Equatorial

Equatorial, an important instrument for observing and following a celestial body in any part of its diurnal course. It consists, in the usual form, of two axes, one fixed square across the end of the other, thus $\backslash Gamma$. The cross axis (called the declination axis) revolves in bearings fixed to the upright one (called the polar axis), which again turns in bearings supported by a fixed pillar, resting on a solid foundation. Across one end of the cross axis is rigidly fixed a telescope, and on the other end slides a counterbalancing weight. Thus the telescope can be turned freely in any direction, and clamped by suitable mechanism. As erected for use, the upright axis is fixed to point directly to the pole of the heavens. It being then revolved, the other axis will trace with its ends the line of the celestial Equator (q.v.). By this motion the telescope sweeps the sky from east to west, and by rotating the cross axis it sweeps from north to south. The amount of rotation of each axis is measured by a graduated circle fixed to it, and thus the position of any heavenly body observed is known. Conversely, by setting the circles and observing the time, the telescope may be set so as to point to any star, even if it be one invisible to the naked eye. The hour circle, on the polar axis, measures the right ascension of the star, and the other, or declination circle, its declination. The equatorial can hardly be called an astronomical instrument. It is rather a convenient method of mounting large astronomical telescopes. For if the telescope be pointed to any star and clamped, while the polar axis is regularly revolved by clockwork, the star remains steadily in the field of view. See TELESCOPE.