<p>Using the following information from <a href="https://theskylive.com/sky/stars/alcyone-eta-tauri-star" rel="nofollow noreferrer">The Skylive</a>, and simple arithmetic I tried to calculate longitude of Alcyone in the year 41050 BCE at a place: 29.9457 N , 78.1642 E.</p>
<p>Galactic Longitude 166.67°,
Galactic Latitude -23.45°</p>
<p>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.</p>
<p>Where did I miss?</p>
<p>Any help would be appreciated.</p>

CC BY-SA 4.0
2021-03-30T19:12:55.840Z
Why do you think that the earth's precession affects galactic longitude?
Dr Chuck
CC BY-SA 4.0
2021-03-30T20:26:43.527Z
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?
Pierre Paquette
CC BY-SA 4.0
2021-03-30T22:13:58.247Z
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.
Atul
CC BY-SA 4.0
2021-03-30T22:20:26.183Z
@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.
Atul
CC BY-SA 4.0
2021-03-31T00:23:58.863Z
@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.
Pierre Paquette
CC BY-SA 4.0
2021-03-31T13:53:46.547Z
Thank you @PierrePaquette for the tip. I checked the effect of proper motion. With the given value of RA: 19.34 ± 0.39 mas/yr, for the period of interest it accounts for only 0.24°. I understand from these comments that my method is incorrect. I have to wait for some enlightened soul for guidance as to how to proceed. BTW I used the Galactic longitude as a reference in the calculations. Thanks again.
Atul
<coordinate>
How to calculate longitude of Alcyone - η Tauri in the year 41050 BCE?
Atul
# How to calculate longitude of Alcyone - η Tauri in the year 41050 BCE?

(Asked by: username_0 on 2021-03-30)

Using the following information from [The Skylive](https://theskylive.com/sky/stars/alcyone-eta-tauri-star), 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.

**Tags:** coordinate

> **Comments:**
> **username_1**: Why do you think that the earth's precession affects galactic longitude?  
> **username_2**: 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?  
> **username_0**: 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.  
> **username_0**: @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.  
> **username_2**: @username_0: 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.  
> **username_0**: Thank you @PierrePaquette for the tip. I checked the effect of proper motion. With the given value of RA: 19.34 ± 0.39 mas/yr, for the period of interest it accounts for only 0.24°. I understand from these comments that my method is incorrect. I have to wait for some enlightened soul for guidance as to how to proceed. BTW I used the Galactic longitude as a reference in the calculations. Thanks again.  

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## 0 answers

