What is a reference system in terms of astronomy?

This is an intriguing question. The obvious natural reference is somewhere on Earth, but the issue is that Earth is in constant motion—it revolves around the Sun, around itself, through the Milky Way, etc.

The coordinate system then typically refers to Earth's position at a given point in time; in this case, Earth's position at the year 2000's spring equinox was chosen.

This system is then moved via a complex coordinate transformation to the local location on Earth at the current time. In addition, even though it is very tiny in comparison to Earth's motion, the movement of stars and galaxies must also be factored into the computation.

Astronomers can locate objects in the sky and follow their movements over time with the help of reference systems, which are frameworks used to precisely define the positions and motions of celestial objects. These systems usually involve a set of coordinates, such as right ascension and declination or celestial longitude and latitude, relative to a particular point or plane, like the celestial equator or the ecliptic.

A reference system in astronomy is a framework used to define the positions and motions of celestial objects relative to one another. It typically involves a set of coordinate axes, such as the celestial equator and the ecliptic, along with reference points or objects such as stars, galaxies, or planets. These systems provide a standardized way to measure and describe the locations and movements of celestial bodies in the sky. Examples include the equatorial coordinate system, which uses the celestial equator and the poles of the celestial sphere as reference points, and the ecliptic coordinate system, which uses the plane of Earth's orbit around the Sun as a reference.