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Introduction    Corrections to Sextant   Time   Finding GHA and Declination   Assumed Position and Local Hour Angle   Computed Altitude and Azimuth    Interpolation    Altitude Intercept    Using Position Plotting Sheets    Plotting Lines of Position

 

TIME

   THUS FAR IN THE SIGHT REDUCTION PROCESS YOU HAVE TWO IMPORTANT PIECES OF INFORMATION. YOU HAVE "OBSERVED ALTITUDE," (Ho), WHICH IS YOUR SEXTANT READING CORRECTED FOR THE VARIOUS ERRORS DISCUSSED IN THE PREVIOUS LESSON.

   THE SECOND PIECE OF INFORMATION YOU HAVE IS THE TIME THAT YOU TOOK THE SIGHT. YOU NEED THE EXACT TIME OF THE SIGHT IN ORDER TO LOOK UP SOME ADDITIONAL DATA IN THE NAUTICAL ALMANAC. BEFORE YOU CAN DO THAT, HOWEVER, YOU MUST MAKE SURE THAT YOUR TIME IS CORRECTED FOR ANY "WATCH ERRORS" IN YOUR TIME PIECE, AND YOU MUST CONVERT YOUR "LOCAL TIME" TO THE DATE AND STANDARD TIME OF GREENWICH, ENGLAND. THIS "GREENWICH MEAN TIME" (GMT) WILL ENABLE YOU TO FIND THE INFORMATION YOU NEED IN THE NAUTICAL ALMANAC.

      WHEN YOU TAKE YOUR SEXTANT READING YOU READ YOUR WATCH OR CHRONOMETER TO THE NEAREST SECOND. YOU KNOW THE LOCAL DATE, OF COURSE. TO USE THE ALMANAC TO FIND THE G.P., YOU NEED THE DATE AND TIME AT GREENWICH, ENGLAND. THAT TIME IS ALMOST ALWAYS DIFFERENT FROM THE READING OF YOUR TIMEPIECE. QUITE OFTEN THE DATE AT GREENWICH IS ALSO DIFFERENT .

   THIS LESSON IS ABOUT MAKING THAT CONVERSION. YOU MUST GET THE GREENWICH MEAN TIME (GMT) AND DATE RIGHT! IF THIS IS WRONG, EVERYTHING WILL BE WRONG.

   THE TIME CONVERSION IS SIMPLE. YET, MORE MISTAKES ARE MADE IN THIS PART OF THE PROBLEM THAN IN ANY OTHER.  MASTER THIS PART BEFORE YOU DO ANYTHING ELSE. FORCE YOURSELF TO SLOW DOWN, WORK ALL TIME PROBLEMS CAREFULLY, AND DOUBLE‑CHECK. WHEN YOU CATCH YOURSELF GETTING "INTO AUTOMATIC" AND WORKING TIME PROBLEMS RAPIDLY, STOP. CHECK. EVERY TIME. FOR THE REST OF YOUR LIFE.

   WHEN YOU TAKE THE TEST, YOU MAY USE THE NAUTICAL ALMANAC AND A SET OF SIGHT REDUCTION TABLES. IF YOU GET STUCK, YOU CAN REFER TO THE DIRECTIONS IN THEM AND GET SOME HELP ON ABOUT ANY PART OF THE PROBLEM EXCEPT THE TIME. WHEN CONVERTING YOUR DATE AND TIME TO GMT, AND GREENWICH DATE, YOU'LL BE ON YOUR OWN.

    ANOTHER THING: A TIME ERROR OF ONLY 4  SECONDS CAN MAKE A FIX WRONG BY AS MUCH AS A MILE. YOU COULD GET AWAY WITH THAT AT SEA. IN THE EXAM ROOM THAT WON'T CUT IT.   

****************************************************************************************************

   WE KEEP TIME BY THE SUN. LONG AGO, EVERY TOWN HIRED A MAN TO WATCH THE SUN. WHEN IT REACHED ITS HIGHEST POINT IN THE SKY (AS IT CROSSED THE LOCAL MERIDIAN) HE WOULD RING THE BELL OR FIRE THE COURTHOUSE CANNON AS A SIGNAL FOR THE GENTRY TO SET THEIR WATCHES TO 12:00.

   THEY HAD TO RESET THEIR WATCHES EVERY DAY. EVEN IF A WATCH RAN AT A PERFECTLY REGULAR RATE, THE SUN'S APPARENT MOTION AROUND THE EARTH ISN'T PERFECTLY REGULAR. AT A VILLAGE FORTY MILES WEST, THE NOON GUN WOULD GO OFF ABOUT TWO MINUTES AND FORTY SECONDS LATER, WHEN THE SUN CROSSED ITS LOCAL MERIDIAN. THIS CAUSED NO PROBLEMS WHEN THE MOST RAPID MEANS OF COMMUNICATION WAS A GALLOPING HORSE.

    LATER CAME THE RAILROADS. THEY FOUND IT IMPOSSIBLE TO WRITE A TRAIN SCHEDULE WHEN NO TWO TOWNS AGREED ON WHAT TIME IT WAS. THEY HAD TO STANDARDIZE BY INVENTING TIME ZONES. THEY ALSO AVERAGED OUT THOSE IRREGULARITIES OF THE SUN'S APPARENT MOTION, OR TOOK A "MEAN". WE NOW  WORK WITH A "MEAN" SUN WHICH GETS AROUND THE WORLD IN EXACTLY TWENTY FOUR HOURS. EVERYONE IN THE SAME ZONE SETS HIS WATCH THE SAME. ASHORE, IT'S KNOWN AS "STANDARD TIME". AT SEA WE CALL IT ZONE TIME (Z.T.). YOU CAN DEPEND ON THE FELLOW WHO WRITES THE EXAM TO GO BY THE BOOK. THE BOOK SAYS:

WATCH TIME (W.T.) IS THE READING OF YOUR WATCH AT THE INSTANT YOU MAKE THE SEXTANT READING. THE WATCH IS SET TO THE STANDARD TIME OF THE TIME ZONE THE BOAT IS IN.

ZONE TIME (Z.T.) IS THE CORRECT TIME OF THE ZONE YOU'RE IN.

WATCH ERROR (W.E.) IS THE AMOUNT OF TIME THE WATCH IS SLOW (S) OR FAST (F). IF THE WATCH IS SLOW, YOU ADD THE ERROR. IF IT'S FAST, SUBTRACT.

1.         WATCH TIME OF SHOT WAS 0812 AND 15 SECONDS. WATCH ERROR WAS 12 SECONDS (F).

 W.T.    08h-12m-15s 

 W.E.                   -12s (F)

 Z.T.      08h-12m-03s

 

2. YOU MADE A SEXTANT OBSERVATION OF A STAR. THE WATCH READ  19h 32m 54s. YOUR WATCH WAS 1 MINUTE 13 SECONDS SLOW.  

W.T.     19h 32m 54s

W.E.        (+) 1m 13s (S)

Z.T.      19h 34m 07s

   TO CONVERT YOUR ZONE TIME AND LOCAL DATE TO GREENWICH TIME AND DATE YOU NEED TO UNDERSTAND TIME ZONES, ZONE DESCRIPTIONS, AND HOW TO FIGURE WHAT TIME ZONE YOU'RE IN. THE TIME ZONES AREN'T MARKED ON THE CHART, AND THE PROBLEMS ON THE TEST USUALLY DON'T TELL YOU.

   THE "MEAN" SUN BY WHICH WE KEEP STANDARD TIME CIRCLES THE EARTH, OR TRAVELS 360° IN TWENTY‑FOUR HOURS. THAT WORKS OUT TO 15° OF LONGITUDE PER HOUR. THE TIME ZONES ARE FIFTEEN DEGREES OF LONGITUDE, SO THE TIME IN EACH ZONE DIFFERS FROM THE TIME OF ITS NEIGHBORS BY EXACTLY ONE HOUR. WHEN IT'S NOON IN YOUR ZONE, IT'S ALREADY 1300 IN THE ZONE EAST OF YOU BUT ONLY 1100 IN ZONE WEST OF YOU .  

   THE TIME ZONE DIAGRAM HERE “VIEW IMAGE 8” SHOWS HOW THE ZONES ARE LAID OUT. THE CENTER OF THE GREENWICH TIME ZONE IS THE GREENWICH MERIDIAN, LO 0°. ALL THE OTHER ZONES ARE ON 15° (OF LONGITUDE) CENTERS. THE BOUNDARIES BETWEEN ZONES ARE HALFWAY BETWEEN THE CENTERS, OR 7° 30' EITHER SIDE OF THE CENTERS.

   THE CENTRAL MERIDIAN OF EVERY TIME ZONE IS THE "STANDARD" MERIDIAN OF THAT ZONE. THE LONGITUDE OF EVERY STANDARD MERIDIAN OTHER THAN THE GREENWICH MERIDIAN CAN BE DIVIDED BY 15°  WITH NO REMAINDER.

    WHEN THE SUN IS DIRECTLY OVER THE STANDARD MERIDIAN OF A ZONE, THE TIME EVERYWHERE IN THAT ZONE IS 1200, NOON, BY DEFINITION.

   IF YOU COVER THE SPOT MARKED "A". “VIEW IMAGE 8” YOUR "SUN" IS DIRECTLY OVER THE STANDARD MERIDIAN OF THE GREENWICH ZONE. GREENWICH MEAN TIME (GMT) IS 1200.

   LOOK AT THE ZONE CENTERED AT LONGITUDE 90° WEST. “VIEW IMAGE 8” IT WON'T BE 1200 THERE UNTIL THE SUN REACHES POINT "B". THE SUN HAS NINETY DEGREES TO GO AT FIFTEEN DEGREES PER HOUR BEFORE IT REACHES POINT "B". THAT WILL TAKE SIX HOURS. SO WHAT TIME IS ZONE TIME IN THE 90° WEST ZONE? IT MUST BE SIX HOURS BEFORE NOON, OR 0600.

   NOW COVER POINT "B". “VIEW IMAGE 8” THE SUN IS NOW DIRECTLY OVER THE MERIDIAN OF LO 90° WEST, SO IT IS 1200 IN THAT ENTIRE TIME ZONE EXTENDING FROM LO 82° 30' W TO LO 97° 30' W. WHAT IS GMT? SIX HOURS AGO, IT WAS 1200 GMT. NOW, IT MUST BE 1800 GMT.

   NOTICE THAT THE ZONE THE SUN IS OVER NOW IS MARKED "+6". THE "+6" IS THE ZONE DESCRIPTION (Z.D.). THE Z.D. IS THE NUMBER OF HOURS TO BE ADDED TO OR SUBTRACTED FROM ZONE TIME TO GET GMT. ALL Z.D.'S FOR ZONES IN WEST LONGITUDE HAVE A PLUS SIGN, AND ARE TO BE ADDED TO Z.T. TO GET GMT. USING THE "INSTRUCTIONS" IN THE +6 ZONE DESCRIPTION, WE ADD 6 HOURS TO THE ZONE TIME (1200) TO GET A GMT OF 1800.

   ALL ZONE DESCRIPTIONS IN EAST LONGITUDE HAVE MINUS SIGNS. THEY ARE THE WHOLE HOURS TO BE SUBTRACTED FROM Z.T. TO GET GMT.

NOW COVER POINT "C". “VIEW IMAGE 8” THE SUN IS NOW OVER LO 120° EAST, SO IT MUST BE 1200 IN THIS ZONE.

   TO GET THE TIME IN GREENWICH, WE SIMPLY FOLLOW THE INSTRUCTION TO SUBTRACT 8 HOURS FROM THE ZONE TIME, RESULTING IN A GMT OF 0400. THE ZONE DESCRIPTION IS "- 8". DOES THAT MAKE SENSE? SURE IT DOES. COUNT UP HOW MANY ZONES OF 15° THE SUN WILL HAVE TO COVER BEFORE IT REACHES LO 0° WHEN IT WILL BE NOON AGAIN IN THE GREENWICH ZONE. EIGHT.. IT WON'T BE 1200 IN GREENWICH FOR EIGHT HOURS YET. GMT IS 0400.

  HERE ARE SOME EXAMPLES OF CORRECTING ZONE TIME FOR WATCH ERROR AND THEN APPLYING THE ZONE DESCRIPTION TO GET GMT.

   NOTICE THAT THE GREENWICH DATE CAN BE THE SAME AS YOURS, OR A DAY EARLIER OR A DAY LATER. IF Z.T. + Z.D. ADD UP TO MORE THAN 24 HOURS, GMT MUST BE THE "NEXT" DAY, ONE DAY LATER THAN YOUR ZONE.   SUBTRACT 24 HOURS FROM YOUR TOTAL AND ADD ONE DAY TO THE DATE (EXAMPLE 2). IF Z.T. IS SMALLER THAN A MINUS Z.D., ADD 24 HOURS TO Z.T., SUBTRACT THE Z.D. AND SUBTRACT ONE DAY FROM THE DATE (EXAMPLE 3).

   REMEMBER THAT IT WAS MINUTES AND SECONDS (60'S) THAT WERE BEING BORROWED OR CARRIED.

   NEXT YOU NEED TO KNOW HOW TO FIGURE OUT YOUR ZONE DESCRIPTION FROM YOUR DR LONGITUDE AND PROCEED WITH THIS:

   TO FIND YOUR Z.D., DIVIDE YOUR DR LONGITUDE BY 15° . IF THE REMAINDER IS MORE THAN 7°30', ADD 1 TO THE ANSWER. IF LONGITUDE IS WEST, THE Z.D. IS PLUS.(+). IF LONGITUDE IS EAST, Z.D. IS MINUS (-).

EXAMPLES:  

		+6	ZONE DESCRIPTION
1. DR LONG. 93° 15' W	15°	93°  15'W	DR LONG.
		3°  15	REMAINDER

 

		+6   (+7)	ZONE DESCRIPTION
2. DR LONG. 98° 27' W	15°	98°   27	DR LONG.
		90
		8°   27’	REMAINDER

 

		-7	ZONE DESCRIPTION
3. DR LONG. 112° 15' E	15°	112 °    15' E	DR LONG.
		105 °
		7 °    15 '	REMAINDER

 

		-3     (-4)	ZONE DESCRIPTION
4. DR LONG. 53°  17' E	15°	53°    17 '	DR LONG.
		45 °
		8°   17 '	REMAINDER

  ALL ZONE DESCRIPTIONS IN WEST LONGITUDE ARE PLUS (+) ALL ZONE DESCRIPTIONS IN EAST LONGITUDE ARE MINUS (-)

    IT ISN'T SO HARD TO FIGURE OUT A ZONE DESCRIPTION AND CONVERT ZONE TIME TO GMT. IN REAL LIFE NAVIGATION IT ISN'T MUCH OF A PROBLEM. YOU ALWAYS KNOW ZONE TIME AND YOU MIGHT STAY IN THE SAME TIME ZONE FOR DAYS. YOU GET THE ROUTINE FIGURED OUT AND IT NEVER CHANGES MORE THAN ONE HOUR AT A TIME. THE CONFUSION COMES IN THE ARTIFICIAL SITUATION OF WORKING PROBLEMS. THEY WILL HAVE YOU JUMPING ALL OVER THE WORLD IN A MATTER OF MINUTES AND SHIFTING BACK AND FORTH BETWEEN EAST AND WEST LONGITUDE.

   AND IT CAN GET WORSE. COAST GUARD PROBLEMS SOMETIMES GIVE YOU LESS INFORMATION THAN YOU WOULD HAVE AT SEA, THAT IS, LEAVE OUT ZONE TIME. THESE PROBLEMS DO GIVE YOU A CHRONOMETER READING AND SOME CLUE TO TELL YOU WHETHER THE LOCAL TIME WAS A.M. OR P.M. HOWEVER, IT TAKES AN EXTRA STEP TO SOLVE THE TIME PART OF THE PROBLEM WHEN IT'S PRESENTED THIS WAY.  

   NOW LET'S DEAL WITH CHRONOMETER TIME. A CHRONOMETER IS JUST A HIGH-PRICED SEA-GOING WATCH BUILT ESPECIALLY FOR NAVIGATORS. NAVIGATORS NEED TO KNOW GMT. SO WE SET TO CHRONOMETER TO GMT AND DON'T MESS WITH IT. NO CHRONOMETER IS PERFECT. THEY'LL ALL RUN A BIT SLOW OR FAST. SO:

CHRONOMETER TIME (C.T.)--WHAT THE CHRONOMETER READS, AS SET TO GMT.

CHRONOMETER ERROR (C.E.)--THE AMOUNT OF TIME IT'S SLOW OR FAST.

CORRECTED CHRONOMETER. TIME (C.C.T.) --THE CHRONOMETER TIME CORRECTED BY ADDING "SLOW" C.E.'S AND SUBTRACTING FAST C.E.'S.

   IF THE CHRONOMETER IS SET TO GMT, WHY DOESN'T C.T. + OR - C.E. = GMT? BECAUSE THE DIAL OF THE CHRONOMETER IS A REGULAR TWELVE HOUR CLOCK FACE.

   IF THE PROBLEM SAYS THAT THE C.C.T. IS 02h-15m-OOs, IS THAT 0215 OR 1415 AT GREENWICH? IT WOULD BE OBVIOUS IF YOU WERE THERE. WHEN YOU'RE HALFWAY AROUND THE WORLD FROM GREENWICH, OR IN THE EXAM ROOM, IT ISN'T SO OBVIOUS. A MISTAKE AT THIS POINT THROWS YOUR TIME OFF BY TWELVE HOURS.

   YOU CAN DECIDE WHETHER IT'S A.M. OR P.M. AND WHAT THE DATE IS AT GREENWICH BY DRAWING A SIMPLE "TIME DIAGRAM". 

THE TIME DIAGRAM IS JUST A SIMPLE SKETCH LIKE THIS:

”VIEW IMAGE 9”

  IT REPRESENTS THE EARTH VIEWED FROM THE SOUTH POLE, WITH WEST GOING COUNTER CLOCKWISE AROUND THE CIRCLE. THE "G" AT THE TOP INDICATES THE GREENWICH MERIDIAN, AND THE "M" MARKS THE BOAT'S LONGITUDE. THE UPPER MERIDIANS OF GREENWICH AND THE VESSEL ARE SHOWN AS SOLID LINES. WHEN THE MEAN SUN IS OVER THE UPPER MERIDIAN OF A PLACE, THE ZONETIME IS NOON AT THAT PLACE. THE LOWER MERIDIANS ARE 180 DEGREES FROM THE UPPER ONES AND ARE DRAWN AS BROKEN LINES ON THE DIAGRAM. WHEN THE SUN IS OVER THE LOWER MERIDIAN OF "G" OR "M", IT IS MIDNIGHT AT "G" OR "M".

   THE TIME DIAGRAM CAN SHOW VERY CLEARLY WHETHER THE C.C.T. IS AM OR PM WHEN YOU ADD THE SUN TO IT. THE TRICK IS TO GET THE SUN LOCATED ON THE DIAGRAM FROM A CLUE GIVEN IN THE PROBLEM.

   HERE'S HOW YOU DO IT. YOU WILL KNOW YOUR DR LONGITUDE SO YOU CAN SPOT YOUR POSITION (M) ON THE DIAGRAM IN RELATION TO THE GREENWICH MERIDIAN (G).

   1.  SUPPOSE THE PROBLEM SAYS, "YOUR DR POSITION IS 22° S 45° W. YOU MAKE A MORNING OBSERVATION OF A STAR AT C.C.T. 09-06-12. YOU KNOW THAT THE ONLY TIME YOU CAN SHOOT A MORNING STAR IS ABOUT SUNRISE, SO THE SUN WOULD BE ABOUT NINETY DEGREES EAST OF YOU. WHEN YOU PUT THE SUN IN THAT POSITION ON THE DIAGRAM, “VIEW IMAGE 10”  IT IS OBVIOUS THAT THE SUN HAS NOT YET PASSED OVER GREENWICH {G}. THE "GMT" IS BEFORE NOON, SO GMT WOULD BE THE SAME AS C.C.T.: 09-06-12

 

  2.   SUPPOSE THE PROBLEM INVOLVES A PM OBSERVATION OF POLARIS. THE SUN MUST BE APPROXIMATELY NINETY DEGREES WEST OF THE BOAT. LET'S SAY YOUR DR POSITION IS 140° W. WHEN WE FILL IN THE TIME DIAGRAM, WE GET: “VIEW IMAGE 11”

  3.  IF AM OR PM SUN SIGHTS ARE SPECIFIED, PUT THE SUN ABOUT HALFWAY. BETWEEN M AND THE CELESTIAL HORIZON (90° FROM M) AND IT WILL BE CLOSE ENOUGH TO DETERMINE WHETHER GREENWICH TIME IS AM OR PM. THE DIAGRAM “VIEW IMAGE 12”  SHOWS A VESSEL AT A DR LONGITUDE OF 104° W TAKING AN AM SUN SIGHT AT C.C.T. 02-30-10. THE SUN HAS NOT YET REACHED THE SHIP'S MERIDIAN, SO THE SUN SHOULD BE PLACED 45° EAST OF M.   THE SUN'S POSITION IS WEST OF G, SO IT MUST BE AFTER NOON, RESULTING IN A GMT OF 14-30-10.

 4.   SUPPOSE YOU ARE GIVEN A PM SUN SIGHT ON AUGUST 5, C.C.T. 01-15-30 AT A DR LONGITUDE OF 150° W. CAN YOU DETERMINE THE DATE AND WHETHER GMT IS 01-15-30 OR 13-15-30? FILL OUT THE TIME DIAGRAM WITH THE INFORMATION PROVIDED.   A PM SUN SIGHT MEANS YOUR SUN POSITION SHOULD BE 45° WEST OF M, RESULTING IN A DIAGRAM LIKE THIS: “VIEW IMAGE 13”    THE FACT THAT THE SUN IS JUST PAST "G" (INDICATING MIDNIGHT FOR GREENWICH)  SHOWS THAT GMT IS 01-15-30. BECAUSE IT IS PAST MIDNIGHT, IT ALSO MEANS THAT THE GMT HAS ADVANCED ONE DAY TO AUGUST 6.

  5.  THE TIME DIAGRAM IS THEREFORE A CONVENIENT WAY TO CHECK THE GREENWICH DATE. IF THE SUN LIES ANYWHERE BETWEEN THE TWO LOWER MERIDIANS (BETWEEN " g " AND " m ") THE GREENWICH DATE IS DIFFERENT FROM LOCAL DATE BY ONE DAY. REMEMBER THAT THE SUN IS ALWAYS MOVING WEST. HERE “VIEW IMAGE 14” IT HAS ALREADY PASSED GREENWICH'S LOWER MERIDIAN " g ". IT IS PAST MIDNIGHT AT GREENWICH SO THE DATE IS NOW THE 16TH. IT IS NOT YET MIDNIGHT " m " AT THE VESSEL, SO THE DATE IS STILL THE 15TH.

  6.  IN THIS EXAMPLE “VIEW IMAGE 15” IT IS AFTER MIDNIGHT WHERE THE BOAT IS, BUT NOT YET MIDNIGHT AT GREENWICH. LOCAL DATE IS THE 25TH, BUT IT IS STILL THE 24TH AT GREENWICH.

  MANY TIMES THE TIME DIAGRAM IS ALL YOU NEED TO HELP YOU DECIDE THE GREENWICH DATE AND WHETHER GMT IS AM OR PM. SOMETIMES, HOWEVER, THE ROUGH APPROXIMATION THAT YOU MAKE OF THE SUN'S POSITION ON THE DIAGRAM DOESN'T MAKE THE SITUATION ABSOLUTELY CLEAR.

 7.   IF YOU WERE CONSTRUCTING A DIAGRAM FOR A VESSEL AT A DR POSITION OF 135° EAST. YOU ARE TRYING TO LOCATE THE SUN BY FOLLOWING THE "CLUE" IN THE PROBLEM, WHICH SAYS THE SIGHT IS AN AM SUN. WHEN YOU BUILD YOUR DIAGRAM “VIEW IMAGE 16” , YOU FIND THAT THE SUN'S APPROXIMATE POSITION (45° FROM YOUR "M" POSITION), PLACES IT NEAR "g". HOW CAN YOU TELL IF THE SUN IS BEFORE MIDNIGHT GMT OR AFTER MIDNIGHT GMT?

   THE ANSWER IS DETERMINED BY THE HOUR OF C.C.T. PROVIDED IN THE PROBLEM. IF THE C.C.T. IN THE PROBLEM IS BEFORE 1200 {11-20-37, FOR EXAMPLE), YOU CAN SEE FROM THE ROUGH SKETCH THAT IT IS NEAR MIDNIGHT GMT. GMT MUST THEREFORE BE 23-20-37, WHICH MEANS THE SUN HAS NOT YET CROSSED "G".

IF THE TIME IN THE PROBLEM WERE LISTED AS C.C.T. 12-30-15, THE TIME DIAGRAM ALLOWS YOU TO SEE THAT IT CANNOT BE A HOUR PAST NOON AT GREENWICH, SO GMT MUST BE 00-30-15. WITH THAT IN MIND, THE SUN CAN BE PLACED JUST PAST "g".

  8.  IF YOU WERE WORKING A PROBLEM WITH A "PM STAR" WHEN YOUR VESSEL WAS AT DR LONGITUDE 90° WEST AT A C.C.T. OF 11-47-08. THE "PM STAR" CLUE INDICATES THAT THE SUN IS APPROXIMATELY 90° WEST OF YOUR POSITION , WHICH PUTS IT NEAR "g". “VIEW IMAGE 17”  CAN YOU TELL FROM THE C.C.T. WHETHER THE SUN SHOULD BE PLACED BEFORE OR AFTER THE "G"? LOOKING AT THE ROUGH POSITION OF THE SUN, IT IS OBVIOUS THAT IT IS NEAR MIDNIGHT AT GREENWICH. BECAUSE C.C.T. IS BEFORE 1200, IT MUST BE BEFORE MIDNIGHT AT GREENWICH, MAKING GMT 23-47-08 AND MAKING IT CLEAR THAT THE SUN SHOULD BE PLACED BEFORE "g".

 USE THE TIME DIAGRAM TO SOLVE THESE PRACTICE PROBLEMS:

FIND GMT AND GREENWICH DATE:
	ZT	ZD
1	14 APR.     1330	+6
2	16 JUNE    2315	+6
3	30 MAY    0330	‑7
4	26 SEP.     1452	‑9
5	2 OCT.      2115	+10

 

FIND GMT AND GREENWICH DATE:
	W.T.	W.E.	DATE	DR LONG.
6	03‑24‑41	4m  32s SLOW	10 FEB.	82°  23.5' E
7	22‑14‑18	2m  51s FAST	4  OCT.	101°   47.8' W
8	08‑25‑10	0m  25s FAST	15 MAY	51°   13.5' E
9	13‑45‑29	1m  21s SLOW	6   SEP.	155°   30.0' W
10	06‑15‑03	1m  32s FAST	8   JULY	68°   40.0' W

 

FIND GMT AND GREENWICH DATE:
	C.T.	C.E.	DR LONG.	TYPE OBSERVATION	LOCAL DATE
11	08‑14‑33	2m 51s Slow	33° 14.7' E	AM SUN	4 OCT.
12	05‑43‑15	5m 13s Fast	41° 48.9' W	PM SUN	13 MAY
13	09‑18‑43	2m 11s Fast	155° 27.4' E	AM STAR	10 FEB.
14	12‑14‑56	2m 15s Slow	87° 12.0' E	AM SUN	11 JULY
15	12‑01‑17	3m 20s Fast	45° 15.0' E	PM SUN	15  MAR

 

ANSWERS:
1	14 Apr. 1930	6	9 Feb. 22‑29‑13	11	4 Oct. 08‑17‑24
2	17 June 0515	7	5 Oct. 05‑11‑27	12	13 May 17‑38‑02
3	29 May 2030	8	15 May 05‑24‑45	13	9 Feb 21‑16‑32
4	26 Sep. 0552	9	6 Sep. 23‑46‑50	14	11 July 00‑17‑11
5	3 Oct. 0715	10	8 July 11‑13‑31	15	15 Mar.11‑57‑57

 

  THERE IS NO DOUBT THAT FINDING GMT AND THE GREENWICH DATE IS EASIER IF YOU ARE GIVEN ZONETIME AND THE DR LONGITUDE TO FIGURE THE ZONE DESCRIPTION. MANY TIMES THE TEST PROBLEMS GIVE BOTH ZONETIME AND CHRONOMETER TIME. ZONETIME IS STATED TO THE NEAREST MINUTE, AS "1730". CHRONOMETER TIME INCLUDES THE SECONDS, AS "08-30-17". YOU CAN FIND GMT AND GREENWICH DATE THE EASY WAY, USING Z.T. AND Z.D., BUT DON'T FORGET THE SECONDS WHEN YOU GO TO THE ALMANAC.

   NOW THAT YOU HAVE ESTABLISHED THE CORRECT GREENWICH MEAN TIME (GMT) THAT WE TOOK THE OBSERVED ALTITUDE (HO) OF OUR CELESTIAL BODY, WE CAN GO TO THE NAUTICAL ALMANAC TO FIND THE LOCATION OF THE BODY AT THAT INSTANT. THAT LOCATION WILL BE EXPRESSED IN TERMS OF "GREENWICH HOUR ANGLE" AND DECLINATION. THE NAUTICAL ALMANAC CONTAINS A LISTING OF GHA AND DEC. FOR THE SUN, MOON, AND NAVIGATIONAL STARS AND PLANETS AT ANY SECOND OF THE YEAR. LET'S NOW LEARN HOW TO USE THE NAUTICAL ALMANAC.

 

GHA AND DECLINATION