1902 Encyclopedia > Sextant


SEXTANT, an instrument for measuring angles on the celestial sphere. The name (indicating that the instrument is furnished with a graduated arc equal to a sixth part of a circle) is now only used to designate an instrument employing reflexion to measure an angle; but originally it was introduced by Tycho Brahe, who con-structed several sextants with two sights, one on a fixed, the other on a movable radius, which the observer pointed to the two objects of which the angular distance was to be measured.

In the article NAVIGATION the instruments are described which were in use before the invention of the reflecting sextant. Their imperfections were so evident that the idea of employing reflexion to remove them occurred independently to several minds. Iiooke contrived two reflecting instruments. The first is described in his Posthumous Works (p. 503); it had only one mirror, which reflected the light from one object into a telescope which is pointed directly at the other. Hooke's second plan employed two single reflexions, whereby an eye placed at the side of a quadrant could at the same time see the images formed in two telescopes, the axes of which were radii of the quadrant and which were pointed at the two objects to be measured. This plan is described in Hooke's Animadversions to the Machina Caslestis of Hevelius, published in 1674, while the first one seems to have been communicated to the Royal Society in 1666. Newton had also his attention turned to this subject, but nothing was known about his ideas till 1742, when a description in his own handwriting of an instrument devised by him was found among Halley's papers and printed in the Philosophical Transactions (No. 465). It consists of a sector of brass, the arc of which, though only equal to one-eighth part of a circle, is divided into 90°. A telescope is fixed along a radius of the sector, the object glass being close to the centre and having outside it a plane mirror inclined 45° to the axis of the telescope, and intercepting half the light which would otherwise fall on the object glass. One object is seen through the tele-scope, while a movable radius, carrying a second mirror close to the first, is turned round the centre until the second object by double reflexion is seen in the telescope to coincide with the first.

But long before this plan of Newton's saw the light the sextant in its present form had been invented and had come into practical use. On May 13, 1731, John Hadley gave an account of an "octant," employing double reflexion, and a fortnight later he exhibited the instrument.'*

On the 20th May Halley stated to the society that Newton had invented an instrument founded on the same principle, and had communicated an account of it to the society in 1699, but on search being made in the minutes it was only found that Newton had showed a new instru-ment " for observing the moon and stars for the longitude at sea, being the old instrument mended of some faults," but nothing whatever was found in the minutes concerning the principle of the construction. Halley had evidently only a very dim recollection of Newton's plan, and at a meeting of the Royal Society on December 16, 1731, he declared himself satisfied that Hadley's idea was quite different from Newton's. The new instrument was already in August 1732 tried on board the "Chatham" yacht by order of the Admiralty, and was found satisfactory, but otherwise it does not seem to have superseded the older instruments for at least twenty years. As constructed by Hadley the instrument could only measure angles up to 90°; but in 1757 Captain Campbell of the navy, one of the first to use it assiduously, proposed to enlarge it so as to measure angles up to 120°, in which form it is now generally employed.

Quite independently of Hadley and Newton the sextant was invented by Thomas Godfrey, a poor glazier in Philadelphia. In May 1732 Mr James Logan of that city wrote to Halley that Godfrey had about eighteen months previously showed him a common sea quadrant "to which he had fitted two pieces of looking-glass in such a manner as brought two stars at almost any distance to coincide." The letter gave a full description of the instru-ment ; the principle was the same as that of Hadley's first octant which had the telescope along a radius. At the meeting of the Royal Society on January 31, 1.734, two affidavits sworn before the mayor of Philadelphia were read, proving that Godfrey's quadrant was made about November 1730, that on the 28th November it was brought by G. Stewart, mate, on board a sloop, the "Truman," John Cox, master, bound for Jamaica, and that in August 1731 it was used by the same persons on a voyage to Newfoundland. There can thus be no doubt that Godfrey invented the instrument independently; but the statement of several modern writers that a brother of Godfrey, a captain in the West India trade, sold the quadrant at Jamaica to a Captain or Lieutenant Hadley of the British navy, who brought it to London to his brother, an instrument maker in the Strand, has been proved to be devoid of all foundation. Not only is this totally at variance with all the particulars given in the affidavits, but between 1719 and 1743 there was no officer in the British navy of the name of Hadley, and John Hadley cannot possibly have been in the West Indies at that time, as he was present at many meetings of the Boyal Society between November 1730 and May 1731; besides, neither Hadley nor his brothers were professional instrument makers. A detailed discussion of this question by Prof. Bigaud is found in the Nautical Magazine, vol. ii. No. 21.

The annexed figure gives an idea of the construction of the sextant. ABC is a light framework of brass in the shape of a Sector of 60°, the limb AB having a graduated arc of silver (some-times of gold) inlaid in the brass. It is held in the hand by a small handle at the back, either vertically to measure the altitude of an object, or in the plane passing through two objects the angular distance of which is to be found. CD is a radius movable round C, where a small plane mirror of silvered plate-glass is fixed perpendicular to the plane of the sextant and in the line CD. At D is a vernier read through a small lens, also a clamp and a tangent screw which enable the observer to give the arm CD a very slow motion within certain limits. At E is another mirror " the horizon glass," also perpendicular to the plane of the sextant and parallel to CB. F is a small telescope fixed across CB, parallel to the plane CAI3 and pointed to the mirror E. Dark glasses can be placed outside E and between E and C when ob-serving the sun. As only the lower half of E issilvered, the observer can see the hori-zon in the telescope through the unsilvered half, while the light from the sun or a star S may be reflected from the " index glass " C to the silvered half of E and thence through F to the observer's eye. If CD has been moved so as to make the image of a star or of the limb of the sun coincide with that of the horizon, it is easy to see that the angle SCH (the altitude of the star or solar limb) is equal to twice the angle BCD. The limb AB is always graduated so as to avoid the necessity of doubling the mea-sured angle, a space marked as a degree on the limb being in reality only 30'. The vernier should point to 0° 0' 0" when the two mirrors are parallel, or in other words, when the direct and reflected images of a very distant object are seen to coincide. For the methods of adjusting the mirrors and finding the index error see NAVIGATION (vol. xvii. p. 268).

If the sextant is employed on land, an artificial horizon has to be used. This is generally a basin of mercury protected from the wind by a roof of plate-glass with perfectly parallel faces ; some-times a glass plate is used (with the lower surface blackened), which can be levelled on three screws by a circular level. The telescope is directed to the image of the celestial object reflected from the artificial horizon, and this image is made to coincide with that reflected from the index-glass. In this case the angle BCD will be double the altitude of the star. Towards the end of last and the beginning of this century the sextant was much used on land for determiniug latitudes, but, though in the hands of a skilful observer it can give results far superior to what one might expect from a small instrument held in the hand (or attached to a small stand), it has on shore been quite superseded by the portable altazimuth or theodolite, while at sea it continues to be indispensable.

The principle of the sextant has been applied to the construction of reflecting circles, on which the index arm is a diameter with a vernier at each end to eliminate the error of eccentricity. The circles constructed by Pistor and Martins of Berlin have a glass prism instead of the horizon glass and are extremely convenient. (J. L. E. D.)


- Thus Marshall Ward has lately drawn attention to the association of parasitism with the disappearance of sexual reproduction in Fungi (Quart. Jour. Micr. Sci., xxiv.).

3 Hadley described two different constructions: in one the telescope was fixed along a radius as in Newton's form, in the other it was placed in the way afterwards universally adopted; an octant of the first construction was made as early as the summer of 17?0, according to a statement made to the Royal Society by Hadley's brother George on Feb. 7, 1734.

The above article was written by: J. L. E. Dreyer, Ph.D., Astronomer, Armagh Observatory.

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