Units and quantities can be combined together using the regular Python numeric operators. For example:
>>> from astropy import units as u
>>> fluxunit = u.erg / (u.cm ** 2 * u.s)
>>> fluxunit
Unit("erg / (cm2 s)")
>>> 52.0 * fluxunit
<Quantity 52.0 erg / (cm2 s)>
>>> 52.0 * fluxunit / u.s
<Quantity 52.0 erg / (cm2 s2)>
Users are free to define new units, either fundamental or compound using the def_unit function. For example:
>>> bakers_fortnight = u.def_unit('bakers_fortnight', 13 * u.day)
The addition of a string gives the new unit a name that will show up when the unit is printed.
Creating a new fundamental unit is simple:
>>> titter = u.def_unit('titter')
>>> chuckle = u.def_unit('chuckle', 5 * titter)
>>> laugh = u.def_unit('laugh', 4 * chuckle)
>>> guffaw = u.def_unit('guffaw', 3 * laugh)
>>> rofl = u.def_unit('rofl', 4 * guffaw)
>>> death_by_laughing = u.def_unit('death_by_laughing', 10 * rofl)
>>> rofl.to(titter, 1)
240.0
By default, custom units are not searched by methods such as find_equivalent_units. However, they can be enabled by calling add_enabled_units:
>>> kmph = u.def_unit('kmph', u.km / u.h)
>>> (u.m / u.s).find_equivalent_units()
[]
>>> u.add_enabled_units([kmph])
<astropy.units.core._UnitContext object at ...>
>>> (u.m / u.s).find_equivalent_units()
Primary name | Unit definition | Aliases
[
kmph | 0.277778 m / s | ,
]