BEQUEST OF REV. CANON SDADDMG, D. TORONTO. 1*01 GLOSSAEY OF NAVIGATION GLOSSARY OF NAVIGATION BY THE REV. J. B. HARBORD, M.A. ST JOHN'S COLLEGE, CAMBRIDGE CHAPLAIN AND NAVAL INSTRUCTOR, R. N. WILLIAM BLACKWOOD AND SONS EDINBURGH AND LONDON MDCCCLXIII SOLD BY J. D. POTTER, ADMIRALTY CHART AGENT, LONDON PREFACE. IN offering this little book to our young naval officers, the author is guided by a wish to help them in what he knows by experience they consider the most troublesome part of their studies. He would give this help by teaching them to regard scientific and technical terms not as necessary evils, but as very useful ser- vants, by the rational use of which a definiteness of conception may be acquired not otherwise attainable. It is also his hope in some small measure to aid in banishing a prevailing looseness of phraseology, and in bringing about a consistency of usage in nautical terms. Under each term, besides what is necessary to explain it fully in its different bearings, will be found an analysis of what is to be learned on the subject by systematic reading. In the arrange- ment of the articles the strictly alphabetical order has sometimes been departed from as regards the subdivisions of a general head, and what appears in each case the natural order adopted. Most of the works on Navigation in common use have been consulted in this but the author drawing up Glossary ; must par- ' ticularly mention his obligations to Lieutenant Eaper's Practice of Navigation.' In the articles belonging to the most important ancillary science he is deeply indebted to Sir John F. W. Her- ' schel's Outlines of Astronomy.' " H.M.S. LIVERPOOL," July 1863. GLOSSARY OF NAVIGATION, AGO ADJ Acceleration of Sidereal on Mean Solar Time. The change of the mean sun's right ascension in a mean solar day, in consequence of which, in the interval of his coming successively to the meridian, the first point of Aries to have in its diurnal hence appears hastened forward revolution ; the name. To explain this, let MPV (fig. 1) Fig L Fig 2. be the sidereal time, when the mean sun is on the meridian of Greenwich on a parti- cular day. When the mean sun comes again to the meridian after a mean solar day, he will have moved in right ascension to the this east through MM' say (fig. 2). The sidereal time corresponding to second the first of mean noon will be M'PV instead of MPV ; point Aries thus appearing to have hastened forward in its diurnal revolution, and the sidereal time to be "accelerated" with reference to mean solar time. The angle M'PM, which is the acceleration, is a portion of side- real time. The amount of acceleration for any given interval of mean solar time will enable us to deduce the sidereal time. In the Nautical " Almanac, pp. 520, 521, is given a Table for Converting Intervals of Mean Solar Time into Equivalent Intervals of Sidereal Time." Similar tables are also given in treatises on navigation. See RETARDATION OP MEAN SOLAR ON SIDEREAL TIME. Achernar. The Arabic name for the bright star a Eridini. See ERIDINDS. Adjustments of Instruments. All nautical instruments are liable to get out of order, their several parts not retaining their relative positions, 1 A ADJ ALD owing to unequal expansion, violence, or like causes. To guard before observing against resulting errors, there are methods of testing whether the is to affected and the instrument in order in the several points subject be ; instrument is provided with means of adjustment, chiefly in the form of screws or sliding weights, by which it may be restored to its correct state. Adjusting screws and weights ought not to be touched more than is abso- lutely necessary, and then with great care. When two such screws work oppositely to each other, one must not be tightened without the other being at the same time loosened. Sometimes, instead of making the ad- justment, the error may be acknowledged and allowed for in observing. " " The term adjustments is often loosely applied to all sources of incorrect- ness, and means of obviating their effects, in using instruments. These are, however, properly of three distinct kinds : (1) Imperfections in the instru- its for of the ment, which should cause rejection ; (2) Adjustments parts instrument liable to temporary derangement, but which can be restored to order the attached Errors of the which by machinery ; (3) instrument, are acknowledged, determined by experiment, and allowed for. It would be well if the term "adjustments" were always strictly limited to the second of these. Adjustments of the Azimuth Compass. The adjustments of the azimuth " compass are commonly said to be the following : (1) The magnetic " axis line of the needle should coincide with the longitudinal ; (2) The in the centre of of the card pivot should be the graduated circumference ; " line of should over the The (3) The sight" pass directly pivot ; (4) eye- vane and should each be vertical needle with card sight-vane ; and (5) The should work upon its pivot horizontally. But of these (1) is properly treated as an error of the instrument ; (2) (3) and (4) as imperfections ; (5) only strictly coming under the head of adjustments. See COMPASS, IMPERFEC- TIONS, ADJUSTMENTS, ERRORS. Adjustments of the Sextant. The adjustments of the sextant are : (1) The "index-glass" should be perpendicular to the plane of the arc; " " of the arc (2) The horizon-glass should be perpendicular to the plane ; (3) The "line of collimation" of the telescope should be parallel to the of the arc and For distant when the zero of the vernier plane ; (4) objects, coincides with the zero of the arc, the horizon-glass and the index-glass should be parallel. The last is generally in practice treated as an error. See SEXTANT, IMPERFECTIONS, ADJUSTMENTS, ERRORS. Age of the Moon. The "age of the moon" is reckoned through a luna- tion or lunar month, the mean length of which is about 29^ days, from new moon to new moon. It is given in the Nautical Almanac, p. iv., for every mean noon at Greenwich. Age of the Tide. The interval between the transit of the moon at which a tide originates and the appearance of the tide itself. Called also Retard of the Tide. See under TIDE. Alam^V The Arabic name for the bright star y Andromeda. See ANDROMEDA. " Aldebaran (Arabic, Ain al Thaur, The Bull's Eye"). The large and bright star of the first magnitude situated in the eye of the constellation Taurus ("The Bull"), in modem catalogues known as a Tauri, but still generally called by its name Aldebaran. It is a very important star to the navigator, being one of those whose "lunar distances" are calculated and tabulated in the Nautical Almanac. It is also easily found. The 2 ALG ALT two remarkable groups, the Pleiades and Hyades, at once point out Taurus, and Aldebaran is among the small stars of the latter, conspicuous by its ruddy colour. It is at about the same distance from Orion's Belt on the one side that Sirius is on the other, and a line drawn from the Pole Star a little to the westward of will no star Capella passm through great till it 4* comes to Aldebaran. 1863, R. A. 28 , Dec. N. 16 14'. See TAURUS. Algebraic Sum. The connection of a series of quantities with their proper algebraic sign. Thus the algebraic sum of + a, b, and + c is (a 6 + c). The principle may be conveniently applied in nautical astro- nomy. Thus, if we call the declination of objects situated in the northern celestial hemisphere positive, and that of objects in the southern negative, then these may often be connected with other quantities without making two cases of the problem. Algenib. The Arabic name for the bright star y Pegasi. See PEGASUS. Algol. The Arabic name for the bright star /3 Persei ; also known as " " /3 Medusae. It is remarkable as being a variable star, changing from d h the second or third magnitude to the fifth in the period of 2 20 50. See PERSEUS. Alioth. The Arabic name for the bright star Capella, a Aurigce. See AURIGA. Almucantars or Almicanthers. The Arabic term for Parallels of Alti- tude. These parallels were conceived to be drawn through every degree of the meridian. Obsolete. Alphard. The Arabic name for the bright star a Hydras. See HYDRA. Alpheratz. The Arabic name for the bright star a Andromedce. See ANDROMEDA. Altair. The Arabic name for the bright star a A quilce. See AQUILA. Altitude of a Celestial Body (L. altitude, height). The angular dis- tance of the body from the horizon. It is measured by the arc of a circle of azimuth (which is hence generally called a "circle of altitude") passing through the place of the body, or by the corresponding angle at the centre of the sphere. The term altitude may be considered to apply not only to elevation above the horizon, but also to depression below it, and in that case it is reckoned from the horizon, from to 90 to the zenith to The positive ( + ), and the nadir negative ( ). complement of the alti- tude is the zenith distance. Azimuth and altitude are the horizon co- ordinates for describing the points of the celestial concave relatively to the position of an observer on the earth's surface. See AZIMUTH and ALTITUDE. Altitude of a Terrestrial Object above the Sea Horizon. The angle included between two lines drawn from the eye of the observer, the one to the horizon, the other to the object.