Low-Level Unit Conversion

Conversion of quantities from one unit to another is handled using the Quantity.to method. This page describes some low-level features for handling unit conversion that are rarely required in user code.

There are two ways of handling conversions between units.

Direct Conversion

In this case, given a source and destination unit, the values in the new units are returned.

>>> from astropy import units as u
>>> u.pc.to(u.m, 3.26)
1.0059308915661856e+17

This converts 3.26 parsecs to meters.

Arrays are permitted as arguments.

>>> u.h.to(u.s, [1, 2, 5, 10.1])
array([  3600.,   7200.,  18000.,  36360.])

Incompatible Conversions

If you attempt to convert to a incompatible unit, an exception will result:

>>> cms = u.cm / u.s
>>> cms.to(u.km)  
Traceback (most recent call last):
  ...
UnitConversionError: 'cm / s' (speed) and 'km' (length) are not convertible

You can check whether a particular conversion is possible using the is_equivalent method:

>>> u.m.is_equivalent(u.pc)
True
>>> u.m.is_equivalent("second")
False
>>> (u.m ** 3).is_equivalent(u.l)
True