Show ow arctic explorers i make use of sextant C IND L r 1 t I N 1 H T rah N the sextant whose service in polar trips Is has been reported by the explorers is an instrument small enough to be conveniently held in the hand and la Is equally well adapted for fop measuring the altitude of celestial objects order to obtain the latitude and local time or for measuring the tha angle between the moon and sun or the moon and a fixed star to ascertain the longitude it Is called sextant because the measure is lg recorded on an ahre arc of 60 degrees one sixth of a circle says the now new york world it consists of a frame usually of metal stiffened by tross cross braces the arc at the bottom of the frame Is marked off with double the number of degrees actually measured this is done because the fixed and movable glasses attached to the tha in tt st rumont give a double reflection of the objects observed and thus form fin angle with reference to each other squall equal to only halt half the angular distance between such objects one of which Is seen directly and the other by reflection the are arc of degrees thus records the actual angle midway on the frame on one side Is a telescope and opposite on ofa the other leg of the frame lg Is a glass transparent in the upper halt half arid and sll sit in the lower halt half doth both the telescope ET E T in the accompanying acure and the glass 11 II 11 in the figure tre ire firmly attached to the frame at the top of the frame Is a mirror C in tho the figure which la Is movable hy by means of an arm HM H 11 M in the figure to which it la Is fastened C Is called the index glass and the arm RM 1151 R 11 M 51 revolves around it at at Is a shifting shit ting scale for making fractional measurements an and d called a vernier the takes the instrument in his hand and holds the telescope horizontally looking through the telescope he may see the horizon through the transparent surface of the horizon glass 11 then it if wishing to bring the sun into line he manipulates the mirror C as a child handles a bit of looking glass for tho the purpose of catching the suns glare and throwing it into the eyes of a companion ile he turns the arm RM R M until the mirror 0 catches its reflection and throws it back to the silvered surface of jilie glass if when the sun Is thus made to coincide with the horizon the section of the graduated are arc over which the arm RM R M 31 has passed indicates the measure of the angle in degrees which Is exactly determined by the movable fractional scale or vernier arabian astronomers are credited with having used a sextant as far back bach as the year with a radius of 59 feet 9 inches tho the modern instrument was invented independently about 1730 by thomas godfrey of philadelphia and capt hadley of the british navy |