parallax

[Gr. parallax is =(1) an alternation, (2) change, (3) parallax (see below); Gr. par all as so to make things alternate: para from beside, and all as so =to change; Ital. paralla e; Ger. & Fr. parallaxe.] 1. Astron.: The difference between the position of a heavenly body as viewed from a station on the earth's surface and as it would be if seen from the center of our planet. It is measured by an angle, of which the point is formed by the heavenly body, while the straight lines inclosing it touch the observer's position and the center of the earth. ( Airy : Pop. Astronomy , ed. 6th, p. 142.) The parallax of the sun is between 8 and 9", and that of the moon about a degree and a half. ( Ibid ., pp. 209, 210.) 12. Optics: The difference in the position of an object produced when the wires are not at a proper distance from the object-gla . (1) Angle of parallax : Optics: The angle formed by two lines drawn from an object, one to the center of each eye. (2) Annual parallax : (a) The term sometimes applied to a parallax, if any is visible, of a star when viewed from two opposite points of the earth's orbit. The annual par allax of the stars is not in any case certainly 2", and in every case but one, unquestionably le than a single second. (5) Sometimes the difference in the position of a star as viewed from the sun and from the earth at the extremity of her orbit; in other words, with a base-line of half her orbit. (3) Binocular parallax : Optics: The difference between the position of an object as seen with the one eye and with the other, the head remaining unmoved. (4) Concentric parallax : The same as Diurnal parallax. (5) Diurnal parallax : The same as Geocentric parallax (q. v.). (6) Geocentric parallax : Astron.: The parallax of a heavenly body when in the horizon. The term is specially used of the 3003 moon. When in the horizon the parallax is greater than when the moon is in any other position. (7) Horizontal equatorial parallax : Astron.: As the radii of the earth are not all equal, it is needful to decide which to use in calculating the moon's horizontal parallax. The inser tion of the term equatorial means that the equator is the particular radius which has been employed, as it actually is by most astronomers. The moon's horizontal equatorial parallax varies from 54 to 1°1'15", the difference arising from the elliptic orbit in which the luminary moves. ( Airy : Pop . Astron ., p. 165.) The distance of the moon is computed by means of her equatorial horizontal parallax. Theo retically speaking, that of the sun can be ascertained in a similar way; but, the figures being small, a minute error in them would make a great difference in the results of computations founded on them. Hence other methods are adopted. [SUN, TRANSIT.] (8) Parallax in altitude : Astron.: The parallax of a heavenly body when it has risen to a certain altitude above the horizon. păr-al-lel, *păr-a-lell, a . & s . [O. Fr. par allele , from Lat. parallel us ; Gr. parallèlos =parallel, side by side, from para beside, and allēlos =one another; Sp. paralelo ; Ital. parallelo , paralello .] IMG:content-0641.png:[blocks in formation] 2. Figuratively : (1) Conformity or resemblance continued through many particulars; resemblance; correspondence in e ential points; similarity. "The parallel holds in the gainle ne , as well as laboriousne of the work." - Decay of Piety . (2) A comparison made; as, to draw a parallel between two events. "Such a parallel is as absolutely erroneous as anything can be."-Whitney: Language , ch. xi. (3) An equal; a counterpart; something equal or similar to another in all e ential points. "The hatred of which Jeffreys was the object was without a parallel in our history."- Macaulay : Hist . Eng ., ch. xiv. II. Technically : 1. Fort .: A wide trench parallel to the attacked work, for protecting the besieging troops. The parallels connect the approaches or zigzags by which advances are made on besieged works. 2. Geog .: A line on a globe or map marking the latitude; a circle or part of a circle parallel to the equator. 3. Print.: A reference-mark (||). parallel-bar, s. 1. Steam-eng.: A rod in the side-lever engine, forming a connection with the pump rods and studs along the center line of the levers. 2. ( Pl .): Bars fixed at a certain height above the ground, and parallel to each other, on which to practice gymnastic exercises. parallel-circles, subst . pl . Those circles of the sphere whose planes are parallel to each other; every system of such circles has a common axis, and, consequently, their poles are also common. [CONCENTRIC.]