Distance¶

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

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:

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.
parallax : Quantity or Angle: The parallax in angular units.
dtype : dtype, optional: See Quantity.
copy : bool, optional: See Quantity.
order : {‘C’, ‘F’, ‘A’}, optional: See Quantity.
subok : bool, optional: See Quantity.
ndmin : int, optional: See Quantity.
allow_negative : bool, optional: Whether to allow negative distances (which are possible is some cosmologies). Default: False.

Raises:

`~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))
>>> 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] [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).
Returns:	z : float The redshift of this distance given the provided `cosmology`.

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Distance¶