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*******************************
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 meta-data describing their provenance and uncertainties.
Getting Started
===============
To use the constants in S.I. 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 the constants you
need, you can simply 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 easily 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 2010
>>> print(const.c.to('km/s'))
299792.458 km / s
>>> print(const.c.to('pc/yr')) # doctest: +FLOAT_CMP
0.306601393788 pc / yr
and you can use them in conjunction with unit and other non-constant
`~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
0.367669392028 N
It is possible to convert most constants to cgs using e.g.::
>>> const.c.cgs
<Quantity 29979245800.0 cm / s>
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
<Quantity 4.8032045057134676e-08 Fr>
Reference/API
=============
.. automodapi:: astropy.constants
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