I'm a physics graduate student hoping to plot ~10°x10° sections of the sky in right ascension and declination (eventually I will want to plot points on this section at appropriate locations given RA and Dec, preferably in Python). I've seen square plots on the order of arcseconds, where curvature has a small effect, but that won't apply to these larger plots. I'm not familiar with astronomy journals or a module which aids in this sort of plotting.

Is there a commonly accepted method for displaying intermediate-sized sections of a sky-map? How is it implemented?

  • $\begingroup$ The sky is curved, paper is flat. You have to decide on some projection or other and then execute the transformation. Is that all you are asking about? $\endgroup$
    – ProfRob
    Oct 22, 2016 at 15:36
  • $\begingroup$ I'm imagining a square-ish plot with appropriately curved edges, depending on declination range. The problem is that I have no example of what is actually used, and if it is used, I have no idea how it is implemented. $\endgroup$
    – Sam
    Oct 22, 2016 at 16:06
  • $\begingroup$ Any projection used to map the Earth's surface should work with minimal modifications. Try looking through those. $\endgroup$
    – user21
    Oct 22, 2016 at 16:38
  • $\begingroup$ "physics graduate student" it's remarkable you're not familiar with ordinary projections dude! $\endgroup$
    – Fattie
    Oct 22, 2016 at 21:16
  • 1
    $\begingroup$ @JoeBlow Somehow I got through a physics bachelors degree without learning this... and so did everyone I graduated with... and so did everyone in my current program. $\endgroup$
    – Sam
    Oct 23, 2016 at 2:58

1 Answer 1


I don't think there's a journal standard for this. An easy and commonly used method is to plot latitude $\delta$ versus longitude $\alpha$, where the aspect ratio is scaled by $\mbox{cos}(\delta_0)$ so the projection is orthonormal at the image center $(\alpha_0, \delta_0)$. Depending on your region size and on $|\delta_0|$, this may or may not come out looking distorted. You might be able to pick a coordinate system (i.e. equatorial or galactic) where the latitude is never large enough to give major distortion.

Another common thing to do, arguably better when you're imaging extended objects, is to use a tangent-plane projection, also called a gnomonic projection. This projection depends on the chosen center, which is usually the image center. It is included in some plotting and analysis packages, but none that I use regularly. I recently coded this up for the Astropy package but haven't really tested it yet.

Projections like Aitoff, Hammer, and Mollweide have been included in many plotting packages, but usually only for whole-sky projections. You could try to write your own versions but I'm not sure it would be worth the trouble.

(Whatever you do, make sure longitude increases to the left since you're looking at the sky not the ground...)

  • $\begingroup$ If the chart is < 10$^\circ$ wide and $|\delta_0|$ < 60$^\circ$, an equirectangular projection as in the first paragraph is fine. If the chart is between 60$^\circ$ and 180$^\circ$ wide, the stereographic projection gives better results than the gnomonic. $\endgroup$
    – Mike G
    Oct 27, 2016 at 15:38

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