes

astropy.units.imperial Module

This package defines colloquially used Imperial units. They are available in the astropy.units.imperial namespace, but not in the top-level astropy.units namespace, e.g.:

>>> import astropy.units as u
>>> mph = u.imperial.mile / u.hour
>>> mph
Unit("mi / h")

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

>>> import astropy.units as u
>>> u.imperial.enable()  
Available Units
Unit Description Represents Aliases SI Prefixes
ac International acre \(\mathrm{43560\,ft^{2}}\) acre No
BTU British thermal unit \(\mathrm{1.0550559\,kJ}\) btu No
cal Thermochemical calorie: pre-SI metric unit of energy \(\mathrm{4.184\,J}\) calorie No
cup U.S. \(\mathrm{0.5\,pint}\)   No
deg_F Degrees Fahrenheit   Fahrenheit No
foz U.S. \(\mathrm{0.125\,cup}\) fluid_oz, fluid_ounce No
ft International foot \(\mathrm{12\,inch}\) foot No
fur Furlong \(\mathrm{660\,ft}\) furlong No
gallon U.S. \(\mathrm{3.7854118\,\mathcal{l}}\)   No
hp Electrical horsepower \(\mathrm{745.69987\,W}\) horsepower No
inch International inch \(\mathrm{2.54\,cm}\)   No
kcal Calorie: colloquial definition of Calorie \(\mathrm{1000\,cal}\) Cal, Calorie, kilocal, kilocalorie No
kip Kilopound: force \(\mathrm{1000\,lbf}\) kilopound No
kn nautical unit of speed: 1 nmi per hour \(\mathrm{\frac{nmi}{h}}\) kt, knot, NMPH No
lb International avoirdupois pound: mass \(\mathrm{16\,oz}\) lbm, pound No
lbf Pound: force \(\mathrm{\frac{ft\,slug}{s^{2}}}\)   No
mi International mile \(\mathrm{5280\,ft}\) mile No
mil Thousandth of an inch \(\mathrm{0.001\,inch}\) thou No
nmi Nautical mile \(\mathrm{1852\,m}\) nauticalmile, NM No
oz International avoirdupois ounce: mass \(\mathrm{28.349523\,g}\) ounce No
pint U.S. \(\mathrm{0.5\,quart}\)   No
psi Pound per square inch: pressure \(\mathrm{\frac{lbf}{inch^{2}}}\)   No
quart U.S. \(\mathrm{0.25\,gallon}\)   No
slug slug: mass \(\mathrm{32.174049\,lb}\)   No
st International avoirdupois stone: mass \(\mathrm{14\,lb}\) stone No
tbsp U.S. \(\mathrm{0.5\,foz}\) tablespoon No
ton International avoirdupois ton: mass \(\mathrm{2000\,lb}\)   No
tsp U.S. \(\mathrm{0.33333333\,tbsp}\) teaspoon No
yd International yard \(\mathrm{3\,ft}\) yard No

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, both 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.

These units are not available in the top-level astropy.units namespace. To use these units, you must import the astropy.units.cds module:

>>> from astropy.units import cds
>>> q = 10. * cds.lyr  

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 \(\mathrm{\%}\)   No
--- dimensionless and unscaled \(\mathrm{}\)   No
\h Planck constant \(\mathrm{6.62607 \times 10^{-34}\,J\,s}\)   Yes
A Ampere \(\mathrm{A}\)   Yes
a year \(\mathrm{a}\)   Yes
a0 Bohr radius \(\mathrm{5.2917721 \times 10^{-11}\,m}\)   Yes
AA Angstrom \(\mathrm{\mathring{A}}\) Å, Angstrom, Angstroem Yes
al Light year \(\mathrm{lyr}\)   Yes
alpha Fine structure constant \(\mathrm{0.0072973526\,}\)   Yes
arcm minute of arc \(\mathrm{{}^{\prime}}\) arcmin Yes
arcs second of arc \(\mathrm{{}^{\prime\prime}}\) arcsec Yes
atm atmosphere \(\mathrm{101325\,Pa}\)   Yes
AU astronomical unit \(\mathrm{AU}\) au Yes
bar bar \(\mathrm{bar}\)   Yes
barn barn \(\mathrm{barn}\)   Yes
bit bit \(\mathrm{bit}\)   Yes
byte byte \(\mathrm{byte}\)   Yes
C Coulomb \(\mathrm{C}\)   Yes
c speed of light \(\mathrm{2.9979246 \times 10^{8}\,\frac{m}{s}}\)   Yes
cal calorie \(\mathrm{4.1854\,J}\)   Yes
cd candela \(\mathrm{cd}\)   Yes
Crab Crab (X-ray) flux     Yes
ct count \(\mathrm{ct}\)   Yes
D Debye (dipole) \(\mathrm{D}\)   Yes
d Julian day \(\mathrm{d}\)   Yes
deg degree \(\mathrm{{}^{\circ}}\) °, degree Yes
dyn dyne \(\mathrm{dyn}\)   Yes
e electron charge \(\mathrm{1.6021766 \times 10^{-19}\,C}\)   Yes
eps0 electric constant \(\mathrm{8.8541878 \times 10^{-12}\,\frac{F}{m}}\)   Yes
erg erg \(\mathrm{erg}\)   Yes
eV electron volt \(\mathrm{eV}\)   Yes
F Farad \(\mathrm{F}\)   Yes
G Gravitation constant \(\mathrm{6.67408 \times 10^{-11}\,\frac{m^{3}}{kg\,s^{2}}}\)   Yes
g gram \(\mathrm{g}\)   Yes
gauss Gauss \(\mathrm{G}\)   Yes
geoMass Earth mass \(\mathrm{M_{\oplus}}\) Mgeo Yes
H Henry \(\mathrm{H}\)   Yes
h hour \(\mathrm{h}\)   Yes
hr hour \(\mathrm{h}\)   Yes
Hz Hertz \(\mathrm{Hz}\)   Yes
inch inch \(\mathrm{0.0254\,m}\)   Yes
J Joule \(\mathrm{J}\)   Yes
JD Julian day \(\mathrm{d}\)   Yes
jovMass Jupiter mass \(\mathrm{M_{\rm J}}\) Mjup Yes
Jy Jansky \(\mathrm{Jy}\)   Yes
K Kelvin \(\mathrm{K}\)   Yes
k Boltzmann \(\mathrm{1.3806485 \times 10^{-23}\,\frac{J}{K}}\)   Yes
l litre \(\mathrm{\mathcal{l}}\)   Yes
lm lumen \(\mathrm{lm}\)   Yes
Lsun solar luminosity \(\mathrm{L_{\odot}}\) solLum Yes
lx lux \(\mathrm{lx}\)   Yes
lyr Light year \(\mathrm{lyr}\)   Yes
m meter \(\mathrm{m}\)   Yes
mag magnitude \(\mathrm{mag}\)   Yes
mas millisecond of arc \(\mathrm{marcsec}\)   No
me electron mass \(\mathrm{9.1093836 \times 10^{-31}\,kg}\)   Yes
min minute \(\mathrm{min}\)   Yes
MJD Julian day \(\mathrm{d}\)   Yes
mmHg millimeter of mercury \(\mathrm{133.32239\,Pa}\)   Yes
mol mole \(\mathrm{mol}\)   Yes
mp proton mass \(\mathrm{1.6726219 \times 10^{-27}\,kg}\)   Yes
Msun solar mass \(\mathrm{M_{\odot}}\) solMass Yes
mu0 magnetic constant \(\mathrm{1.2566371 \times 10^{-6}\,\frac{N}{A^{2}}}\) µ0 Yes
muB Bohr magneton \(\mathrm{9.27401 \times 10^{-24}\,\frac{J}{T}}\)   Yes
N Newton \(\mathrm{N}\)   Yes
Ohm Ohm \(\mathrm{\Omega}\)   Yes
Pa Pascal \(\mathrm{Pa}\)   Yes
pc parsec \(\mathrm{pc}\)   Yes
ph photon \(\mathrm{ph}\)   Yes
pi π \(\mathrm{3.1415927\,}\)   Yes
pix pixel \(\mathrm{pix}\)   Yes
ppm parts per million \(\mathrm{1 \times 10^{-6}\,}\)   Yes
R gas constant \(\mathrm{8.3144598\,\frac{J}{K\,mol}}\)   Yes
rad radian \(\mathrm{rad}\)   Yes
Rgeo Earth equatorial radius \(\mathrm{6356800\,m}\)   Yes
Rjup Jupiter equatorial radius \(\mathrm{71492000\,m}\)   Yes
Rsun solar radius \(\mathrm{R_{\odot}}\) solRad Yes
Ry Rydberg \(\mathrm{R_{\infty}}\)   Yes
S Siemens \(\mathrm{S}\)   Yes
s second \(\mathrm{s}\) sec Yes
sr steradian \(\mathrm{sr}\)   Yes
Sun solar unit \(\mathrm{Sun}\)   Yes
T Tesla \(\mathrm{T}\)   Yes
t metric tonne \(\mathrm{1000\,kg}\)   Yes
u atomic mass \(\mathrm{1.660539 \times 10^{-27}\,kg}\)   Yes
V Volt \(\mathrm{V}\)   Yes
W Watt \(\mathrm{W}\)   Yes
Wb Weber \(\mathrm{Wb}\)   Yes
yr year \(\mathrm{a}\)   Yes
µas microsecond of arc \(\mathrm{\mu arcsec}\)   No

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.
molar_mass_amu() Returns the equivalence between amu and molar mass.
pixel_scale(pixscale) Convert between pixel distances (in units of pix) and angular units, given a particular pixscale.
plate_scale(platescale) Convert between lengths (to be interpreted as lengths in the focal plane) and angular units with a specified platescale.

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(*args, **kwargs) Logarithmic physical units expressed in magnitudes
Magnitude

Class Inheritance Diagram

Inheritance diagram of astropy.units.function.logarithmic.Decibel, astropy.units.function.logarithmic.DecibelUnit, astropy.units.function.logarithmic.Dex, astropy.units.function.logarithmic.DexUnit, astropy.units.function.core.FunctionQuantity, astropy.units.function.core.FunctionUnitBase, astropy.units.function.logarithmic.LogQuantity, astropy.units.function.logarithmic.LogUnit, astropy.units.function.logarithmic.MagUnit, astropy.units.function.logarithmic.Magnitude

astropy.units.deprecated Module

This package defines deprecated units.

These units are not available in the top-level astropy.units namespace. To use these units, you must import the astropy.units.deprecated module:

>>> from astropy.units import deprecated
>>> q = 10. * deprecated.emu  

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

>>> from astropy.units import deprecated
>>> deprecated.enable()  
Available Units
Unit Description Represents Aliases SI Prefixes
emu Biot: CGS (EMU) unit of current \(\mathrm{Bi}\)   No
Prefixes for earthMass Earth mass prefixes \(\mathrm{5.9723647 \times 10^{24}\,kg}\) M_earth, Mearth Only
Prefixes for earthRad Earth radius prefixes \(\mathrm{6356800\,m}\) R_earth, Rearth Only
Prefixes for jupiterMass Jupiter mass prefixes \(\mathrm{1.8981872 \times 10^{27}\,kg}\) M_jup, Mjup, M_jupiter, Mjupiter Only
Prefixes for jupiterRad Jupiter radius prefixes \(\mathrm{71492000\,m}\) R_jup, Rjup, R_jupiter, Rjupiter Only

Functions

enable() Enable deprecated units so they appear in results of find_equivalent_units and compose.

astropy.units.required_by_vounit Module

This package defines SI prefixed units that are required by the VOUnit standard but that are rarely used in practice and liable to lead to confusion (such as msolMass for milli-solar mass). They are in a separate module from astropy.units.deprecated because they need to be enabled by default for astropy.units to parse compliant VOUnit strings. As a result, e.g., Unit('msolMass') will just work, but to access the unit directly, use astropy.units.required_by_vounit.msolMass instead of the more typical idiom possible for the non-prefixed unit, astropy.units.solMass.

Available Units
Unit Description Represents Aliases SI Prefixes
Prefixes for solLum Solar luminance prefixes \(\mathrm{3.828 \times 10^{26}\,W}\) L_sun, Lsun Only
Prefixes for solMass Solar mass prefixes \(\mathrm{1.9884754 \times 10^{30}\,kg}\) M_sun, Msun Only
Prefixes for solRad Solar radius prefixes \(\mathrm{6.957 \times 10^{8}\,m}\) R_sun, Rsun Only

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.