a. and sb. Also 7–8 æquatorial, 8–9 equatoreal. [f. L. æquātor (see prec.) + -(I)AL; in Fr. équatorial.]
A. adj.
1. Of or pertaining to the equator; situated or existing on or about the equator.
1713. Derham, Phys.-Theol., II. i. note 1 (1714), 39 (R.). A prolate Sphæroid, making the Polar about 34 Miles shorter than the Equatorial Diameter.
1789. Herschel, Saturn, in Phil. Trans., LXXX. 16. The arrangement of the belts … has always followed the direction of the ring, which is what I have called being equatorial.
1794. G. Adams, Nat. & Exper. Philos., III. xxxii. 334. Their surfaces will be higher … in the equatoreal, than in the polar regions.
1860. Maury, Phys. Geog. Sea, v. § 296. Panama is in the region of equatorial calms.
1870. Yeats, Nat. Hist. Comm., 65. Equatorial grains are maize and rice.
b. Pertaining to the ‘equator’ of a magnet, or of any spherical or spheroidal body.
1664. Power, Exp. Philos., III. 168. Those aequatorial parts of the Magnet, which before respected the East.
1837. Brewster, Magnet., 268. The middle of a copper wire … was applied to the equatorial groove.
1861. J. R. Greene, Man. Anim. Kingd., Cœlent., 227. In Cestum … these [a pair of symmetrical tentacles] do not … issue from the equatorial region, thence turning away from the mouth.
2. Equatorial instrument or telescope: an apparatus consisting essentially of a telescope attached by an arm to an axle revolving in a direction parallel to the plane of the equator. By a uniform motion given to this axle (in large instruments by clockwork) the telescope follows the diurnal apparent motion of any point in the heavens to which it is directed. Equatorial circle: a graduated circle (otherwise called hour-circle, right-ascension-circle) revolving in a plane parallel to the equator, forming part of the equatorial instrument.
1791. Jefferson, in Harper’s Mag. (1885), March, 535/2. He is to pay for equatorial instrument.
1793. Sir G. Shuckburgh, Equator. Instr., in Phil. Trans., LXXXIII. 72. The idea of an equatorial telescope was again renewed by three several artists in this kingdom.
1868. Airy, Pop. Astron., ii. 39. For causing the Equatoreal instrument to revolve uniformly.
B. sb. = Equatorial instrument: see A. 2.
1793. Sir G. Shuckburgh, Equator. Instr., in Phil. Trans., LXXXIII. 84. The equatorial is a machine calculated to observe the heavenly bodies in every part of the hemisphere.
1847. Whewell, Hist. Induct. Sc., VII. vi. § 1 (ed. 2). Transit instruments, equatorials, heliometers.
1879. Lockyer, Elem. Astron., vi. 224. An equatorial.
b. attrib. in equatorial clock, a clock for driving an equatorial.
1884. Britten, Watch & Clockm. Hand-bk., 66, 102.
