What is a reference system in terms of astronomy?
This is a very interesting question.
The natural reference is of course some point on Earth, the problem is that Earth is moving in many ways: it rotates around itself, it rotates around the Sun, it moves in the Milky Way, etc.
Then, usually, the coordinate system is referred to the position of the Earth in a specific instant. What was selected is the position of Earth at the spring equinox of the year 2000.
This system is then transported to the local place on Earth at the current time with a complicate transformation of coordinates.
On top of this there is also a movement of the stars and galaxies that has to add to the calculation, even if it is very small compared with the motion of the Earth.