Distance¶

class
astropy.coordinates.
Distance
[source] [edit on github]¶ Bases:
astropy.units.quantity.SpecificTypeQuantity
A onedimensional distance.
This can be initialized in one of four ways:
 A distance
value
(array or float) and aunit
 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 aQuantity
. 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
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 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
orAngle
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 aunit
orcosmology
is provided whenz
is not given, orvalue
is given as well asz
.
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
objectz
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

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
orNone
The cosmology to assume for this calculation, or
None
to use the current cosmology (seeastropy.cosmology
for details).
Returns:  z : float
The redshift of this distance given the provided
cosmology
.
 cosmology :
 A distance