Abstract: The sidereal clock comprises a digital clock portion having a 60-Hz pulse source operating at line frequency of 60-Hz. It contains a means for periodically changing logic state at the 50/60-Hz select input of the prescale counter so that the digital clock portion is periodically changed during a period equal to 2922 clock pulses from the 60 Hz operation mode to the 50 Hz operation mode for a time interval equal to 40 clock pulses. During this time interval the prescale counter actually registers 6 counts for every 5 pulses received so that the digital clock portion runs fast by a factor of 2930/2922 and measures sidereal time. The means for changing logic state at the 50/60-Hz select input may comprise a ripple counter connected electrically to a plurality of gating circuits of first and second gating integrated circuits and a monostable multivibrator all of which are energized by the DC of the digital clock portion.