Equatorial
an astronomical instrument designed to note the course of the stars as they move through the sky.
A strong axis is constructed and perma- EQUINOX nently fixed in a slanting position so as to point exactly to the North pole of the heavens. It turns on its axis, carrying with it a telescope which, if it retained its relative position to that of the revolving portion of the instrument, would enable an observer looking through it to see no more than a single great circle of the sky. It is not, however, fixed to the revolving portion of the instrument, but may be moved up or down so that with it an astronomer can follow the entire course of a circumpolar star in its passage around the sky. It is of importance to ascertain not only the course of a star, but the apparent rapidity of its movement. This end is attained by attaching to the axis of the equatorial a racked wheel in which works an endless screw or worm, the whole put in motion by an apparatus furnished with centrifugal balls, like those of the governor of a steam engine, and which render the motion uniform. a