The present definition of U.T.1 is a complex one which introduces the old concept of the ”Fictitious Mean Sun” which has been
suggested by Newcomb (1895). The conventional right ascension of the Fictitious Mean Sun brings the basic relationship between Sidereal Time, arising directly from observations, and U.T.1, as it is internationally adopted. Unfortunately, this basic relationship needs some effort of understanding for the common user. It is the reason
why B. Guinot (1979) proposed to adopt another point instead of the vernal equinox on the celestial equator, that he called the
‘non-rotating origin σ’. This point obeys to a clear kinematical
concept. Moreover, it should bring a new conceptual definition of U.T.1 very easy to understand. The position of σ on the celestial sphere can be easily determined by the Eulerian angles ψ and θ which are positionning the instantaneous axis of rotation of the Earth relatively to an inertial plane of reference. It can also be realized by the way of a quantity 's’ depending on the only motion
of the instantaneous equator. Formulation and developpement of ‘s’ are successively given.