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 following function 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          | 9.109383e-31 kg |            ,
  M_p          | 1.672622e-27 kg |            ,
  g            | 1.000000e-03 kg | gram       ,
  kg           | irreducible     | kilogram   ,
  lb           | 4.535924e-01 kg | pound      ,
  oz           | 2.834952e-02 kg | ounce      ,
  solMass      | 1.989100e+30 kg |            ,
  t            | 1.000000e+03 kg | tonne      ,
  ton          | 9.071847e+02 kg |            ,
  u            | 1.660539e-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 unit that is equal to None: that indicates that no unit was specified in the data or by the user.

To obtain the dimensionless unit, use the dimensionless object:

>>> from astropy import units as u
>>> u.dimensionless
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 in Python code is preferred for readability:

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

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