[AstroPy] Query for Messier objects in a given region

Eric Jensen ejensen1 at swarthmore.edu
Mon Apr 12 08:23:33 EDT 2021

Hi Jim,

> On Apr 11, 2021, at 8:25 PM, Jim Singh <jimmyboysingh at gmail.com> wrote:
> What you suggest is certainly an option, but in the first instance I wanted to see if I could get away without having to download and store any reference data locally. Also, do not RA/Dec values drift over time, so real-time DB access assures the latest for one's plots?

The direction of Earth’s pole is constantly changing due to precession, but the coordinates of objects in catalogs are tabulated at a fixed reference date (called the *equinox* of the coordinates, currently usually the year 2000).  So you won’t get “fresher” coordinates by downloading them each time - they will always be the same.  (And it will always be slower to download the list than to read it from a local file.). Also, precession doesn’t change the positions of objects *relative to each other* - it just slowly moves the grid. 

For high-proper-motion objects (like asteroids or other solar system objects) you do need to update coordinates for the current date (to correct for the objects’ intrinsic space motion rather than the slow drift of the pole), but that’s not an issue for Messier objects or bright stars - those objects’ proper motions are negligible for your purposes. 


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