# Distance¶

class astropy.coordinates.Distance[source] [edit on github]

A one-dimensional distance.

This can be initialized in one of four ways:

• A distance value (array or float) and a unit
• A Quantity object
• A redshift and (optionally) a cosmology.
• Providing a distance modulus
Parameters: value : scalar or Quantity. The value of this distance. unit : UnitBase The units for this distance, if value is not a Quantity. Must have dimensions of 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 or None A cosmology that will be used to compute the distance from z. If None, the current cosmology will be used (see astropy.cosmology for details). distmod : float or Quantity 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. dtype : dtype, optional copy : bool, optional order : {‘C’, ‘F’, ‘A’}, optional subok : bool, optional ndmin : int, optional allow_negative : bool, optional Whether to allow negative distances (which are possible is some cosmologies). Default: False. ~astropy.units.UnitsError If the unit is not a distance. ValueError If value specified is less than 0 and allow_negative=False. If z is provided with a unit or cosmology is provided when z is not given, or value is given as well as z.

Examples

>>> from astropy import units as u
>>> from astropy import cosmology
>>> from astropy.cosmology import WMAP5, WMAP7
>>> cosmology.set_current(WMAP7)
>>> d1 = Distance(10, u.Mpc)
>>> d2 = Distance(40, unit=u.au)
>>> d3 = Distance(value=5, unit=u.kpc)
>>> d4 = Distance(z=0.23)
>>> d5 = Distance(z=0.23, cosmology=WMAP5)
>>> d6 = Distance(distmod=24.47)
>>> d7 = Distance(Distance(10 * u.Mpc))


Attributes Summary

 distmod The distance modulus as a Quantity z Short for self.compute_z()

Methods Summary

 compute_z([cosmology]) The redshift for this distance assuming its physical distance is a luminosity distance.

Attributes Documentation

distmod

The distance modulus as a Quantity

z

Short for self.compute_z()

Methods Documentation

compute_z(cosmology=None)[source] [edit on github]

The redshift for this distance assuming its physical distance is a luminosity distance.

Parameters: cosmology : Cosmology or None The cosmology to assume for this calculation, or None to use the current cosmology (see astropy.cosmology for details). z : float The redshift of this distance given the provided cosmology.