CHAPTER 18TIMETIMEINNAVIGATION1800.SolarTimemoved to point C in its orbit. Thus, duringthe courseofa daythe sun appears to move eastward withrespect to the stars.The earth'srotation on its axis causes the sun and otherThe apparent positions of the stars are commonly reck-celestialbodies toappeartomoveacross the skyfromeastoned with reference to an imaginary point called the vernalto west each day.If a person located on the earth's equatorequinox,the intersection ofthe celestialequator and theeclip-measured thetime interval between two successivetransitstic.Theperiod oftheearth'srotationmeasured with respecttooverhead ofa very distant star, he wouldbe measuringthethe vernal equinox is called a sidereal day. The period withperiod ofthe earth'srotation.Ifhe then made a similarmea-respect to the sun is called an apparent solar day.surement of the sun,the resultingtimewould be about4When measuringtimebytheearth's rotation,using theminutes longer.This is due to the earth's motion around theactual position ofthe sun results in apparent solartimesun, which continuously changes the apparent place of theUse of the apparent sun as a time reference results insun among the stars.Thus, during the course of a day thetime of non-constantrate for at least three reasons.First,rev-sun appears to move a littleto the east among thestars soolution of the earth in its orbit is not constant. Second, timethat the earth must rotate on its axis throughmore than 3600is measured along the celestial equator and thepath of thein order to bring the sun overhead againreal sun is not along the celestial equator. Rather, its path isSeeFigure1800.If the sun is on the observer's meridianalong the ecliptic, which is tilted at an angle of23°27 withwhen the earth is at point A in its orbit around the sun, it willrespect to the celestial equator.Third, rotation of the earthnotbe onthe observer'smeridian aftertheearthhas rotatedon itsaxis isnotconstant.through360°becausetheearth will havemoved along its or-Toobtaina constantrateoftime,the apparent sun isre-bit to point B. Before the sun is again on the observer'splaced by a fictitious mean sun. This mean sun movesmeridian, the earth must turn still more on its axis. The sunwill be on the observer's meridian again when the earth haseastwardalongthecelestialequatorata uniform speedequalSEGMENTOFELLPTICALPATHOFEARTHSORBITABOUTSUNCBSERVERSMERIDIANFigure 1800.Apparent eastward movement of the sun with respect to the stars287
287 CHAPTER 18 TIME TIME IN NAVIGATION 1800. Solar Time The earth’s rotation on its axis causes the sun and other celestial bodies to appear to move across the sky from east to west each day. If a person located on the earth’s equator measured the time interval between two successive transits overhead of a very distant star, he would be measuring the period of the earth’s rotation. If he then made a similar measurement of the sun, the resulting time would be about 4 minutes longer. This is due to the earth’s motion around the sun, which continuously changes the apparent place of the sun among the stars. Thus, during the course of a day the sun appears to move a little to the east among the stars so that the earth must rotate on its axis through more than 360° in order to bring the sun overhead again. See Figure 1800. If the sun is on the observer’s meridian when the earth is at point A in its orbit around the sun, it will not be on the observer’s meridian after the earth has rotated through 360° because the earth will have moved along its orbit to point B. Before the sun is again on the observer’s meridian, the earth must turn still more on its axis. The sun will be on the observer’s meridian again when the earth has moved to point C in its orbit. Thus, during the course of a day the sun appears to move eastward with respect to the stars. The apparent positions of the stars are commonly reckoned with reference to an imaginary point called the vernal equinox, the intersection of the celestial equator and the ecliptic. The period of the earth’s rotation measured with respect to the vernal equinox is called a sidereal day. The period with respect to the sun is called an apparent solar day. When measuring time by the earth’s rotation, using the actual position of the sun results in apparent solar time. Use of the apparent sun as a time reference results in time of non-constant rate for at least three reasons. First, revolution of the earth in its orbit is not constant. Second, time is measured along the celestial equator and the path of the real sun is not along the celestial equator. Rather, its path is along the ecliptic, which is tilted at an angle of 23° 27' with respect to the celestial equator. Third, rotation of the earth on its axis is not constant. To obtain a constant rate of time, the apparent sun is replaced by a fictitious mean sun. This mean sun moves eastward along the celestial equator at a uniform speed equal Figure 1800. Apparent eastward movement of the sun with respect to the stars
288TIMEExample2:See Figure 1801.Determine the time of the up-to the average speed of the apparent sun along the ecliptic.permeridianpassageof the sun on April16,1995Thismeansun,thereforeprovidesauniformmeasureoftime which approximates the average apparent time.TheSolution:FromFigure 1801,uppermeridian passagespeed of themean sunalong the celestial equator is15°perofthe sun on April 16, 1995, is given as 1200. The dividinghourofmeansolartimelinebetweenthevaluesforupperand lowermeridianpas-sage on April 16th indicates that the sign of the equation of1801.EquationOfTimetimechangesbetveen lowermeridianpassageanduppermeridian passage on thisdate; the question,therefore,be-comes: does it become positive or negative? Note that onMeansolartime,ormeantimeasitiscommonlyApril18,1995,uppermeridianpassageisgivenas1159,called, is sometimesahead of and sometimes behindappar-ent solartime.This difference,whichnever exceeds aboutindicating thaton April 18,1995,the equation of time ispositive.All valuesfor the equation oftime on the same side16.4minutes, is called the equation of time.ofthedividingline asAprilI8tharepositive.Therefore,theThe navigatormost often deals with theequation of timeequationof timefor uppermeridianpassageof the sunonwhen determining the time of upper meridian passage of theApril 16,1995 is (+)00m05sUpper meridian passagesun.The sun transits theobserver's uppermeridian at local ap-parent noon.Were it not for thedifference in rate between thetherefore, takes placeat 11h59m55smeanandapparent sun,the sun would beon theobserver'sme-ridian when the mean sun indicated 1200 local time.Theapparent solar time ofupper meridian passage, however, is offsetfromexactly1200meansolartime.Thistimedifference,theSUNMOONDayequation of time atmeridian transit is listed on theright handEqn.ofTimeMerMer.Pass.12h00hPass.UppelLowerAgPhasedailypagesoftheNauticalAlmanacThesignof the equation oftime is positiveif thetime000016605of sun's meridianpassage isearlierthan1200 andnegative170020Oif laterthan1200.Therefore14Apparent Time=Mean Time-(equation of time).Figure1801.The equation oftimefor April 16,17,18,1995Example:Determinethe time of the sun's meridianpassage(Local ApparentNoon)on June16,1994.To calculate latitude and longitude at LAN,the navigatorSolution: See Figure 2007in Chapter 20, the Nauticalseldom requires the time of meridian passage to accuraciesAlmanac's right hand daily pagefor June16,1994.Thegreater than one minute.Therefore,use the time listed underequation of time is listed in thebottom righthand cornerofthe"Mer.Pass."column to estimateLANunless extraordinarythe page.There are two ways to solve theproblem,depend-accuracy is required.ing on the accuracy required for the value of meridianpassage.The time of the sun at meridian passage is givento1802.FundamentalSystemsOfTimethe nearest minute in the"Mer.Pass."column.For June16, 1994, this value is 1201.The first fundamental system of time is EphemerisTodetermine the exacttime of meridian passage,useTime (ET).EphemerisTimeis used byastronomers incal-the value given for the equation of time.This value is listedculating the fundamental ephemerides of the sun, moon,immediatelytotheleff of the"Mer.Pass."column ontheand planets.It is notused bynavigators.dailypages.ForJume16,1994,thevalueisgivenas00m37sThe fundamental system of time of most interesttoUsethe"12h"columnbecausetheproblemasked formerid-navigators is Universal Time (UT). UT is the mean solarianpassageatLAN.Thevalueofmeridianpassagefromthetime on the Greenwich meridian, reckoned in days of 24"Mer.Pass."columnindicatesthat meridian passage oc-mean solar hours beginning with Oh at midnight. Universalcurs afer 1200; therefore, add the 37 second correction toTime, in principle, is determined bythe average rate ofthe1200toobtaintheexacttimeofmeridianpassage.Theexactapparent daily motion ofthe sun relative to the meridian oftimeofmeridianpassageforJune16,1994,is12h00m37s.Greenwich, but in practice the numerical measure of Uni-versal Time at any instant is computed from sidereal timeTheequation of time's maximum value approachesUniversal Time is the standard intheapplication ofastron-16m22sinNovember.omy to navigation.Observations of Universal Times areIftheAlmanacliststhetimeofmeridianpassageasmade by observing the times of transit of stars.1200,proceedasfollows.Examinetheequationsoftimelist-The Universal Time determined directly from astro-nomical observations is denoted UTo.Since the earth'sed intheAlmanactofindthe dividing linemarkingwhere theequation of time changes between positiveand negativeval-rotationisnonuniform,correctionsmustbeappliedtoUToues.Examine thetrendofthe values near this dividingline toto obtain a more uniform time.This more uniform time isdetermine the correct signforthe equationoftime.obtainedbycorrectingfortwoknownperiodicmotions
288 TIME to the average speed of the apparent sun along the ecliptic. This mean sun, therefore, provides a uniform measure of time which approximates the average apparent time. The speed of the mean sun along the celestial equator is 15° per hour of mean solar time. 1801. Equation Of Time Mean solar time, or mean time as it is commonly called, is sometimes ahead of and sometimes behind apparent solar time. This difference, which never exceeds about 16.4 minutes, is called the equation of time. The navigator most often deals with the equation of time when determining the time of upper meridian passage of the sun. The sun transits the observer’s upper meridian at local apparent noon. Were it not for the difference in rate between the mean and apparent sun, the sun would be on the observer’s meridian when the mean sun indicated 1200 local time. The apparent solar time of upper meridian passage, however, is offset from exactly 1200 mean solar time. This time difference, the equation of time at meridian transit, is listed on the right hand daily pages of the Nautical Almanac. The sign of the equation of time is positive if the time of sun’s meridian passage is earlier than 1200 and negative if later than 1200. Therefore: Apparent Time = Mean Time – (equation of time). Example 1: Determine the time of the sun’s meridian passage (Local Apparent Noon) on June 16, 1994. Solution: See Figure 2007 in Chapter 20, the Nautical Almanac’s right hand daily page for June 16, 1994. The equation of time is listed in the bottom right hand corner of the page. There are two ways to solve the problem, depending on the accuracy required for the value of meridian passage. The time of the sun at meridian passage is given to the nearest minute in the “Mer. Pass.”column. For June 16, 1994, this value is 1201. To determine the exact time of meridian passage, use the value given for the equation of time. This value is listed immediately to the left of the “Mer. Pass.” column on the daily pages. For June 16, 1994, the value is given as 00m37s. Use the “12h” column because the problem asked for meridian passage at LAN. The value of meridian passage from the “Mer. Pass.” column indicates that meridian passage occurs after 1200; therefore, add the 37 second correction to 1200 to obtain the exact time of meridian passage. The exact time of meridian passage for June 16, 1994, is 12h00m37s. The equation of time’s maximum value approaches 16m22s in November. If the Almanac lists the time of meridian passage as 1200, proceed as follows. Examine the equations of time listed in the Almanac to find the dividing line marking where the equation of time changes between positive and negative values. Examine the trend of the values near this dividing line to determine the correct sign for the equation of time. Example 2: See Figure 1801. Determine the time of the upper meridian passage of the sun on April 16, 1995. Solution: From Figure 1801, upper meridian passage of the sun on April 16, 1995, is given as 1200. The dividing line between the values for upper and lower meridian passage on April 16th indicates that the sign of the equation of time changes between lower meridian passage and upper meridian passage on this date; the question, therefore, becomes: does it become positive or negative? Note that on April 18, 1995, upper meridian passage is given as 1159, indicating that on April 18, 1995, the equation of time is positive. All values for the equation of time on the same side of the dividing line as April 18th are positive. Therefore, the equation of time for upper meridian passage of the sun on April 16, 1995 is (+) 00m05s. Upper meridian passage, therefore, takes place at 11h59m55s. To calculate latitude and longitude at LAN, the navigator seldom requires the time of meridian passage to accuracies greater than one minute. Therefore, use the time listed under the “Mer. Pass.” column to estimate LAN unless extraordinary accuracy is required. 1802. Fundamental Systems Of Time The first fundamental system of time is Ephemeris Time (ET). Ephemeris Time is used by astronomers in calculating the fundamental ephemerides of the sun, moon, and planets. It is not used by navigators. The fundamental system of time of most interest to navigators is Universal Time (UT). UT is the mean solar time on the Greenwich meridian, reckoned in days of 24 mean solar hours beginning with 0h at midnight. Universal Time, in principle, is determined by the average rate of the apparent daily motion of the sun relative to the meridian of Greenwich; but in practice the numerical measure of Universal Time at any instant is computed from sidereal time. Universal Time is the standard in the application of astronomy to navigation. Observations of Universal Times are made by observing the times of transit of stars. The Universal Time determined directly from astronomical observations is denoted UT0. Since the earth’s rotation is nonuniform, corrections must be applied to UT0 to obtain a more uniform time. This more uniform time is obtained by correcting for two known periodic motions. Day SUN MOON Eqn. of Time Mer. Mer. Pass. 00h 12h Pass. Upper Lower Age Phase ms ms hmhmhm d 16 00 02 00 05 12 00 00 26 12 55 16 17 00 13 00 20 12 00 01 25 13 54 17 18 00 27 00 33 11 59 02 25 14 55 18 Figure 1801. The equation of time for April 16, 17, 18, 1995
TIME289One motion, the motion ofthegeographic poles, is the=104m=60°resultof theaxis of rotation continuouslymoving with re-= 15'60s=1mspectto the earth's crust.The corrections for this motion are=1=60"4squitesmall(±15millisecondsforWashington,D.C.).On= 15"1s=0.25'applying the correction to UTo, the result is UT1, which isthe same as Greenwichmean time(GMT)used in celestialnavigation.Therefore anytime interval can be expressed as anThesecondknownperiodicmotion isthevariation inequivalentamount ofrotation,and vice versa. Interconver-the earth's speed ofrotation due to winds, tides, and othersion of these units can be made bythe relationshipsphenomena.As a consequence,theearth suffers an annualindicated above.variation initsspeedofrotation,ofabout±30milliseconds.WhenUT1 is correctedfor themean seasonal variations inTo convert time to arc:theearth'srate ofrotation,the result is UT2Although UT2 was at one time believed to be a uni-1.Multiply thehours by15 to obtain degrees of arc.form time system, it was later determined that there are2.Divide the minutes of time by four to obtainvariations intheearth'srateofrotation,possibly causedbydegrees.random accumulations of matter in the convection coreof3.Multiply the remainder of step 2 by15 to obtainthe earth.Such accumulations would changetheearth'sminutes of arc.momentofinertiaandthusitsrateofrotation4.Divide the seconds of timeby four to obtain min-Thethird fundamental system oftime,Atomic Timeutes ofarc(AT),isbasedontransitionsintheatom.Thebasicprinci-5.Multiplytheremainderby15toobtainsecondsofarcple of the atomic clock is that electromagneticwaves of a6.Add the resulting degrees, minutes, and seconds.particularfrequencyareemittedwhen an atomictransitionoccurs. The frequency of the cesium beam atomic clock isExample 1:Convert 14h2/m39s toarc.9,192,631,770 cycles per second ofEphemeris Time.The advent of atomic clocks having accuracies betterSolution:than 1 part in 10-13 led in 1961 to the coordination of timeandfrequencyemissionsoftheU.S.Naval Observatoryand=210°00'00"(1)14h×15theRoyal GreenwichObservatory.Themaster oscillators(2)=005°0000"(remainder1)21m+4controllingthe signals werecalibrated in terms ofthe cesium(3)1 × 15=000°1500"standard,and corrections determined at the U.S.Naval Ob-=000°09'00"(remainder3)(4)395+4servatory and the Royal Greenwich Observatorywere made(5)3×15=000°00'45"simultaneouslyat all transmitting stations.The result is Co-ordinated Universal Time (UTC)(6)14h21m39s=215°24'45"1803.TimeAnd ArcTo convert arc to time:One day represents one complete rotation of the earth.Each day is divided into 24 hours of 60 minutes; each1.Dividethe degreesby15to obtain hours.2Multiply the remainder from step 1 by four to ob-minutehas60seconds.Time of dayis an indication of the phase ofrotation oftain minutes of time.3.Divide the minutes of arc by 15 to obtain minutesthe earth.That is, it indicates how much of a day has elapsed,oftime.orwhatpartofarotationhasbeencompleted.Thus,atzerohours the day begins. One hour later, the earth has turned4. Multiply the remainder from step 3 by four to ob-through1/24ofaday,or1/24of360°,or360°+24=15tainseconds oftimeSmallerintervalscanalso bestatedinangularunits5.Divide the seconds of arc by15to obtain secondssince1hour or60minutes is equivalentto15,1minuteofoftime.time is equivalent to 15° + 60 = 0.25°= 15', and 1 second6.Addthe resultinghours,minutes,and secondsoftimeisequivalentto15'+60=0.25'=15"Example2:Convert215°24'45"totimeunitsSummarizingintableform:Solution:TimeArc215°+ 15(1)=14h00m00sremainder 51d=24h=360°(2)5×4=00h20m00s=]h=15°60m24'+15(3)=00ho1m00sremainder9
TIME 289 One motion, the motion of the geographic poles, is the result of the axis of rotation continuously moving with respect to the earth’s crust. The corrections for this motion are quite small (± 15 milliseconds for Washington, D.C.). On applying the correction to UT0, the result is UT1, which is the same as Greenwich mean time (GMT) used in celestial navigation. The second known periodic motion is the variation in the earth’s speed of rotation due to winds, tides, and other phenomena. As a consequence, the earth suffers an annual variation in its speed of rotation, of about ± 30 milliseconds. When UT1 is corrected for the mean seasonal variations in the earth’s rate of rotation, the result is UT2. Although UT2 was at one time believed to be a uniform time system, it was later determined that there are variations in the earth’s rate of rotation, possibly caused by random accumulations of matter in the convection core of the earth. Such accumulations would change the earth’s moment of inertia and thus its rate of rotation. The third fundamental system of time, Atomic Time (AT), is based on transitions in the atom. The basic principle of the atomic clock is that electromagnetic waves of a particular frequency are emitted when an atomic transition occurs. The frequency of the cesium beam atomic clock is 9,192,631,770 cycles per second of Ephemeris Time. The advent of atomic clocks having accuracies better than 1 part in 10-13 led in 1961 to the coordination of time and frequency emissions of the U. S. Naval Observatory and the Royal Greenwich Observatory. The master oscillators controlling the signals were calibrated in terms of the cesium standard, and corrections determined at the U. S. Naval Observatory and the Royal Greenwich Observatory were made simultaneously at all transmitting stations. The result is Coordinated Universal Time (UTC). 1803. Time And Arc One day represents one complete rotation of the earth. Each day is divided into 24 hours of 60 minutes; each minute has 60 seconds. Time of day is an indication of the phase of rotation of the earth. That is, it indicates how much of a day has elapsed, or what part of a rotation has been completed. Thus, at zero hours the day begins. One hour later, the earth has turned through 1/24 of a day, or 1/24 of 360°, or 360° ÷ 24 = 15° Smaller intervals can also be stated in angular units; since 1 hour or 60 minutes is equivalent to 15°, 1 minute of time is equivalent to 15° ÷ 60 = 0.25° = 15', and 1 second of time is equivalent to 15' ÷ 60 = 0.25' = 15". Summarizing in table form: Therefore any time interval can be expressed as an equivalent amount of rotation, and vice versa. Interconversion of these units can be made by the relationships indicated above. To convert time to arc: 1. Multiply the hours by 15 to obtain degrees of arc. 2. Divide the minutes of time by four to obtain degrees. 3. Multiply the remainder of step 2 by 15 to obtain minutes of arc. 4. Divide the seconds of time by four to obtain minutes of arc 5. Multiply the remainder by 15 to obtain seconds of arc. 6. Add the resulting degrees, minutes, and seconds. Example 1: Convert 14h21m39s to arc. Solution: To convert arc to time: 1. Divide the degrees by 15 to obtain hours. 2. Multiply the remainder from step 1 by four to obtain minutes of time. 3. Divide the minutes of arc by 15 to obtain minutes of time. 4. Multiply the remainder from step 3 by four to obtain seconds of time. 5. Divide the seconds of arc by 15 to obtain seconds of time. 6. Add the resulting hours, minutes, and seconds. Example 2: Convert 215° 24’ 45" to time units. Solution: Time Arc 1d =24h =360° 60m =1h =15° 4m =1° =60' 60s = 1m = 15' 4s = 1' = 60" 1s = 15" = 0.25' (1) 14h × 15 = 210° 00' 00" (2) 21m ÷ 4 = 005° 00' 00" (remainder 1) (3) 1 × 15 = 000° 15' 00" (4) 39s ÷ 4 = 000° 09' 00" (remainder 3) (5) 3 × 15 = 000° 00' 45" (6) 14h21m39s = 215° 24' 45" (1) 215° ÷ 15 = 14h00m00s remainder 5 (2) 5 × 4 = 00h20m00s (3) 24’ ÷ 15 = 00h01m00s remainder 9
290TIMEofthedate line(east longitude)isIday laterthanthedate im-(4)9×4=00h00m36smediatelytothe east of the line.When solvingproblems,45"+15(5)=00h00m03sconvert local timeto Greenwichtime and then convertthis tolocaltimeontheoppositesideof thedateline215°24'45"(6)=14h2/m39s1806.ZoneTimeSolutions canalsobemadeusing arcto timeconversionAt sea, as well as ashore, watches and clocks are nor-tables inthealmanacs.In theNautical Almanac,thetablemally set to some form of zonetime(ZT).At sea thegiven near the back of the volume is in two parts, permittingnearest meridian exactlydivisibleby15°is usuallyusedasseparate entries with degrees, minutes, and quarter minutesthe time meridian or zone meridian. Thus, within a timeof arc.This table is arranged in this manner because the nav-zone extending 7.5'on each side of thetime meridian theigatorconverts arctotimemoreoftenthanthereversetime is the same, and time in consecutive zones differs byExample 3:Convert 334°1822" to time units, using theexactly onehour.Thetimeischanged as convenient,usual-ly at a whole hour, when crossingthe boundary betweenNautical Almanacarctotime conversiontablezones.Each time zone is identified by the number of timesthe longitude of its zone meridian is divisible by 15°,posi-Solution:tive in west longitude and negative in east longitude.Thisnumber and its sign, called the zone description (ZD), isConvert the22"to the nearest quarter minute of arc forthe number of wholehours that areadded toor subtractedsolution to the nearest second of time.Interpolate if morefrom thezone timetoobtainGreenwichmeantime(GMT)precise results are requiredThe mean sun is the celestial reference point for zone time.See Figure 1806.22h16m00s334°00.00m=Converting ZT to GMT,a positiveZT is added and a000°18.25m00h0|m13s=negative onesubtracted;convertingGMTtoZT,apositiveZD is subtracted, and a negative one added.334°1822"22h17m/3s-Example:TheGMTis15h27m09s1804.TimeAndLongitude(l)ZTatlong.156°24.4W.Required:Suppose a celestial reference point were directly over(2) ZT at long.039°04.8'E.a certain point on the earth.An hour later the earth wouldhaveturnedthrough15°,and thecelestial referencewouldSolutions:be directly over a meridian 15°farther west. Any differenceof longitude betweentwopoints is a measure of the angle(1)GMT15h27m09sthrough which the earth must rotate to separate them.ZD+10h (rev.)Therefore,places east ofanobserverhavelater time,andthose westhave earlier time, and the difference is exactlyZT05h27m09sequal to the difference inlongitude,expressed intime units.Thedifference in time between two places is equal to theGMT(2)15h27m09sdifference of longitude between their meridians,expressedZD-03h (rev.)intimeunits instead ofarc.ZT18h27m09s1805.TheDate Line1807.Chronometer TimeSince time is later towardtheeastandearliertowardtheChronometer time (C) is time indicated by a chronom-west of an observer,time at the lower branch of one's merid-ian is 12 hours earlier or later depending upon the directioneter.Sinceachronometer is setapproximatelytoGMT andofreckoning.Atravelermakinga triparound the worldgainsnot reset until it is overhauled and cleaned about every 3orlosesanentireday.Topreventthedatefrombeing in error,years, there is nearly always a chronometer error (CE), ei-and to provide a starting placefor eachday,a date line isther fast (F) or slow (S).The change in chronometer error infixedby international agreement.This line coincides with the24 hours is called chronometer rate, or daily rate, and des-180th meridian over most of its length.In crossing this line,ignatedgaining orlosing.Witha consistentrateof1sperdaythe date is altered by one day.If a person is traveling east-for three years, the chronometer error would be approxi-mately1gm.Since chronometer error is subjectto change,itward from east longitude to west longitude, time is becominglater, and when the date line is crossed the date becomes 1should bedetermined fromtimetotime,preferablydaily atday earlier.Atanymomentthedate immediatelytothewestsea.Chronometererror isfound byradiotime signal, by
290 TIME Solutions can also be made using arc to time conversion tables in the almanacs. In the Nautical Almanac, the table given near the back of the volume is in two parts, permitting separate entries with degrees, minutes, and quarter minutes of arc. This table is arranged in this manner because the navigator converts arc to time more often than the reverse. Example 3: Convert 334°18’22" to time units, using the Nautical Almanac arc to time conversion table. Solution: Convert the 22" to the nearest quarter minute of arc for solution to the nearest second of time. Interpolate if more precise results are required. 334° 00.00m = 22h16m00s 000° 18.25m = 00h01m13s 334° 18’ 22" = 22h17m13s 1804. Time And Longitude Suppose a celestial reference point were directly over a certain point on the earth. An hour later the earth would have turned through 15°, and the celestial reference would be directly over a meridian 15° farther west. Any difference of longitude between two points is a measure of the angle through which the earth must rotate to separate them. Therefore, places east of an observer have later time, and those west have earlier time, and the difference is exactly equal to the difference in longitude, expressed in time units. The difference in time between two places is equal to the difference of longitude between their meridians, expressed in time units instead of arc. 1805. The Date Line Since time is later toward the east and earlier toward the west of an observer, time at the lower branch of one’s meridian is 12 hours earlier or later depending upon the direction of reckoning. A traveler making a trip around the world gains or loses an entire day. To prevent the date from being in error, and to provide a starting place for each day, a date line is fixed by international agreement. This line coincides with the 180th meridian over most of its length. In crossing this line, the date is altered by one day. If a person is traveling eastward from east longitude to west longitude, time is becoming later, and when the date line is crossed the date becomes 1 day earlier. At any moment the date immediately to the west of the date line (east longitude) is 1 day later than the date immediately to the east of the line. When solving problems, convert local time to Greenwich time and then convert this to local time on the opposite side of the date line. 1806. Zone Time At sea, as well as ashore, watches and clocks are normally set to some form of zone time (ZT). At sea the nearest meridian exactly divisible by 15° is usually used as the time meridian or zone meridian. Thus, within a time zone extending 7.5' on each side of the time meridian the time is the same, and time in consecutive zones differs by exactly one hour. The time is changed as convenient, usually at a whole hour, when crossing the boundary between zones. Each time zone is identified by the number of times the longitude of its zone meridian is divisible by 15°, positive in west longitude and negative in east longitude. This number and its sign, called the zone description (ZD), is the number of whole hours that are added to or subtracted from the zone time to obtain Greenwich mean time (GMT). The mean sun is the celestial reference point for zone time. See Figure 1806. Converting ZT to GMT, a positive ZT is added and a negative one subtracted; converting GMT to ZT, a positive ZD is subtracted, and a negative one added. Example: The GMT is 15h27m09s. Required: (1) ZT at long. 156°24.4’ W. (2) ZT at long. 039°04.8’ E. Solutions: 1807. Chronometer Time Chronometer time (C) is time indicated by a chronometer. Since a chronometer is set approximately to GMT and not reset until it is overhauled and cleaned about every 3 years, there is nearly always a chronometer error (CE), either fast (F) or slow (S). The change in chronometer error in 24 hours is called chronometer rate, or daily rate, and designated gaining or losing. With a consistent rate of 1s per day for three years, the chronometer error would be approximately 18m. Since chronometer error is subject to change, it should be determined from time to time, preferably daily at sea. Chronometer error is found by radio time signal, by (4) 9 × 4 = 00h00m36s (5) 45" ÷ 15 = 00h00m03s (6) 215° 24’ 45" = 14h21m39s (1) GMT 15h27m09s ZD +10h (rev.) ZT 05h27m09s (2) GMT 15h27m09s ZD –03 h (rev.) ZT 18h27m09s
MHNHHAR1TIMTIMEEVENNUMBEREDZONEOODNUMBEREDZONECOUNTRIESWHERESTANDARD.TIMEDIFFERSHALFANHOURFROMNEIGHBORINGZONES0AAz10o30601503012F0Figure1806.TimeZone Chart
TIME 291 Figure 1806. Time Zone Chart