.. _astropy-constants:
*******************************
Constants (`astropy.constants`)
*******************************
.. currentmodule:: astropy.constants
Introduction
============
`astropy.constants` contains a number of physical constants useful in
Astronomy. Constants are `~astropy.units.Quantity` objects with
additional metadata describing their provenance and uncertainties.
Getting Started
===============
To use the constants in International System of Units (SI units), you can
import the constants directly from the `astropy.constants` sub-package::
>>> from astropy.constants import G
Or, if you want to avoid having to explicitly import all of the constants you
need, you can do:
>>> from astropy import constants as const
and then subsequently use, for example, ``const.G``. Constants are fully-fledged
`~astropy.units.Quantity` objects, so you can conveniently convert them to
different units. For example::
>>> print(const.c)
Name = Speed of light in vacuum
Value = 299792458.0
Uncertainty = 0.0
Unit = m / s
Reference = CODATA 2018
>>> print(const.c.to('km/s'))
299792.458 km / s
>>> print(const.c.to('pc/yr')) # doctest: +FLOAT_CMP
0.306601393788 pc / yr
You can then use them in conjunction with unit and other nonconstant
`~astropy.units.Quantity` objects::
>>> from astropy import units as u
>>> F = (const.G * 3. * const.M_sun * 100 * u.kg) / (2.2 * u.au) ** 2
>>> print(F.to(u.N)) # doctest: +FLOAT_CMP
0.3675671602160826 N
It is possible to convert most constants to Centimeter-Gram-Second (CGS) units
using, for example::
>>> const.c.cgs # doctest: +FLOAT_CMP
However, some constants are defined with different physical dimensions in CGS
and cannot be directly converted. Because of this ambiguity, such constants
cannot be used in expressions without specifying a system::
>>> 100 * const.e
Traceback (most recent call last):
...
TypeError: Constant u'e' does not have physically compatible units
across all systems of units and cannot be combined with other
values without specifying a system (eg. e.emu)
>>> 100 * const.e.esu # doctest: +FLOAT_CMP
.. _astropy-constants-prior:
Collections of Constants (and Prior Versions)
=============================================
Constants are organized into version modules. The constants for
``astropy`` 2.0 can be accessed in the ``astropyconst20`` module.
For example:
>>> from astropy.constants import astropyconst20 as const
>>> print(const.e)
Name = Electron charge
Value = 1.6021766208e-19
Uncertainty = 9.8e-28
Unit = C
Reference = CODATA 2014
Physical CODATA constants are in modules with names like ``codata2010``,
``codata2014``, or ``codata2018``:
>>> from astropy.constants import codata2014 as const
>>> print(const.h)
Name = Planck constant
Value = 6.62607004e-34
Uncertainty = 8.1e-42
Unit = J s
Reference = CODATA 2014
Astronomical constants defined (primarily) by the International Astronomical
Union (IAU) are collected in modules with names like ``iau2012`` or ``iau2015``:
>>> from astropy.constants import iau2012 as const
>>> print(const.L_sun)
Name = Solar luminosity
Value = 3.846e+26
Uncertainty = 5e+22
Unit = W
Reference = Allen's Astrophysical Quantities 4th Ed.
>>> from astropy.constants import iau2015 as const
>>> print(const.L_sun)
Name = Nominal solar luminosity
Value = 3.828e+26
Uncertainty = 0.0
Unit = W
Reference = IAU 2015 Resolution B 3
The astronomical and physical constants are combined into modules with
names like ``astropyconst13``, ``astropyconst20``, and ``astropyconst40``
for different versions.
However, importing these prior version modules directly will lead to
inconsistencies with other subpackages that have already imported
`astropy.constants`. Notably, `astropy.units` will have already used
the default version of constants. When using prior versions of the constants
in this manner, quantities should be constructed with constants instead of units.
To ensure consistent use of a prior version of constants in other Astropy
packages (such as `astropy.units`) that import ``constants``, the physical and
astronomical constants versions should be set via ``ScienceState`` classes.
These must be set before the first import of either `astropy.constants` or
`astropy.units`. For example, you can use the CODATA2010 physical constants and the
IAU 2012 astronomical constants:
>>> from astropy import physical_constants, astronomical_constants
>>> physical_constants.set('codata2010') # doctest: +SKIP
>>> physical_constants.get() # doctest: +SKIP
'codata2010'
>>> astronomical_constants.set('iau2012') # doctest: +SKIP
>>> astronomical_constants.get() # doctest: +SKIP
'iau2012'
Then all other packages that import `astropy.constants` will self-consistently
initialize with that prior version of constants.
The versions may also be set using values referring to the version modules:
>>> from astropy import physical_constants, astronomical_constants
>>> physical_constants.set('astropyconst13') # doctest: +SKIP
>>> physical_constants.get() # doctest: +SKIP
'codata2010'
>>> astronomical_constants.set('astropyconst13') # doctest: +SKIP
>>> astronomical_constants.get() # doctest: +SKIP
'iau2012'
.. The doctest should not be skipped, ideally. See https://github.com/astropy/astropy/issues/8781
If either `astropy.constants` or `astropy.units` have already been imported, a
``RuntimeError`` will be raised.
>>> import astropy.units
>>> from astropy import physical_constants, astronomical_constants
>>> astronomical_constants.set('astropyconst13')
Traceback (most recent call last):
...
RuntimeError: astropy.units is already imported
.. note that if this section gets too long, it should be moved to a separate
doc page - see the top of performance.inc.rst for the instructions on how to
do that
.. include:: performance.inc.rst
Reference/API
=============
.. automodapi:: astropy.constants