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The mechanism that moves a telescope so it can track a star or other astronomical object across the sky.
The apparent motion of the sky reflects the rotation of the Earth on its axis relative to the distant stars.
The period of one such rotation is a sidereal day. Since the sidereal day is shorter than a solar day by about 4 minutes,
the telescope needs to be driven so as to make a complete revolution in 23 hours 56 minutes. This is known as the sidereal rate. In practice, the drive mechanism is normally an electric motor, appropriately geared. Formerly, mechanical clockwork drives were constructed, powered by falling weights for example. Such drives are still operational on some historical instruments.
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