E`qua*to"ri*al (?), a. [Cf. F.
équatorial.] Of or pertaining to the equator; as,
equatorial climates; also, pertaining to an equatorial
instrument.
E`qua*to"ri*al, n. (Astron.)
An instrument consisting of a telescope so mounted as to have
two axes of motion at right angles to each other, one of them
parallel to the axis of the earth, and each carrying a graduated
circle, the one for measuring declination, and the other right
ascension, or the hour angle, so that the telescope may be directed,
even in the daytime, to any star or other object whose right
ascension and declination are known. The motion in right ascension is
sometimes communicated by clockwork, so as to keep the object
constantly in the field of the telescope. Called also an
equatorial telescope.
&fist; The term equatorial, or equatorial
instrument, is sometimes applied to any astronomical instrument
which has its principal axis of rotation parallel to the axis of the
earth.
