  A reference frame in event space is a smooth field of orthonormal bases.
Every reference frame is equipped by anholonomic coordinates. Using anholonomic
coordinates allows to find out relative speed of two observers and appropriate
Lorentz transformation.
  Synchronization of a reference frame is an anholonomic time coordinate.
Simple calculations show how synchronization influences time measurement in the
vicinity of the Earth.
  Measurement of Doppler shift from the star orbiting the black hole helps to
determine mass of the black hole. According observations of Sgr A, if non
orbiting observer estimates age of S2 about 10 Myr, this star is 0.297 Myr
younger.
