Time transfer and synchronization

The second is certainly the unit of time... but which second are we talking about?

The notion of time is a complex one, and there are many different ways of thinking about it. Each of these ways has its own "time scale". These scales are not equivalent, and they need to be linked together.

Universal time as defined by the IAU (International Astronomical Union), noted UT ₁ , is based on the work of Euler, who in 1736 showed that, according to Newtonian mechanics, the angular velocity of the Earth's rotation in space is invariant. This observation made it possible to construct a fragmentation or "time scale" based on the observation of the diurnal motion of the stars....