As an outsider to the field, I am surprised to learn that astronomers commonly use the 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.
