This is the first piece for handling deep sky objects such as stars, galaxies, clusters, nebulae, etc.

Disclaimer: For now, Astronoby won't store the initial data of these objects. Depending on the used catalogues, there could be between hundreds and billions of objects. The Messier catalogue has 110 objects, the NGC contains 7840 objects, the Hipparcos catalogue has 118218 objects and the Gaia DR3 contains 1.8 billion sources.
Also, each catalogue might expose data differently, which would increase complexity for Astronoby.
In the future, it is possible Astronoby wold make it easier to fetch data from external data sources. We'll see.

This first piece handle astrometric position of deep sky objects.

What

Given J2000 equatorial coordinates and optionally additional attributes, a DeepSkyObject object is instantiated and exposes the astrometric method, which, the same way as for solar system bodies, describes the astrometric reference frame of the object.

How

J2000 equatorial coordinates can be found on the internet, the suggested source is the SIMBAD Astronomical Database.

If only instant and equatorial_coordinates key arguments are provided, Astronoby will simply instantiate the object with no transformation.

If the following optional key attributes are provided, Astronoby will compute the propagated astrometric position which applies correction of the object's proper motion:

• proper_motion_ra (Astronoby::AngularVelocity): object's proper motion in right ascension
• proper_motion_dec (Astronoby::AngularVelocity): object's proper motion in declination
• parallax (Astronoby::Angle): stellar parallax
• radial_velocity (Astronoby::Velocity): variation over time of the object's velocity

Example

Vega (Alpha Lyrae) data coming from SIMBAD: https://simbad.u-strasbg.fr/simbad/sim-basic?Ident=vega

time = Time.utc(2025, 10, 1)
instant = Astronoby::Instant.from_time(time)
equatorial_coordinates = Astronoby::Coordinates::Equatorial.new(
  right_ascension: Astronoby::Angle.from_hms(18, 36, 56.33635),
  declination: Astronoby::Angle.from_dms(38, 47, 1.2802),
)
proper_motion_ra = Astronoby::AngularVelocity.from_milliarcseconds_per_year(200.94)
proper_motion_dec = Astronoby::AngularVelocity.from_milliarcseconds_per_year(286.23)
parallax = Astronoby::Angle.from_degree_arcseconds(130.23 / 1000.0)
radial_velocity = Astronoby::Velocity.from_kilometers_per_second(-13.5)

dso = Astronoby::DeepSkyObject.new(
  instant: instant,
  equatorial_coordinates: equatorial_coordinates,
  proper_motion_ra: Astronoby::AngularVelocity.from_milliarcseconds_per_year(200.94),
  proper_motion_dec: Astronoby::AngularVelocity.from_milliarcseconds_per_year(286.23),
  parallax: Astronoby::Angle.from_degree_arcseconds(130.23 / 1000.0),
  radial_velocity: Astronoby::Velocity.from_kilometers_per_second(-13.5)
)

astrometric = dso.astrometric

astrometric.equatorial.right_ascension.str(:hms)
# => "18h 36m 56.7788s"

astrometric.equatorial.declination.str(:dms)
# => "+38° 47′ 8.65″"

astrometric.ecliptic.latitude.str(:dms)
# => "+61° 44′ 4.8773″"

astrometric.ecliptic.longitude.str(:dms)
# => "+285° 19′ 11.8989″"

Precision

Compared to other astronomy libraries such as Astropy or Skyfield, when provided the additional coordinates, the precision provided is below 10th of an arcsecond.

As you can see on the example below, the different in right ascension is around 0.4 hour arcseconds, while the difference in declination is around 7 degree arcseconds.
None of these differences can be seen with a naked eye and extremely hard to notice in an amateur telescope even with great magnification. But still, precision up to professional standards can be achieved.

Velocity

On this first version, the astrometric velocity is mathematically right but astronomically wrong.

Astronoby currently computed the object's intrinsic velocity, which is related to its actual motion. In Astronoby, an astrometric position is geocentric, which means the astrometric velocity is supposed to be observer-relative and corrected with the Earth's own motion in the Solar System.

To reduce the complexity and because an ephemeride would be necessary, I decided to only compute the intrinsic velocity for now and I will see how I could fix it in the future when DeepSkyObject will support apparent and topocentric positions.