Image 1 Caption: A 16th century engraving of Ptolemy (c.100--c.170 CE) being guided in his observations by Urania (AKA Astronomia), muse of astronomy---astronomy is the only science with a muse.

Ptolemy is wearing a crown because the Middle Ages and Renaissance tended assimilate him to the Ptolemaic dynasty (305--30 BCE) that ruled Hellenistic Eygpt---Cleopatra (69--30 BCE) was the last monarch (a queen regnant) of that dynasty.

The Fixed Stars:

Ptolemy may be observing the fixed stars----which Isaac Newton (1643--1727) theorized as defining the the singular fundamental inertial frame: i.e., Newton's absolute space. This theory was wrong. For the modern view of inertial frame, see Mechanics file: frame_basics.html.

Explication of the term fixed stars:

1. The fixed stars (in Latin: stellae fixae) are the background of extrasolar objects (i.e., astronomical objects beyond the Solar System) that do NOT to move relative to each other in the night sky on short enough time scales compared to the foreground of Solar System astronomical objects (i.e., Solar System objects) that do.

2. Traditionally, fixed stars are the stars relatively near the Sun in our small region of the Milky Way. They include naked-eye stars, particularly those making up the constellations. Of course, the fixed stars are NOT actually fixed (i.e., unmoving in 3-dimensional space). They are just very slowly moving relative to the Sun and each other over the course of a human lifetime. But to the Ancients (e.g., Ptolemy) and even to the time of Newton and for some to the 1920s, they seemed truly fixed.

   Yours truly usually means the traditional fixed stars when yours truly says fixed stars.

   

3. Image 2 Caption: An artist conception extraterrestrial map of the observable universe with the particle horizon (the defined boundary of the observable universe) at the comoving radius of the observable universe = 14.25 Gpc = 46.48 Gly (current value). The cosmological value just given and those below are according to the Λ-CDM model which is the favored cosmological model circa 2022, but it may NOT be favored at the end of the 2020s.

4. The traditional fixed stars constitute a reference frame with respect to which inertial frames do NOT rotate to good approximation. So absolute rotation can be measured with respect to the fixed stars to good approximation and this was how such measurements were always done in the past in the belief that absolute rotation was being measured. The "past" meaning before the advent of general relativity (1915) and general relativistic cosmology (c.1920s).

5. Note in our modern understanding absolute rotation is rotation relative observable universe (i.e., to the bulk mass-energy of observable universe).

6. Nowadays, we determine absolute rotation to high accuracy/precision when we need to by measuring rotation relative to distant astronomical objects, mostly quasars (see Wikipedia: International Celestial Reference System and its realizations: Realizations).

7. Yours truly sometimes uses the expression "relative to the fixed stars" when yours truly means "relative to the observable universe". It's an old habit yours truly is trying to break.

Images:

1. Credit/Permission: Anonymous artist, appearing in Gregor Reisch (c.1467--1525), Margarita Philosophica 1508 (uploaded to Wikipedia by User:Zachariel, 2011) / Public domain.
   Image link: Wikipedia: File:Ptolemy urania.jpg.
   
2. Credit/Permission: © Andrew Colvin (AKA User:Azcolvin429), 2010 / Creative Commons CC BY-SA 3.0.
   Image link: Wikipedia: File:Observable Universe with Measurements 01.png.