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 | 5.97236e+24 kg | M_earth, Mearth ,
g | 0.001 kg | gram ,
jupiterMass | 1.89819e+27 kg | M_jup, Mjup, M_jupiter, Mjupiter ,
kg | irreducible | kilogram ,
solMass | 1.98848e+30 kg | M_sun, Msun ,
t | 1000 kg | tonne ,
u | 1.66054e-27 kg | Da, Dalton ,
]
Prefixes¶
Most units can be used with prefixes, with both the standard SI prefixes and
the IEEE 1514 binary prefixes (for bit
and byte
) supported:
Available decimal prefixes | ||
---|---|---|
Symbol | Prefix | Value |
Y | yotta- | 1e24 |
Z | zetta- | 1e21 |
E | exa- | 1e18 |
P | peta- | 1e15 |
T | tera- | 1e12 |
G | giga- | 1e9 |
M | mega- | 1e6 |
k | kilo- | 1e3 |
h | hecto- | 1e2 |
da | deka-, deca | 1e1 |
d | deci- | 1e-1 |
c | centi- | 1e-2 |
m | milli- | 1e-3 |
u | micro- | 1e-6 |
n | nano- | 1e-9 |
p | pico- | 1e-12 |
f | femto- | 1e-15 |
a | atto- | 1e-18 |
z | zepto- | 1e-21 |
y | yocto- | 1e-24 |
Available binary prefixes | ||
---|---|---|
Symbol | Prefix | Value |
Ki | kibi- | 2 ** 10 |
Mi | mebi- | 2 ** 20 |
Gi | gibi- | 2 ** 30 |
Ti | tebi- | 2 ** 40 |
Pi | pebi- | 2 ** 50 |
Ei | exbi- | 2 ** 60 |
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 | 6.957e+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():
... print(u.m.find_equivalent_units())
Primary name | Unit definition | Aliases
...
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([imperial.knot]):
... print(u.m.find_equivalent_units())
Primary name | Unit definition | Aliases
...