In any experiment, specifying a physical quantity of interest always relies on a reference frame. For example, identifying the time at which an event happens only makes sense relative to a clock.

According to one of the most fundamental principles in physics, an observer on a moving train uses the same laws to describe a ball on the platform as an observer standing on the platform - physical ...

VLBI is the only technology that permits determination of positions on Earth with a direct relationship to a celestial reference system. The International Celestial Reference Frame (ICRF) is an ...