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 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

valuescalar or Quantity.: The value of this distance.
unitUnitBase: The units for this distance, if value is not a Quantity. Must have dimensions of distance.
zfloat: 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.
cosmologyCosmology 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).
distmodfloat 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.
parallaxQuantity or Angle: The parallax in angular units.
dtypedtype, optional: See Quantity.
copybool, optional: See Quantity.
order{‘C’, ‘F’, ‘A’}, optional: See Quantity.
subokbool, optional: See Quantity.
ndminint, optional: See Quantity.
allow_negativebool, optional: Whether to allow negative distances (which are possible is some cosmologies). Default: False.

Raises

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.cosmology import WMAP5, 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))
>>> d8 = Distance(parallax=21.34*u.mas)

Attributes Summary

`distmod`	The distance modulus as a `Quantity`
`parallax`	The parallax angle as an `Angle` object
`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

parallax¶: The parallax angle as an Angle object

z¶: Short for self.compute_z()

Methods Documentation

compute_z(cosmology=None)[source]¶

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

Parameters

cosmologyCosmology or None: The cosmology to assume for this calculation, or None to use the current cosmology (see astropy.cosmology for details).

Returns

zfloat: The redshift of this distance given the provided cosmology.

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Distance¶