Star map references

star charting libraries

1. starplot
   python library from NASA
   it's easy to use and provides lots of functionalities.
   it will be a good choice to make a celestial image.
   https://starplot.dev/

2. drawing star chart into SVG
   https://github.com/codebox/star-charts
   This sample draws star chart directly from stardata.csv.
   I think this can work offline.
   even though stardata.csv contains not quite a lot of star databases, i think it's a very nice try.
   I didn't look into the code yet but I wonder how it handles the projection.

3. using astropy, skyfield
   I found a stackoverflow thread discussing an issue but it includes a sample python code using skyfield and astropy which is a good reference code to use astropy and skyfield.
   https://stackoverflow.com/questions/78778792/plotting-star-maps-with-equatorial-coordinates-system

star map coordinate system

Most star maps and catalogs use the equitorial coordinate system which uses RA(right ascension) and Dec (declination) as coordinates.

• Hipparcos Catalog: equatorial
• Gia catalog: equatorial
• Yale bright star catalog: equatorial
• tycho-2 catalog: equatorial

Ecliptic coordinate system is rarely used (it is not that it is never used)

locate sky area

1. your position (latitude, longitude)
2. get LST (local sidereal time) -- use library like astropy, ephem (python), NOVAS.COM, AASharp (c#)
3. LST will give what RA is directly overhead
   Visible RA range is roughly LST plus minus 6 hours.

calculate LST

1. Astropy

from astropy.time import Time
from astropy.coordinates import EarthLocation, LST

# Create location
observatory = EarthLocation(lat=40*u.deg, lon=-75*u.deg)

# Get current time
current_time = Time.now()

# Calculate LST
lst = current_time.sidereal_time('apparent', observatory.lon)

2. Ephm

import ephem

# Create observer
observer = ephem.Observer()
observer.lat = '40'    # North
observer.lon = '-75'   # West
observer.date = ephem.now()

# Get LST
lst = observer.sidereal_time()

3. NOVAS.COM (c#)
   install ASCOM.AstrometryTools using NuGet package manager

using ASCOM.Tools.Novas31;

var novas = new Novas();
var jdutc = novas.JulianDate(year, month, day, hour);
var lst = novas.SiderealTime(jdutc, longitude, 0);
 --> this usage is not correct.
 --> instead, Novas class provides static methods.
 --> Novas.JulianDate(), Novas.SiderealTime()
 --> but usage is a little different.
 --> refer to https://ascom-standards.org/library/html/T_ASCOM_Tools_Novas31_Novas.htm page.

this requires NOVAS-C (or libnovas).

4. AASharp(c#)
   1) installing
   a) Visual Studio
   Tools > NuGet package manager > package manager console
   $ install-package AASharp

   this will install AASharp package
   b) or at the project folder
   $ dotnet add package AASharp

2) How to use it

using AASharp;

double dayvalue = DateTime.UtcNow.Day + DateTime.UtcNow.Hour/24.0 + DateTime.UtcNow.Minute/24.0/60.0 + DateTime.UtcNow.Second/24.0/60.0/60.0 + DateTime.UtcNow.Millisecond / 24.0/60.0/60.0/1000.0;

double JD = AASDate.DateToJD(year, month, dayvalue, true);
double GMST = AASSidereal.MeanGreenwichSiderealTime(JD);
double GAST = AASSidereal.ApparentGreenwichSiderealTime(JD);
// GMST = Greenwitch Mean Sidereal time in degree (0-360)
// GAST = Apparent Sidereal time in degree(0-360)
// to change it to local, we must add longitude

in most cases, we use Apprent Sidereal time when calculating celestial object position.