# [AstroPy] help with SkyCoord

Ivan Valtchanov ivvv68 at gmail.com
Thu Jan 21 03:03:03 EST 2021

```I may have misunderstood what was the goal.

So, in this case you may try with
centre.directional_offset_by(phi,theta) method in SkyCoord. You fix
the centre and the offset and calculate the SkyCoord for points in a
range of azimuth angles phi. At each phi yu will have a new SkyCoord.

Cheers,
Ivan

On Thu, 21 Jan 2021 at 08:22, Mohammad Shameoni Niaei
<m.shemuni at gmail.com> wrote:
>
> Hello Emmanuella.
>
> Thank you for sending this question. I found it challenging.
>
> As Ivan suggested, I tried to understand regions, CircleSkyRegion and find the boundaries to no avail.
> I even tried to solve the angular distance equation for known one coordinates and distance. That failed too.
>
> But I think I have a solution. We can use Vectors for this problem. Let's say we have a vector (v1) pointed to x, y, z (Spherical form would be r, theta, phi. Where theta and phi is our reference point and r can be anything but better to be 1.) We can create another vector (v2) with angular distance of your R (Where R is your radius) and rotate the v2 about v1. see: stackexchange
>
> I once created a simple Vector library, which is installable with pip, named v3d .
>
> You can easily do the same calculations with numpy if v3d is bugged or is not trustable enough for you.
>
> An example is in attachment
>
> angular distance equation: https://en.wikipedia.org/wiki/Angular_distance#Equation
> v3d: https://pypi.org/project/v3d/
>
> On Thu, Jan 21, 2021 at 1:28 AM Ivan Valtchanov <ivvv68 at gmail.com> wrote:
>>
>> Hi Emmanuella,
>>
>> Have a look at astropy regions package:
>>
>> hope this helps,
>> Ivan
>>
>> On Wed, 20 Jan 2021 at 19:11, Emanuella Puddu <manupu08 at gmail.com> wrote:
>> >
>> > Hi everyone!
>> > I have to draw a circle around an object's center, a circle made of points which have the same distance from the object. This all in spherical projection on the sky (package SkyCoord). Does anyone have an idea about how this can be done?
>> > Many thanks in advance for the help ...
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>
>
>
> --
> Mohammad Shameoni Niaei
> Astronomer
> Atatürk University, Astrophysics Research and Application Center.
> ERZURUM-TURKEY
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```