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- Standard units

# Standard units

Standard units are defined in the `astropy.units` package as object instances.

All units are defined in term of basic ‘irreducible’ units. The irreducible units include:

> - Length (meter)
> - Time (second)
> - Mass (kilogram)
> - Current (ampere)
> - Temperature (Kelvin)
> - Angular distance (radian)
> - Solid angle (steradian)
> - Luminous intensity (candela)
> - Stellar magnitude (mag)
> - Amount of substance (mole)
> - Photon count (photon)

(There are also some more obscure base units required by the FITS standard that are no longer recommended for use.)

Units that involve combinations of fundamental units are instances of `CompositeUnit`. In most cases, one does not need to worry about the various kinds of unit classes unless one wants to design a more complex case.

There are many units already predefined in the module. One may use the `find_equivalent_units` method to list all the existing predefined units of a given type:

    >>> from astropy import units as u
    >>> u.g.find_equivalent_units()
      Primary name | Unit definition | Aliases
    [
      M_e          | 9.10938e-31 kg  |                                  ,
      M_p          | 1.67262e-27 kg  |                                  ,
      earthMass    | 5.9742e+24 kg   | M_earth, Mearth                  ,
      g            | 0.001 kg        | gram                             ,
      jupiterMass  | 1.8987e+27 kg   | M_jup, Mjup, M_jupiter, Mjupiter ,
      kg           | irreducible     | kilogram                         ,
      solMass      | 1.9891e+30 kg   | M_sun, Msun                      ,
      t            | 1000 kg         | tonne                            ,
      u            | 1.66054e-27 kg  | Da, Dalton                       ,
    ]

## The dimensionless unit

In addition to these units, `astropy.units` includes the concept of the dimensionless unit, used to indicate quantities that don’t have a physical dimension. This is distinct in concept from a unit that is equal to `None`: that indicates that no unit was specified in the data or by the user.

For convenience, there is a unit that is both dimensionless and unscaled: the `dimensionless_unscaled` object:

    >>> from astropy import units as u
    >>> u.dimensionless_unscaled
    Unit(dimensionless)

Dimensionless quantities are often defined as products or ratios of quantities that are not dimensionless, but whose dimensions cancel out when their powers are multiplied. For example:

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

For compatibility with the supported unit string formats, this is equivalent to `Unit('')` and `Unit(1)`, though using `u.dimensionless_unscaled` in Python code is preferred for readability:

    >>> u.dimensionless_unscaled == u.Unit('')
    True
    >>> u.dimensionless_unscaled == u.Unit(1)
    True

Note that in many cases, a dimensionless unit may also have a scale. For example:

    >>> (u.km / u.m).decompose()
    Unit(dimensionless with a scale of 1000.0)
    >>> (u.km / u.m).decompose() == u.dimensionless_unscaled
    False

To determine if a unit is dimensionless (but regardless of the scale), use the `physical_type` property:

    >>> (u.km / u.m).physical_type
    u'dimensionless'
    >>> # This also has a scale, so it is not the same as u.dimensionless_unscaled
    >>> (u.km / u.m) == u.dimensionless_unscaled
    False
    >>> # However, (u.m / u.m) has a scale of 1.0, so it is the same
    >>> (u.m / u.m) == u.dimensionless_unscaled
    True

## Enabling other units

By default, only the “default” units are searched by `find_equivalent_units` and similar methods that do searching. This includes SI, CGS and astrophysical units. However, one may wish to enable the imperial or other user-defined units.

For example, to enable Imperial units, simply do:

    >>> from astropy.units import imperial
    >>> imperial.enable()
    >>> u.m.find_equivalent_units()
      Primary name | Unit definition | Aliases
    [
      AU           | 1.49598e+11 m   | au, astronomical_unit ,
      Angstrom     | 1e-10 m         | AA, angstrom          ,
      cm           | 0.01 m          | centimeter            ,
      ft           | 0.3048 m        | foot                  ,
      fur          | 201.168 m       | furlong               ,
      inch         | 0.0254 m        |                       ,
      lyr          | 9.46073e+15 m   | lightyear             ,
      m            | irreducible     | meter                 ,
      mi           | 1609.34 m       | mile                  ,
      micron       | 1e-06 m         |                       ,
      mil          | 2.54e-05 m      | thou                  ,
      nmi          | 1852 m          | nauticalmile, NM      ,
      pc           | 3.08568e+16 m   | parsec                ,
      solRad       | 6.95508e+08 m   | R_sun, Rsun           ,
      yd           | 0.9144 m        | yard                  ,
    ]

This may also be used with the `with` statement, to temporarily enable additional units:

    >>> from astropy import units as u
    >>> from astropy.units import imperial
    >>> with imperial.enable():
    ...     u.m.find_equivalent_units()
    ...

To enable just specific units, use `add_enabled_units`:

    >>> from astropy import units as u
    >>> from astropy.units import imperial
    >>> with u.add_enabled_units_context([imperial.knot]):
    ...     u.m.find_equivalent_units()
    ...