I'm developing my own planetarium software and I using Stellarium to check if the values a get from my software are correct or not.

All values are roughly equal, being different only in the seconds.

These are Stellarium coordiantes followed by my coordinates.

Note, this is a CSV txt file. I don't know if I can attached to this question so I have pasted here:

Star;RA J2000;DEC J2000;RA J2016;DEC J2016;Hour Angle;AZ;Alt.
Betelgeuse (Stellarium);5h55m10.34s;7d24m25.6s;5h56m2.38s;7d24m22.0s;23h52m25.55s;+176d38m27.3s;+55d59m52.0s
Betelgeuse (My software);5h55m10.34s;7d24m25.596s;5h56m2.32s;7d24m31.74s;23h52m25.6662s;+176d38m29.643s;+56d00m01.836s
Rigel (Stellarium);5h14m32.28s;-8d12m05.9s;5h15m18.50s;-8d11m12.8s;00h33m09.44s;+190d42m52.5s;+39d51m52.8s
Rigel (My software);5h14m32.279s;-8d12m05.9s;5h15m18.46s;-8d11m03.24s;00h33m09.5262s;+190d42m55.996s;+39d52m02.033s
Bellatrix (Stellarium);5h25m7.86s;+6d20m58.7s;5h25m59.49s;+6d21m37.1s;00h22m28.45s;+189d40m43.7s;+54d38m14.4s
Bellatrix (MySoftware);5h25m7.9s;+6d20m59s;5h25m59.42s;+6d21m46.71s;00h22m28.5662s;+189d40m48.830s;+54d38m23.685s
Mintaka (Stellarium);5h32m00.41s;-0d17m56.7s;5h32m49.55s;-0d17m27.8s;0h15m38.38s;+185d52m14.8s;+48d11m30.9s
Mintaka (MySoftware);5h32m00.42s;-0d17m56.6988s;05h32m49.51s;-0d17m18.1958s;00h15m38.4762s;+185d52m18.043s;+48d11m40.40s
Alnilam (Stellariumm);5h36m12.82s;-1d12m06.9s;5h37m01.63s;-1d11m44.00s;00h11m26.31s;+184d13m21.1s;+47d21m31.3s
Alnilam (My software);5h36m12.82s;-1d12m06.912s;5h37m01.57s;-1d11m34.2547s;00h11m26.4162s;+184d13m24.435s;+47d21m40.929s
Alnitak (Stellarium);5h40m45.54s;-1d56m33.2s;5h41m34.07s;-1d56m16.6s;00h06m53.87s;+182d30m42,7s;+46d40m01.3s
Alnitak (My software);5h40m45.55s;-1d56m33.2160s;5h41m34.03s;-1d56m06.8887;00h06m53.9562s;+182d30m45.212s;+46d40m10.931s
Saiph (Stellarium);5h47m45.40s;-9d40m10.6s;5h48m31.01s;-9d40m03.7s;23h59m56.93s;+179d59m01.5s;+38d57m56.3s
Saiph (My software);5h47m45.38s;-9d40m10.5960s;5h48m30.9500s;-9d39m54.0031s;23h59m57.0362s;+179d59m03.631s;+38d58m05.992s

Is that different a problem?

I don't know which value is the correct one (the one from Stellarium or the one from my software). The maximum difference is about 10 seconds.

And the difference between declinations (Stellarium and mine) is always greater that the difference between right ascensions.

  • $\begingroup$ This is possibly more a computing question than an astronomy question. Computer mathematics is (generally) only an approximation - hence errors creep in and over time these can grow. To avoid this you can use arbitrary precision integer libraries but you'll likely take a huge performance hit. $\endgroup$ Commented Jan 9, 2016 at 19:40
  • $\begingroup$ I don't think you can expect arcsecond accuracy from either Stellarium or a program you've written, unless your program uses a library or functionality that is known to be precise to the arcsecond. $\endgroup$
    – user21
    Commented Jan 9, 2016 at 20:32
  • $\begingroup$ stackoverflow.com/questions/16293146/… is a related question showing different libraries, even fairly precise ones, can disagree on RA/DEC. $\endgroup$
    – user21
    Commented Jan 10, 2016 at 4:22

1 Answer 1


Stellarium 0.14 or later has a checkbox under Configuration, Tools, Planetarium options: Include nutation. If I uncheck it, the RA and Dec of date for those stars on 2016/1/1 match yours within 0.2 arcsecond.

Some libraries distinguish between astrometric and apparent positions, aberration making up to 20 arcseconds difference.


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