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Using the following information from The Skylive, and simple arithmetic I tried to calculate longitude of Alcyone in the year 41050 BCE at a place: 29.9457 N , 78.1642 E.

Galactic Longitude 166.67°, Galactic Latitude -23.45°

I could calculate it to be 330° against the expected value of 360°. I used J(2000) value of 25771 years for the precessional cycle of earth's axis.

Where did I miss?

Any help would be appreciated.

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    $\begingroup$ Why do you think that the earth's precession affects galactic longitude? $\endgroup$ – Dr Chuck Mar 30 at 19:12
  • $\begingroup$ As @DrChuck wrote, Earth’s precession doesn’t affect galactic longitude. I have two questions: 1) Where did you get the “expected value of 360°” from? and 2) Have you taken into account the star’s proper motion? $\endgroup$ – Pierre Paquette Mar 30 at 20:26
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    $\begingroup$ Thank you for reading the question.I am sorry @DrChuck the question has been misunderstood. It is about calculating longitude of the star. Never claimed that Galactic longitude changes due to precession of earth's axis. $\endgroup$ – Atul Mar 30 at 22:13
  • $\begingroup$ @PierrePaquette Hope my comment above clarifies to some extent. Answers to your queries 1) Value expected after one complete precessional rotation of earth's axis. 2) No, not at this stage. $\endgroup$ – Atul Mar 30 at 22:20
  • $\begingroup$ @Atul: As DrChuck and I said, Earth’s precession has nothing to do with the galactic longitude of an object. And you need to take its proper motion into account. I fail to see how you’d expect it to have a longitude of 360° “after one complete precessional rotation of [E]arth’s axis.” If Earth’s precession had anything to do, after a full rotation, objects would be back in their initial place… But like I said twice now, it has nothing to do with that. $\endgroup$ – Pierre Paquette Mar 31 at 0:23

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