.. _astropy-constants: ******************************* Constants (`astropy.constants`) ******************************* .. currentmodule:: astropy.constants Introduction ============ `astropy.constants` contains a number of physical constants useful in Astronomy. A `~astropy.constants.Constant` is a |Quantity| object with additional metadata describing its provenance and uncertainty. Getting Started =============== You can import a :class:`~astropy.constants.Constant` directly from the :mod:`astropy.constants` sub-package:: >>> from astropy.constants import G >>> print(G) Name = Gravitational constant Value = 6.6743e-11 Uncertainty = 1.5e-15 Unit = m3 / (kg s2) Reference = CODATA 2018 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 >>> print(const.G) Name = Gravitational constant ... Constants can be used in :ref:`quantity_arithmetic` operations and :ref:`quantity_and_numpy` just like any other |Quantity|:: >>> 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 Unit Conversion =============== .. EXAMPLE START Converting Constants to Different Units Explicitly :ref:`quantity_unit_conversion` is often not necessary, but can be done if needed:: >>> 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 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 .. EXAMPLE END .. _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 The version modules contain physical and astronomical constants, and both sets can also be chosen independently from each other. 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 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 :mod:`astropy.units`) that import :mod:`astropy.constants`, the physical and astronomical constants versions should be set via :class:`~astropy.utils.state.ScienceState` classes. These must be set before the first import of either :mod:`astropy.constants` or :mod:`astropy.units`. For example, you can use the CODATA2010 physical constants together with 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 these prior versions 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 :mod:`astropy.constants` or :mod:`astropy.units` have already been imported, a :class:`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