What is the space relative to which motion is measured?

Answer 1

The space relative to which motion is measured is typically referred to as the reference frame. A reference frame is a coordinate system in which the position and motion of objects can be described. It can be fixed or in motion, but it provides a point of reference to measure the position, velocity, and acceleration of an object. Different reference frames can yield different measurements of motion depending on their relative motion to the observed object.