Distance¶

class
astropy.coordinates.
Distance
(value=None, unit=None, z=None, cosmology=None, distmod=None, parallax=None, dtype=None, copy=True, order=None, subok=False, ndmin=0, allow_negative=False)[source]¶ Bases:
astropy.units.quantity.SpecificTypeQuantity
A onedimensional distance.
This can be initialized by providing one of the following:
Distance
value
(array or float) and aunit
Quantity
object with dimensionality of lengthRedshift and (optionally) a
Cosmology
Distance modulus
Parallax
 Parameters
 valuescalar or
Quantity
[:ref: ‘length’] The value of this distance.
 unit
UnitBase
[:ref: ‘length’] The unit for this distance.
 z
float
A redshift for this distance. It will be converted to a distance by computing the luminosity distance for this redshift given the cosmology specified by
cosmology
. Must be given as a keyword argument. cosmology
Cosmology
orNone
A cosmology that will be used to compute the distance from
z
. IfNone
, the current cosmology will be used (seeastropy.cosmology
for details). distmod
float
orQuantity
The distance modulus for this distance. Note that if
unit
is not provided, a guess will be made at the unit between AU, pc, kpc, and Mpc. parallax
Quantity
orAngle
The parallax in angular units.
 dtype
dtype
, optional See
Quantity
. copybool, optional
See
Quantity
. order{‘C’, ‘F’, ‘A’}, optional
See
Quantity
. subokbool, optional
See
Quantity
. ndmin
int
, optional See
Quantity
. allow_negativebool, optional
Whether to allow negative distances (which are possible in some cosmologies). Default:
False
.
 valuescalar or
 Raises
UnitsError
If the
unit
is not a length unit.ValueError
If value specified is less than 0 and
allow_negative=False
.If
cosmology
is provided whenz
is not given.If either none or more than one of
value
,z
,distmod
, orparallax
were given.
Examples
>>> from astropy import units as u >>> from astropy.cosmology import WMAP5 >>> Distance(10, u.Mpc) <Distance 10. Mpc> >>> Distance(40*u.pc, unit=u.kpc) <Distance 0.04 kpc> >>> Distance(z=0.23) <Distance 1184.01657566 Mpc> >>> Distance(z=0.23, cosmology=WMAP5) <Distance 1147.78831918 Mpc> >>> Distance(distmod=24.47*u.mag) <Distance 783.42964277 kpc> >>> Distance(parallax=21.34*u.mas) <Distance 46.86035614 pc>
Attributes Summary
The distance modulus as a
Quantity
The parallax angle as an
Angle
objectShort for
self.compute_z()
Methods Summary
compute_z
([cosmology])The redshift for this distance assuming its physical distance is a luminosity distance.
Attributes Documentation

z
¶ Short for
self.compute_z()
Methods Documentation

compute_z
(cosmology=None, **atzkw)[source]¶ The redshift for this distance assuming its physical distance is a luminosity distance.
 Parameters
 cosmology
Cosmology
orNone
The cosmology to assume for this calculation, or
None
to use the current cosmology (seeastropy.cosmology
for details). **atzkw
keyword arguments for
z_at_value()
 cosmology
 Returns
 z
Quantity
The redshift of this distance given the provided
cosmology
.
 z
Warning
This method can be slow for large arrays. The redshift is determined using
astropy.cosmology.z_at_value()
, which handles vector inputs (e.g. an array of distances) by elementwise calling ofscipy.optimize.minimize_scalar()
. For faster results consider using an interpolation table;astropy.cosmology.z_at_value()
provides details.See also
astropy.cosmology.z_at_value()
Find the redshift corresponding to a
astropy.cosmology.FLRW.luminosity_distance()
.