[AstroPy] Query for Messier objects in a given region

[Jim Singh]

Thanks Eric and Doug, as a 'proof of concept' I believe I've got the real
time plotting of both Bright stars and M objects around a given planet,
working now. However, in practice I will access a local Messier table like
you suggest, for the reasons you mention.

I am fetching the RA,Dec of a given planet thusly:
  planetName = "Neptune"
  c = (get_body(planetName, Time(datetime.utcnow()), location=None,
ephemeris=None)
  planetRA = c3.ra.hourangle
  planetDec = c3.dec.deg

So far the planet-neighbourhood patterns match (within acceptable bounds)
what I'm seeing through the finderscope (at least for the planets currently
visible), and with the SkEye app. My understanding from what you've said is
that if I keep using the RA and Dec of stars and M objects based on the
equinox 'J2000' reference (per the _RAJ2000', '_DEJ2000' Bright Star
catalog columns and downloaded coords columns from SIMBAD), then years from
now for this particular use case the agreement between the observed and
plot will remain as sound as it is today.

Regards,
Jim

On Tue, Apr 13, 2021 at 4:43 AM Doug Burke <dburke.gw at gmail.com> wrote:

>
> Jim,
>
> I second Eric's suggestion that you store the database contents for
> searches/queries that are not likely to change if possible. I know of one
> person who recently had to take down a really useful Astronomy-based web
> service because they were being flooded with requests for data just like
> the Messier catalog - i.e. something that doesn't change often - which
> ended up causing the hosting service to stop supporting it. There's also
> the environmental cost of repeatedly requesting information that doesn't
> change (something I know my code should be more aware of).
>
> Doug
>
> On Mon, Apr 12, 2021 at 8:24 AM Eric Jensen <ejensen1 at swarthmore.edu>
> wrote:
>
>> 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.
>>
>> Eric
>>
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