Unit manipulation

Documentation

For more information about the features presented below, you can read the astropy.units docs.

Representing units and quantities

Astropy includes a powerful framework for units that allows users to attach units to scalars and arrays, and manipulate/combine these, keeping track of the units.

Since we may want to use a number of units in expressions, it is easiest and most concise to import the units module with:

from astropy import units as u

though note that this will conflict with any variable called u.

Units can then be accessed with:

>>> u.m
Unit("m")

>>> u.pc
Unit("pc")

>>> u.s
Unit("s")

>>> u.kg
Unit("kg")

We can create composite units:

>>> u.m / u.kg / u.s**2
Unit("m / (kg s2)")

The most useful feature about the units is the ability to attach them to scalars or arrays, creating Quantity objects:

>>> 3. * u.m
<Quantity 3.0 m>

>>> import numpy as np

>>> np.array([1.2, 2.2, 1.7]) * u.pc / u.year
<Quantity [ 1.2  2.2  1.7] pc / (yr)>

Combining and converting units

Quantities can then be combined:

>>> q1 = 3. * u.m

>>> q2 = 5. * u.cm / u.s / u.g**2

>>> q1 * q2
<Quantity 15.0 cm m / (g2 s)>

and converted to different units:

>>> (q1 * q2).to(u.m**2 / u.kg**2 / u.s)
<Quantity 150000.0 m2 / (kg2 s)>

The units and value of a quantity can be accessed separately via the value and unit attributes:

>>> q = 5. * u.pc

>>> q.value
5.0

>>> q.unit
Unit("pc")

Advanced features

The units of a quantity can be decomposed into a set of base units using the decompose() method. By default, units will be decomposed to S.I.:

>>> (3. * u.cm * u.pc / u.g / u.year**2).decompose()
<Quantity 929.5706802193289 m2 / (kg s2)>

To decompose into c.g.s. units, one can do:

>>> (3. * u.cm * u.pc / u.g / u.year**2).decompose(u.cgs.bases)
<Quantity 9295.70680219329 cm2 / (g s2)>

Using physical constants

The astropy.constants module contains physical constants relevant for Astronomy, and these are defined with units attached to them using the astropy.units framework. If we want to compute the Gravitational force felt by a 100. * u.kg space probe by the Sun, at a distance of 3.2au, we can do:

>>> from astropy.constants import G

>>> F = (G * 1. * u.M_sun * 100. * u.kg) / (3.2 * u.au)**2

>>> F
<Quantity 6.517421874999999e-10 m3 solMass / (AU2 s2)>

>>> F.to(u.N)
<Quantity 0.05792707869188191 N>

The full list of available physical constants is shown here (and additions are welcome!).

Practical Exercises

Level 1

What is 1 barn megaparsecs in teaspoons?

Click to Show/Hide Solution

>>> (1. * u.barn * u.Mpc).to(u.tsp)
<Quantity 0.626035029893 tsp>

Level 2

What is 3 nm^2 Mpc / m^3 in dimensionless units?

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>>> (3. * u.nm**2 * u.Mpc / u.m**3).decompose()
<Quantity 92570.327444 >

or to just get the numerical value:

>>> (3. * u.nm**2 * u.Mpc / u.m**3).decompose().value
92570.327444015755

Level 3

Try and convert 3 Angstroms to keV using the units framework. You will need to look through the documentation for astropy.units to see how this can be made to work.

Click to Show/Hide Solution

>>> (3 * u.angstrom).to(u.keV)
...
UnitsException: 'angstrom' (length) and 'keV' (energy) are not convertible

>>> (3 * u.angstrom).to(u.keV, equivalencies=u.spectral())
<Quantity 4.13280643067 keV>