# [AstroPy] Heliocentric Velocity Correction

David Berry d.berry at jach.hawaii.edu
Mon Jun 11 06:27:43 EDT 2012

```On 11 June 2012 10:36, gonghang.naoc <ghang.naoc at gmail.com> wrote:
>
>    Thank you.
>>
>> I just want to get  the radial velocity  a star  in heliocentric
>> coordinate.I think the the heliocentric correction depends on the position
>> of the observatory,but the input parameters of astrolib.baryvel  are just
>> dje and deq.The result is a 3d velocity.My question is  whether the velocity
>> is the velocity of the  center of the earth and if not,whether I can input
>> the observatory parameters,i.e. longitude,latitude,elevation.

The heliocentric correction depends on the rest frame from which you
start. If you start with a topocentric velocity (i.e. relative to the
observer), then the correction will need to take account of the
observer's position. If you start from a geocentric velocity (i.e.
relative to the centre of the earth) then the correction will not
include the observers position.

I read the documentation for the astrolib.baryvel function as meaning
that the returned velocity is the velocity of the centre of the earth.
It can't really be anything else given that it just takes an epoch and
equinox as input.

So you want the full 3D velocity of the star, rather than just the

David

>>
>> On Mon, Jun 11, 2012 at 4:24 PM, David Berry <d.berry at jach.hawaii.edu>
>> wrote:
>>>
>>> Sorry - I'm not sure I understand your question. I thought you were
>>> trying to convert spectral positions (frequencies, velocities,
>>> wavelengths, etc) into the heliocentric rest frame from some other
>>> rest frame.
>>>
>>> The exact set of properties you need to specify when converting
>>> spectral positions depends on your original rest frame. I used
>>> topocentric as an example, and so included the observatory position.
>>> If instead I had used (for instance) geocentric, then the observatory
>>> position would not have been needed.
>>>
>>> But I see that astrolib.baryvel just returns the velocity of the earth
>>> in the barycentric or heliocentric rest frame, so now I'm not sure
>>> what it is you are trying to do. What velocity is it you are trying to
>>> correct?
>>>
>>> David
>>>
>>> On 11 June 2012 08:58, gonghang.naoc <ghang.naoc at gmail.com> wrote:
>>> > Hi,according to the link below,the 3d velocity we get from astrolib has
>>> > nothing to do with the observatory position.My question is  the 3d
>>> > velocity
>>> > is just the velocity of earth center?As the author claimed the
>>> > precision
>>> > level is cm/s, is it necessary to  input the  longtitude, latitude and
>>> > elevation of the observatory?
>>> >
>>> > Thank you!
>>> >
>>> >
>>> > http://www.astro.ucla.edu/~ianc/python/astrolib.htmlIDL> baryvel, jd,
>>> > 2000,
>>> > vh, vb, /jpl ;JPL ephemeris ==> vh = [-17.07236, -22.81126, -9.889419]
>>> > ;Heliocentric km/s ==> vb = [-17.08083, -22.80484, -9.886409]
>>> > ;Barycentric
>>> > km/s
>>> >
>>> >
>>> >
>>> >
>>> > On Sat, Jun 9, 2012 at 4:20 PM, gonghang.naoc <ghang.naoc at gmail.com>
>>> > wrote:
>>> >>
>>> >> I have checked
>>> >> http://www.astro.ucla.edu/~ianc/python/astrolib.html
>>> >> Is there a whole astrolib module I can download and install? Or is
>>> >> there
>>> >> any other python module that can makes heliocentric velocity
>>> >> correction you
>>> >> suggest?
>>> >> Thank you very much!
>>> >>
>>> >>
>>> >> On Sat, Jun 9, 2012 at 10:52 AM, gonghang.naoc <ghang.naoc at gmail.com>
>>> >> wrote:
>>> >>>
>>> >>> Dear pythoners:
>>> >>>           Is there any python nodule that can make heliocentric
>>> >>> velocity
>>> >>> correction well?
>>> >>>           Thank you!
>>> >>
>>> >>
>>> >
>>> >
>>> > _______________________________________________
>>> > AstroPy mailing list
>>> > AstroPy at scipy.org
>>> > http://mail.scipy.org/mailman/listinfo/astropy
>>> >
>>
>>
>
>
> _______________________________________________
> AstroPy mailing list
> AstroPy at scipy.org
> http://mail.scipy.org/mailman/listinfo/astropy
>

```