An exact knowledge of the instantaneous Earth rotation rate is indispensable for accurate navigation and point positioning. Fluctuations in the `Length of sidereal Day' (LoD) are caused by a momentum exchange between the fluids of the Earth, namely the atmosphere, hydrosphere and cryosphere and the solid Earth. Since a multitude of very different globally distributed and independent mass transport phenomena are involved, the resultant effect on the Earth rotation is not predictable and has to be measured continuously. Here we report successful observations of the minute variations in the rotation rate of the Earth ΔΩe at the level of 5 ppb, by an inertial self-contained measurement technique. Our active optical ring laser interferometer `G' is operated in the Sagnac configuration and is rigidly strapped down to the Earth's crust. This large scale gyroscope integrates over one hour for each data point, increasing the data rate of LoD estimates by more than one order of magnitude, thus enlarging the measurement bandwidth. It is emphasized that a single local sensor, namely a large ring laser interferometer, is now able to measure effects which previously required entire global networks of GNSS receivers and VLBI interferometers, which themselves then provide a single measurement per day.