The time-scale used in the Barycentric Celestial Reference System, the unit of which is the SI second. TCB differs from Terrestrial Time (TT) because of the movement of the Earth in the Barycentric Celestial Reference System and the Earth's immersion in the gravitational potential of the Sun and other Solar System bodies. These effects cause an overall rate change of about 0.5 s per year on which are superimposed smaller periodic variations. The time-scale Barycentric Dynamical Time (TDB) is a convenient linear transformation of TCB that keeps in step with TT on the average.
Subjects: Astronomy and Astrophysics.