CHAPTER 16INSTRUMENTSFORCELESTIALNAVIGATIONTHE MARINE SEXTANTThe index mirror of the sextant is at B,the horizon glass at C1600.DescriptionAndUseand theeye of theobserver atD.Construction lines EFandCF areperpendicular to the index mirror and horizon glass,Themarine sextantmeasures the anglebetween tworespectively.Lines BG and CG are parallel to these mirrorspoints by bringing the direct rayfrom one point and a dou-Therefore, angles BFC and BGC are equal because theirble-reflected ray from theother into coincidence.Itssides are mutually perpendicular.Angle BGC is the inclina-principal use is to measure the altitudes of celestial bodiestion of thetwo reflecting surfaces.The ray of light AB isabovethevisible seahorizon.Itmayalsobeusedtomeasurereflected at mirror B,proceeds to mirror C,where it is againvertical angles tofind therangefrom an object of knownreflectedandthencontinuesontotheeveoftheobserveratheight. Sometimes it is turned on its side and used for mea-D.Sincethe angle of reflection is equal to the angle ofsuring the angulardistance between twoterrestrial objects.incidence,A marine sextant can measure angles up to approxi-mately120°.Originally,theterm“sextant"was applied toABE =EBC,andABC =2EBCthe navigator's double-reflecting, altitude-measuring in-BCF = FCD, and BCD = 2BCF.strument only if its arc was 60°in length, or 1/6ofa circle,permittingmeasurementofanglesfrom0°to120o.Inmod-Since an exterior angle of a triangle equals the sum ofern usagethe term is applied to all modern navigationalthe two non adjacent interior angles,altitude-measuring instruments regardless ofangular rangeABC=BDC+BCD,andEBC=BFC+BCForprinciplesofoperation.Transposing,BDC = ABC-BCD, and BFC = EBC-BCF1601.Optical Principles Of A SextantSubstituting 2EBC for ABC, and 2BCF for BCD in theWhenaplanesurfacereflects alightray,the angle ofrefirst of these equations,flection equals the angle of incidence.The angle between theBDC=2EBC-2BCF,0rBDC=2(EBC-BCF)first and final directions of a rayof light that has undergonedouble reflection in the same plane is twice the angle the twoSinceBFC=EBC-BCF,andBFC=BGC,thereforereflecting surfacesmake with each other (Figure 1601).InFigure1601,AB isarayoflightfromacelestialbodyBDC=2BFC=2BGC.That is.BDC,theanglebetween thefirstand last direc-Ations of the ray of light,is equal to 2BGC,twice the angleof inclination of the reflecting surfaces.AngleBDC is thealtitude ofthecelestialbodyIf the two mirrors are parallel, the incident ray from anyobserved bodymustbeparalleltothe observer's lineofsightthrough the horizon glass. In that case, the body's altitudewould bezero.The anglethatthesetworeflectingsurfacesmakewitheachotherisone-halftheobserved angle.Thegraduations on the arc reflect this half anglerelationship be-tween the angle observed and themirrorsangle1602.MicrometerDrum SextantGFigure1602 shows a modern marinesextant,called amicrometer drum sextant. In most marine sextants, brassFigure1601.Opticalprincipleofthemarinesextantor aluminum comprise theframe,A.Frames come in vari-273
273 CHAPTER 16 INSTRUMENTS FOR CELESTIAL NAVIGATION THE MARINE SEXTANT 1600. Description And Use The marine sextant measures the angle between two points by bringing the direct ray from one point and a double-reflected ray from the other into coincidence. Its principal use is to measure the altitudes of celestial bodies above the visible sea horizon. It may also be used to measure vertical angles to find the range from an object of known height. Sometimes it is turned on its side and used for measuring the angular distance between two terrestrial objects. A marine sextant can measure angles up to approximately 120°. Originally, the term “sextant” was applied to the navigator’s double-reflecting, altitude-measuring instrument only if its arc was 60° in length, or 1/6 of a circle, permitting measurement of angles from 0° to 120°. In modern usage the term is applied to all modern navigational altitude-measuring instruments regardless of angular range or principles of operation. 1601. Optical Principles Of A Sextant When a plane surface reflects a light ray, the angle of reflection equals the angle of incidence. The angle between the first and final directions of a ray of light that has undergone double reflection in the same plane is twice the angle the two reflecting surfaces make with each other (Figure 1601). In Figure 1601, AB is a ray of light from a celestial body. The index mirror of the sextant is at B, the horizon glass at C, and the eye of the observer at D. Construction lines EF and CF are perpendicular to the index mirror and horizon glass, respectively. Lines BG and CG are parallel to these mirrors. Therefore, angles BFC and BGC are equal because their sides are mutually perpendicular. Angle BGC is the inclination of the two reflecting surfaces. The ray of light AB is reflected at mirror B, proceeds to mirror C, where it is again reflected, and then continues on to the eye of the observer at D. Since the angle of reflection is equal to the angle of incidence, Since an exterior angle of a triangle equals the sum of the two non adjacent interior angles, ABC = BDC+BCD, and EBC = BFC+BCF. Transposing, BDC = ABC-BCD, and BFC = EBC-BCF. Substituting 2EBC for ABC, and 2BCF for BCD in the first of these equations, BDC = 2EBC-2BCF, or BDC=2 (EBC-BCF). Since BFC=EBC - BCF, and BFC = BGC, therefore BDC = 2BFC = 2BGC. That is, BDC, the angle between the first and last directions of the ray of light, is equal to 2BGC, twice the angle of inclination of the reflecting surfaces. Angle BDC is the altitude of the celestial body. If the two mirrors are parallel, the incident ray from any observed body must be parallel to the observer’s line of sight through the horizon glass. In that case, the body’s altitude would be zero. The angle that these two reflecting surfaces make with each other is one-half the observed angle. The graduations on the arc reflect this half angle relationship between the angle observed and the mirrors’ angle. 1602. Micrometer Drum Sextant Figure 1602 shows a modern marine sextant, called a micrometer drum sextant. In most marine sextants, brass Figure 1601. Optical principle of the marine sextant. or aluminum comprise the frame, A. Frames come in variABE = EBC, and ABC = 2EBC. BCF = FCD, and BCD = 2BCF
274INSTRUMENTSFORCELESTIALNAVIGATIONous designs,mostaresimilartothis.TeethmarktheouterItismountedontheframe,perpendiculartotheplaneoftheedge of the limb,B;each tooth marks one degree of alti-sextant.The index mirror and horizon glass are mounted sotude.Thealtitudegraduations.C.alongthelimb.markthethat their surfaces are parallel when the micrometer drum isarc.Some sextants have an arcmarked in a strip of brassset at oo, if the instrument is in perfect adjustment.Shadesilver, or platinum inlaid in the limbglasses, K, of varying darkness are mounted on the sexTheindexarm,D.isamovablebarofthesamematerialtant's frame in front of the index mirror and horizon glassas theframe.It pivots about the center of curvature of theThey can be moved into theline of sight as needed to reducelimb.The tangent screw,E, is mounted perpendicularly onthe intensity of light reaching the eye.theendoftheindexarm,whereitengagestheteethoftheThe telescope,L, screws into an adjustable collar inlimb.Because theobserver can move the index arm throughline withthe horizon glass and parallel to theplane of thethe length of the arc by rotating the tangent screw, this isinstrument.Most modern sextants areprovided with onlysometimes called an“endlesstangent screw."Contrastthisonetelescope.When onlyonetelescopeisprovided,itisofwith the limited-range device on older instruments. The re-the“erect imagetype,"either as shown or witha wider“ob-lease, F,is a spring-actuated clamp thatkeeps the tangentiectglass"(farendoftelescope),whichgenerallyisshorterscrew engaged with the limb'steeth.The observer can disen-in lengthand gives a greater field of view.Thesecondtele-gage thetangent screw and movethe index arm alongthescope, if provided, may be the“inverting type."Thelimbfor roughadjustment.The end of thetangent screwinvertingtelescope,having onelens less than the erecttypemounts a micrometer drum, G, graduated in minutes of al-absorbsless light, butat theexpense of producing an invert-titude.Onecompleteturnofthedrummovestheindexarmed image.Asmall coloredglasscapissometimesprovided.onedegreealongthearc.Nexttothemicrometerdrumandtobe placed over the“eyepiece"(near end oftelescope)tofixedontheindexarmisavernier,H,thatreadsinfractionsreduceglare.With this inplace, shade glasses are generallyof a minute.The vernier shown isgraduated into tenpartsnot needed.A"peep sight,or clear tube which servesto di-permitting readings to /1o of a minute of arc (0.1).Somerect the lineof sight of the observerwhen no telescope issextants(generallyof Europeanmanufacture)haveverniersused, maybefittedgraduated into only five parts, permitting readings to 0.2'Sextants are designed to be held in the right hand.The index mirror, I, is a piece of silvered plate glassSome have a small light on the index arm to assist in read-mountedontheindexarm,perpendiculartotheplaneoftheing altitudes.The batteries for this light are fitted inside ainstrument,withthe center of the reflecting surfacedirectlyrecess in the handle, M. Not clearly shown in Figure 1602over the pivot of the index arm.The horizon glass, J, is apiece of optical glass silvered on its half nearer the frame.are the tangent screw, E, and the three legs.Figure 1602.U.S. Navy Mark 2 micrometer drum sextant
274 INSTRUMENTS FOR CELESTIAL NAVIGATION ous designs; most are similar to this. Teeth mark the outer edge of the limb, B; each tooth marks one degree of altitude. The altitude graduations, C, along the limb, mark the arc. Some sextants have an arc marked in a strip of brass, silver, or platinum inlaid in the limb. The index arm, D, is a movable bar of the same material as the frame. It pivots about the center of curvature of the limb. The tangent screw, E, is mounted perpendicularly on the end of the index arm, where it engages the teeth of the limb. Because the observer can move the index arm through the length of the arc by rotating the tangent screw, this is sometimes called an “endless tangent screw.” Contrast this with the limited-range device on older instruments. The release, F, is a spring-actuated clamp that keeps the tangent screw engaged with the limb’s teeth. The observer can disengage the tangent screw and move the index arm along the limb for rough adjustment. The end of the tangent screw mounts a micrometer drum, G, graduated in minutes of altitude. One complete turn of the drum moves the index arm one degree along the arc. Next to the micrometer drum and fixed on the index arm is a vernier, H, that reads in fractions of a minute. The vernier shown is graduated into ten parts, permitting readings to 1/10 of a minute of arc (0.1'). Some sextants (generally of European manufacture) have verniers graduated into only five parts, permitting readings to 0.2'. The index mirror, I, is a piece of silvered plate glass mounted on the index arm, perpendicular to the plane of the instrument, with the center of the reflecting surface directly over the pivot of the index arm. The horizon glass, J, is a piece of optical glass silvered on its half nearer the frame. It is mounted on the frame, perpendicular to the plane of the sextant. The index mirror and horizon glass are mounted so that their surfaces are parallel when the micrometer drum is set at 0°, if the instrument is in perfect adjustment. Shade glasses, K, of varying darkness are mounted on the sextant’s frame in front of the index mirror and horizon glass. They can be moved into the line of sight as needed to reduce the intensity of light reaching the eye. The telescope, L, screws into an adjustable collar in line with the horizon glass and parallel to the plane of the instrument. Most modern sextants are provided with only one telescope. When only one telescope is provided, it is of the “erect image type,” either as shown or with a wider “object glass” (far end of telescope), which generally is shorter in length and gives a greater field of view. The second telescope, if provided, may be the “inverting type.” The inverting telescope, having one lens less than the erect type, absorbs less light, but at the expense of producing an inverted image. A small colored glass cap is sometimes provided, to be placed over the “eyepiece” (near end of telescope) to reduce glare. With this in place, shade glasses are generally not needed. A “peep sight,” or clear tube which serves to direct the line of sight of the observer when no telescope is used, may be fitted. Sextants are designed to be held in the right hand. Some have a small light on the index arm to assist in reading altitudes. The batteries for this light are fitted inside a recess in the handle, M. Not clearly shown in Figure 1602 are the tangent screw, E, and the three legs. Figure 1602. U.S. Navy Mark 2 micrometer drum sextant
275INSTRUMENTSFORCELESTIALNAVIGATIONThereare two basicdesigns commonlyused for mountingberesting exactly on the horizon,tangentto the lowerlimband adjusting mirrors on marine sextants.On the U.S.NavyThe noviceobserverneeds practicetodetermine the exactMark3andcertainothersextants.themirrorismountedsothatpoint of tangency.Beginners often err by bringing the imit can bemoved againstretaining or mounting springs withinagedown toofar.its frame. Only one perpendicular adjustment screw is re-Some navigatorsgettheirmost accurate observationsquired.On theU.S.Navy Mark2and other sextants the mirrorby letting the body contact the horizon by its own motion,isfixedwithin itsframe.Twoperpendicularadjustmentscrewsbringing it slightlybelowthehorizon ifrising,andaboveifarerequired.One screwmustbe loosened beforetheothersetting.At the instant the horizon is tangent to the disk, thescrew bearingon the same surface is tightened.navigator notes the time. The sextant altitude is the uncor-rected readingofthesextant.1603.Vernier Sextant1605.SextantMo0nSightsMost recent marine sextants areof the micrometerWhen observing the moon, follow the same proceduredrum type, but at least two older-type sextants are still inas for the sun.Because ofthe phases of the moon, the upperuse.Thesedifferfromthemicrometerdrumsextantprinci-pally in the manner in which the final reading is made.Theylimb of themoon is observed moreoften than that of theare called vernier sextants.sun.When the terminator (the line between light and darkareas)is nearlyvertical, be careful in selecting the limb toThe clamp screw vernier sextant is the older of theshoot. Sights of themoon are best made during either day-two.Inplaceofthemodernreleaseclamp,aclampscrew islight hours or that part oftwilight in which themoon isleastfitted on the underside of the index arm.To move the indexluminous. At night, false horizons may appear below thearm,theclamp screwis loosened,releasingthearm.Whenmoon because the moon illuminates the waterbelow it.the armis placed at theapproximate altitude ofthebody be-ing observed, the clamp screw istightened.Fixed to theclamp screwand engaged withthe indexarm is alongtan-1606.SextantStarAndPlanetSightsgent screw.When this screw is turned,the index arm movesslowly,permitting accurate setting.Movement of the indexUse one of these three methods when making the initialarmbythetangentscrew is limitedtothelengthofthescrewaltitude approximation on a star or planet:(severaldegreesofarc).Beforeanaltitudeismeasured.thisscrew should be settotheapproximatemid-point of itsMethod 1. Set the index arm and micrometer drum onrange.Thefinal reading ismadeonavernierset intheindexO° and direct the line of sight at the body to be observedarmbelowthearc.A small microscopeor magnifyingglassThen,whilekeeping thereflected image ofthe bodyinthefitted to the index arm is used in making the final reading.mirrored halfof the horizon glass, swing the index arm outThe endless tangent screwvernier sextant is identical tcandrotatetheframeofthesextantdown.Keepthereflectedthe micrometer drum sextant, except that it has no drum, andimageof thebodyin themirror untilthehorizon appears inthefine reading is madeby a vernier along the arc, as with th-theclearpartofthehorizonglass.Then,maketheobservaeclampscrewvernier sextant.Therelease is the same as on thetion.When there is little contrast between brightness of themicrometerdrumsextantandteetharecutintotheundersidesky and the body,this procedure is difficult. If the body is"lost"while it is being broughtdown, itmay not be recov-ofthe limbwhichengagewiththe endlesstangentscrewered without starting over again.1604.Sextant Sun SightsMethod 2.Direct the line of sight at thebody whileholding the sextant upsidedown.Slowlymovethe index-Hold the sextant vertically and direct the sight line at thearm out until the horizon appears in the horizon glass. Theninvert the sextant and take the sight in the usual manner.horizondirectlybelowthe sun.After moving suitable shadeglasses into the line of sight,move the index arm outwardMethod 3.Determine in advance the approximate alti-along the arc until the reflected image appears in the horizontude and azimuth of the body by a starfinder suchas Noglassnearthedirectviewofthehorizon.Rockthesextant2102D.Set the sextant at the indicated altitude and face inslightly to the right and left to ensure it is perpendicular. As thethe direction of the azimuth.The image of the body shouldobserver rocks the sextant,the image ofthe sun appears toappear in the horizon glass with a little searching.move in an arc,and theobservermayhaveto turn slightlytoWhen measuring the altitudeof a star or planet, bringpreventthe imagefrommovingoffthehorizonglassits center down to the horizon. Stars and planets have noThe sextant is vertical when the sun appears at the bot-discernible upper or lower limb;observethecenter of thetom of the arc. This is the correct position for making thepointoflight.Because stars and planets havenodiscernibleobservation.The sun's reflected image appears at the centerlimbandbecausetheirvisibilitymaybe limited,themethodof thehorizonglass,onehalf appears on thesilvered part.of letting a star orplanet intersect thehorizon by its ownand the other half appears on the clear part. Move the indexmotion is notrecommended.As withthe sunand moon,armwiththedrumorvernierslowlyuntilthesunappearstohowever,rock the sextant"to establish perpendicularity
INSTRUMENTS FOR CELESTIAL NAVIGATION 275 There are two basic designs commonly used for mounting and adjusting mirrors on marine sextants. On the U.S. Navy Mark 3 and certain other sextants, the mirror is mounted so that it can be moved against retaining or mounting springs within its frame. Only one perpendicular adjustment screw is required. On the U.S. Navy Mark 2 and other sextants the mirror is fixed within its frame. Two perpendicular adjustment screws are required. One screw must be loosened before the other screw bearing on the same surface is tightened. 1603. Vernier Sextant Most recent marine sextants are of the micrometer drum type, but at least two older-type sextants are still in use. These differ from the micrometer drum sextant principally in the manner in which the final reading is made. They are called vernier sextants. The clamp screw vernier sextant is the older of the two. In place of the modern release clamp, a clamp screw is fitted on the underside of the index arm. To move the index arm, the clamp screw is loosened, releasing the arm. When the arm is placed at the approximate altitude of the body being observed, the clamp screw is tightened. Fixed to the clamp screw and engaged with the index arm is a long tangent screw. When this screw is turned, the index arm moves slowly, permitting accurate setting. Movement of the index arm by the tangent screw is limited to the length of the screw (several degrees of arc). Before an altitude is measured, this screw should be set to the approximate mid-point of its range. The final reading is made on a vernier set in the index arm below the arc. A small microscope or magnifying glass fitted to the index arm is used in making the final reading. The endless tangent screw vernier sextant is identical to the micrometer drum sextant, except that it has no drum, and the fine reading is made by a vernier along the arc, as with theclamp screw vernier sextant. The release is the same as on the micrometer drum sextant, and teeth are cut into the underside of the limb which engage with the endless tangent screw. 1604. Sextant Sun Sights Hold the sextant vertically and direct the sight line at the horizon directly below the sun. After moving suitable shade glasses into the line of sight, move the index arm outward along the arc until the reflected image appears in the horizon glass near the direct view of the horizon. Rock the sextant slightly to the right and left to ensure it is perpendicular. As the observer rocks the sextant, the image of the sun appears to move in an arc, and the observer may have to turn slightly to prevent the image from moving off the horizon glass. The sextant is vertical when the sun appears at the bottom of the arc. This is the correct position for making the observation. The sun’s reflected image appears at the center of the horizon glass; one half appears on the silvered part, and the other half appears on the clear part. Move the index arm with the drum or vernier slowly until the sun appears to be resting exactly on the horizon, tangent to the lower limb. The novice observer needs practice to determine the exact point of tangency. Beginners often err by bringing the image down too far. Some navigators get their most accurate observations by letting the body contact the horizon by its own motion, bringing it slightly below the horizon if rising, and above if setting. At the instant the horizon is tangent to the disk, the navigator notes the time. The sextant altitude is the uncorrected reading of the sextant. 1605. Sextant Moon Sights When observing the moon, follow the same procedure as for the sun. Because of the phases of the moon, the upper limb of the moon is observed more often than that of the sun. When the terminator (the line between light and dark areas) is nearly vertical, be careful in selecting the limb to shoot. Sights of the moon are best made during either daylight hours or that part of twilight in which the moon is least luminous. At night, false horizons may appear below the moon because the moon illuminates the water below it. 1606. Sextant Star And Planet Sights Use one of these three methods when making the initial altitude approximation on a star or planet: Method 1. Set the index arm and micrometer drum on 0° and direct the line of sight at the body to be observed. Then, while keeping the reflected image of the body in the mirrored half of the horizon glass, swing the index arm out and rotate the frame of the sextant down. Keep the reflected image of the body in the mirror until the horizon appears in the clear part of the horizon glass. Then, make the observation. When there is little contrast between brightness of the sky and the body, this procedure is difficult. If the body is “lost” while it is being brought down, it may not be recovered without starting over again. Method 2. Direct the line of sight at the body while holding the sextant upside down. Slowly move the indexarm out until the horizon appears in the horizon glass. Then invert the sextant and take the sight in the usual manner. Method 3. Determine in advance the approximate altitude and azimuth of the body by a star finder such as No. 2102D. Set the sextant at the indicated altitude and face in the direction of the azimuth. The image of the body should appear in the horizon glass with a little searching. When measuring the altitude of a star or planet, bring its center down to the horizon. Stars and planets have no discernible upper or lower limb; observe the center of the point of light. Because stars and planets have no discernible limb and because their visibility may be limited, the method of letting a star or planet intersect the horizon by its own motion is not recommended. As with the sun and moon, however, “rock the sextant” to establish perpendicularity
276INSTRUMENTSFORCELESTIALNAVIGATION1607.TakingASightthenavigator,and a star or planet is more easilyobservedwhen the sky is relatively bright. Near the darker limit ofPredict expected altitudes and azimuths for up to eighttwilight, thetelescope can be moved out, giving a broaderbodies when preparing totake celestial sights.Choose theviewof the clearhalfof theglass,andmakingthelessdisstars and planets that give the best bearing spread. Try to se-tinct horizon more easily discernible. If both eyes are keptlect bodies with a predicted altitude between 30°and 70°open until the last moments of an observation, eye strainTake sights of the brightest stars first in the evening; takewill belessened.Practicewill permit observationsto besights ofthe brightest stars last in the morning.made quickly,reducing inaccuracy due to eye fatigue.When measuring an altitude,have anassistant note andOccasionally,fog.haze,orothershipsinaformationmayobscurethehorizon directlybelowa bodywhichtherecord thetimeifpossible,witha“stand-by"warningwhennavigator wishes to observe.If the arc of the sextant is suf-themeasurementisalmostready,andamark"atthemo-ficiently long,a back sight might be obtained, using thement a sight is made. If a flashlight is needed to see theoppositepointof the horizonas thereference.Forthis thecomparing watch,the assistant should be careful not to in-observerfaces awayfromthebody and observesthesup-terfere with the navigators night vision.plement of the altitude.If the sun ormoon is observed inIf an assistant is notavailable totime the observations,thethis manner, what appears in the horizon glass to be theobserverholdsthewatchinthepalmofhislefthandleavinghislowerlimb isinfacttheupper limb,and viceversa.Inthefingersfreetomanipulatethetangentscrewofthesextant.Aftercase of the sun, it is usually preferableto observe whatapmaking theobservation,henotes the timeas quicklyas possibleThedelaybetween completingthealtitudeobservation and not-pears to be the upper limb.Thearc that appears whenrockingthesextantforabacksightisinverted:thatis,theing the timeshouldnotbemore than one or two secondshighestpoint indicates theposition of perpendicularityIf morethanonetelescopeisfurnishedwiththe sex1608.ReadingTheSextanttant,theerectingtelescopeisusedtoobservethe sun.Awiderfield of view ispresent if thetelescope is not used.Reading a micrometer drum sextant is done in threeThe collar into whichthe sextant telescopefitsmay bead-steps.The degrees areread bynoting the position ofthe ar-justed in or out, in relation to the frame. When moved in.row on the index arm in relation to the arc.The minutes aremore ofthemirrored half of thehorizonglass is visibletoreadbynotingthepositionofthezero on thevernierwith80whmmO60302055HO505Figure1608a.Micrometer drumsextant setat29o42.5
276 INSTRUMENTS FOR CELESTIAL NAVIGATION 1607. Taking A Sight Predict expected altitudes and azimuths for up to eight bodies when preparing to take celestial sights. Choose the stars and planets that give the best bearing spread. Try to select bodies with a predicted altitude between 30° and 70°. Take sights of the brightest stars first in the evening; take sights of the brightest stars last in the morning. Occasionally, fog, haze, or other ships in a formation may obscure the horizon directly below a body which the navigator wishes to observe. If the arc of the sextant is sufficiently long, a back sight might be obtained, using the opposite point of the horizon as the reference. For this the observer faces away from the body and observes the supplement of the altitude. If the sun or moon is observed in this manner, what appears in the horizon glass to be the lower limb is in fact the upper limb, and vice versa. In the case of the sun, it is usually preferable to observe what appears to be the upper limb. The arc that appears when rocking the sextant for a back sight is inverted; that is, the highest point indicates the position of perpendicularity. If more than one telescope is furnished with the sextant, the erecting telescope is used to observe the sun. A wider field of view is present if the telescope is not used. The collar into which the sextant telescope fits may be adjusted in or out, in relation to the frame. When moved in, more of the mirrored half of the horizon glass is visible to the navigator, and a star or planet is more easily observed when the sky is relatively bright. Near the darker limit of twilight, the telescope can be moved out, giving a broader view of the clear half of the glass, and making the less distinct horizon more easily discernible. If both eyes are kept open until the last moments of an observation, eye strain will be lessened. Practice will permit observations to be made quickly, reducing inaccuracy due to eye fatigue. When measuring an altitude, have an assistant note and record the time if possible, with a “stand-by” warning when the measurement is almost ready, and a “mark” at the moment a sight is made. If a flashlight is needed to see the comparing watch, the assistant should be careful not to interfere with the navigator’s night vision. If an assistant is not available to time the observations, the observer holds the watch in the palm of his left hand, leaving his fingers free to manipulate the tangent screw of the sextant. After making the observation, he notes the time as quickly as possible. The delay between completing the altitude observation and noting the time should not be more than one or two seconds. 1608. Reading The Sextant Reading a micrometer drum sextant is done in three steps. The degrees are read by noting the position of the arrow on the index arm in relation to the arc. The minutes are read by noting the position of the zero on the vernier with Figure 1608a. Micrometer drum sextant set at 29° 42.5
277INSTRUMENTSFORCELESTIALNAVIGATION5.Figure 1608b.Vernier sextant set at 29°4230"relationto thegraduations onthemicrometer drum.Thethe index arm and has the small reference mark as its zerofraction ofa minuteis read by noting which mark onthegraduation.On thisvernier,40 graduations coincide with 39vernier most nearly coincides with one of the graduationsgraduations on thearc.Eachgraduation onthe vemnier isequiv-onthemicrometerdrum.Thisissimilartoreadingthetimealentto1/40ofonegraduationof20'on thearc,or0.5,or30"with the hour,minute,and second hands ofa watch.Inboth,In theillustration, theverniergraduation representing 2 1/2the relationship of one part of thereading tothe others(2'3o")mostnearlycoincideswithoneofthegraduationsonthearc.Thereore, thereading is29°42'30",or 29°42.5,asbeshould bekept in mind.Thus, if thehour hand of a watchwere about on“4,"one wouldknowthat the time was aboutfore.When a vernier ofthistype is used,anydoubt asto whichfour o'clock.But if theminutehandwereon"58"onemarkontheverniercoincideswithagraduationonthearccanwouldknowthatthetimewas0358(or1558),not0458(orusuallyberesolvedbynotingthepositionof theverniermark1658).Similarly, ifthe arc indicated a reading ofabout 400oneachsideoftheonethatseemstobeincoincidenceand58'onthemicrometerdrumwereoppositezeroontheNegative readings,suchasa negative index correctionvernier,onewouldknowthatthereadingwas 39o58',notare made in the same manneras positive readings,thevar-40°58.Similarly,anydoubt astothecorrectminutecanbeious figures are added algebraically.Thus, ifthe three partsremoved by noting thefraction of a minutefrom theposiof amicrometer drumreading are(-)1°,56'and0.3,thetion of the vernier.In Figure1608a the reading is 29042.5total reading is(-)1°+ 56'+0.3'=(-)3.7.Thearrowontheindexmarkisbetween29and30the1609.DevelopingObservationalSkillzeroonthevernier isbetween42'and43',and the0.5grad-uation on the vernier coincides with one of thegraduationsonthemicrometerdrumAwell-constructedmarinesextantiscapableofmeasurThe principle of reading a vernier sextant is the same, buting angles with an instrument error not exceeding O.I'.Linesthe reading is made in two steps.Figure 1608b shows a typicalof positionfromaltitudesofthisaccuracywouldnotbeiner-altitude setting.Each degree on the arc of this sextant is grad-ror by more than about 200 yards.However,there arevariousuated into three parts, permitting an initial reading by thesources of error, other than instrumental, in altitudes mea-referencemarkontheindexarmtothenearest20'ofarc.Inthissured by sextant.One of the principal sources is the observer.illustration thereferencemarkliesbetween29°40and30°00The first fix a student celestial navigator plots is likelyindicating a reading between these values.Thereading for theto bedisappointing.Mostnavigators require agreatamountfraction of20'ismade using the vernier,which is engravedonof practicetodeveloptheskill necessaryforgoodobserva-
INSTRUMENTS FOR CELESTIAL NAVIGATION 277 relation to the graduations on the micrometer drum. The fraction of a minute is read by noting which mark on the vernier most nearly coincides with one of the graduations on the micrometer drum. This is similar to reading the time with the hour, minute, and second hands of a watch. In both, the relationship of one part of the reading to the others should be kept in mind. Thus, if the hour hand of a watch were about on “4,” one would know that the time was about four o’clock. But if the minute hand were on “58,” one would know that the time was 0358 (or 1558), not 0458 (or 1658). Similarly, if the arc indicated a reading of about 40°, and 58' on the micrometer drum were opposite zero on the vernier, one would know that the reading was 39° 58', not 40°58'. Similarly, any doubt as to the correct minute can be removed by noting the fraction of a minute from the position of the vernier. In Figure 1608a the reading is 29° 42.5'. The arrow on the index mark is between 29° and 30°, the zero on the vernier is between 42' and 43', and the 0.5' graduation on the vernier coincides with one of the graduations on the micrometer drum. The principle of reading a vernier sextant is the same, but the reading is made in two steps. Figure 1608b shows a typical altitude setting. Each degree on the arc of this sextant is graduated into three parts, permitting an initial reading by the reference mark on the index arm to the nearest 20' of arc. In this illustration the reference mark lies between 29°40' and 30°00', indicating a reading between these values. The reading for the fraction of 20' is made using the vernier, which is engraved on the index arm and has the small reference mark as its zero graduation. On this vernier, 40 graduations coincide with 39 graduations on the arc. Each graduation on the vernier is equivalent to 1/40 of one graduation of 20' on the arc, or 0.5', or 30". In the illustration, the vernier graduation representing 2 1/2' (2'30") most nearly coincides with one of the graduations on the arc. Therefore, the reading is 29°42'30", or 29°42.5', as before. When a vernier of this type is used, any doubt as to which mark on the vernier coincides with a graduation on the arc can usually be resolved by noting the position of the vernier mark on each side of the one that seems to be in coincidence. Negative readings, such as a negative index correction, are made in the same manner as positive readings; the various figures are added algebraically. Thus, if the three parts of a micrometer drum reading are ( - )1°, 56' and 0.3', the total reading is ( - )1° + 56' + 0.3' = ( - )3.7'. 1609. Developing Observational Skill A well-constructed marine sextant is capable of measuring angles with an instrument error not exceeding 0.1'. Lines of position from altitudes of this accuracy would not be in error by more than about 200 yards. However, there are various sources of error, other than instrumental, in altitudes measured by sextant. One of the principal sources is the observer. The first fix a student celestial navigator plots is likely to be disappointing. Most navigators require a great amount of practice to develop the skill necessary for good observaFigure 1608b. Vernier sextant set at 29°42'30