Sidereal time is a measure of the position of the Earth in its rotation around its axis, or time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical to, the motion of stars.
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Conversely, " following " the diurnal motion with the camera, to eliminate it on the photograph, can best be done with an equatorial mount, which requires adjusting the right ascension only; a telescope may have a motor to do that automatically ( sidereal drive ).
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Because the planetary spheres each move unfalteringly for all eternity in uniform circular motion with a given rotational period relative to the supreme diurnal motion of the sphere of fixed stars ( or First Heaven ), they must each love and desire to mimic different unmoved movers corresponding to the given periods.
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The system is as follows : an iron chain with its links joined together to form an endless circuit hangs down from the upper chain-wheel which is concealed by the tortoise-and-cloud ( column supporting the armillary sphere centrally ), and passes also round the lower chain-wheel which is mounted on the main driving-shaft . Whenever one link moves, it moves forward one tooth of the diurnal motion gear-ring and rotates the Component of the Three Arrangers of Time, thus following the motion of the heavens . }}
