<p>As an outsider to the field, I am surprised to learn that astronomers commonly use the <a href="https://en.wikipedia.org/wiki/Equatorial_coordinate_system" rel="nofollow noreferrer">equatorial coordinate system</a>, in which fixed stars are not actually fixed.</p>

<p>Isn't that making life harder than it needs to be? Why would one not define a coordinate system in terms of fixed stars?</p>

<p>For example like this: Fixed star A is defined as (0°, 0°), fixed star B is defined as (0°, X). That would constrain the 3 degrees of freedom of a spherical coordinate system.</p>

<p>Just to be clear, by fixed stars I mean stars that do not change their apparent relative position over long time scales, say a million years. So stars in the Milky Way would likely be excluded.</p>

<p>When making a new observation, these two reference stars may not be visible, which would make calibration difficult. But there could be a reference catalog of fixed stars with precisely known coordinates. Any new observation could be made relative to a visible subset of that catalog. This would be similar in principle to the International Temperature Scale of 1990, which defines temperature relative to a set of reference materials and states.</p>

CC BY-SA 4.0
2019-07-17T07:59:01.960Z
The standard equatorial system *is* defined in terms of “fixed” extragalactic sources: https://en.m.wikipedia.org/wiki/International_Celestial_Reference_System
Peter Erwin
CC BY-SA 4.0
2019-07-17T09:52:50.437Z
I don't understand your question, all this exactly happens in astronomy? Distant quasars provide your calibration, and during one night they, as well as all other stars rotate with the celestial sphere, leaving their coordinates on this rotating coordinate system constant. On long timescales stars then show their proper motion, which is measured again relative to the distant 'non-moving' quasars.
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-07-17T10:56:04.600Z
I think this question is about axial precession, which causes the coordinates to change, necessitating the use of epochs etc.
James K
CC BY-SA 4.0
2019-07-18T02:08:21.383Z
@PeterErwin that seems to be the answer.
uhoh
CC BY-SA 4.0
2019-07-18T02:08:35.893Z
@JamesK I agree.
uhoh
CC BY-SA 4.0
2019-07-18T02:12:22.067Z
[This explanation of precession](https://lco.global/spacebook/sky/equatorial-coordinate-system/) (scroll down to it) is helpful, e.g. "The changes to the coordinates happen slowly enough that successive generations of star catalog are 50 years apart."
uhoh
<star><fundamental-astronomy><coordinate>
Why do astronomers use the equatorial coordinate system that moves, instead of one based on fixed, distant stars?
griffins
# Why do astronomers use the equatorial coordinate system that moves, instead of one based on fixed, distant stars?

(Asked by: username_0 on 2019-07-17)

As an outsider to the field, I am surprised to learn that astronomers commonly use the [equatorial coordinate system](https://en.wikipedia.org/wiki/Equatorial_coordinate_system), in which fixed stars are not actually fixed.

Isn't that making life harder than it needs to be? Why would one not define a coordinate system in terms of fixed stars?

For example like this: Fixed star A is defined as (0°, 0°), fixed star B is defined as (0°, X). That would constrain the 3 degrees of freedom of a spherical coordinate system.

Just to be clear, by fixed stars I mean stars that do not change their apparent relative position over long time scales, say a million years. So stars in the Milky Way would likely be excluded.

When making a new observation, these two reference stars may not be visible, which would make calibration difficult. But there could be a reference catalog of fixed stars with precisely known coordinates. Any new observation could be made relative to a visible subset of that catalog. This would be similar in principle to the International Temperature Scale of 1990, which defines temperature relative to a set of reference materials and states.

**Tags:** star, fundamental-astronomy, coordinate
> **Comments:**
> **username_1**: The standard equatorial system *is* defined in terms of “fixed” extragalactic sources: https://en.m.wikipedia.org/wiki/International_Celestial_Reference_System  
> **username_2**: I don't understand your question, all this exactly happens in astronomy? Distant quasars provide your calibration, and during one night they, as well as all other stars rotate with the celestial sphere, leaving their coordinates on this rotating coordinate system constant. On long timescales stars then show their proper motion, which is measured again relative to the distant 'non-moving' quasars.  
> **username_3**: I think this question is about axial precession, which causes the coordinates to change, necessitating the use of epochs etc.  
> **username_4**: @PeterErwin that seems to be the answer.  
> **username_4**: @JamesK I agree.  
> **username_4**: [This explanation of precession](https://lco.global/spacebook/sky/equatorial-coordinate-system/) (scroll down to it) is helpful, e.g. "The changes to the coordinates happen slowly enough that successive generations of star catalog are 50 years apart."  

---

## 0 answers

