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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

ALTITUDE INTERCEPT   

    NOW THAT YOU HAVE MASTERED THE BASICS OF 229 TO FIND THE COMPUTED ALTITUDE (Hc) AND BEARING (AZIMUTH) OF THE CELESTIAL BODY FROM THE ASSUMED POSITION, YOU NEED TO COMPARE YOUR ACTUAL OBSERVED ALTITUDE (Ho)  TO THE Hc YOU WORKED SO HARD TO GET.

    THIS COMPARISON WILL RESULT IN SOMETHING CALLED AN "ALTITUDE INTERCEPT," THE MYSTERIES OF WHICH ARE TO FOLLOW

   AFTER WORKING WITH THE 229 FOR A WHILE YOU'RE PROBABLY READY FOR AN EASY ONE THAT DOESN'T REQUIRE ANY BOOKS. HERE IT IS.   ALTITUDE INTERCEPT (a) THE DIFFERENCE BETWEEN Hc AND Ho.  

   IT IS NAMED “AWAY" IF Hc IS GREATER THAN Ho, AND "TOWARD" IF Ho IS GREATER THAN Hc. JUST REMEMBER THE WORDS OF THE JAPANESE NAVIGATOR “HOMOTO”.  IF Ho IS MORE THAN Hc YOU ARE TOWARD THE GP OF THE BODY. 

   IT WILL HELP YOU TO REMEMBER, AND TO KNOW WHICH DIRECTION TO PLOT FROM THE A.P., IF YOU UNDERSTAND WHY.

LOOK AT THE DIAGRAM:

 

   LET'S SUPPOSE YOU SHOT THE STAR FROM POSITION “ A “ AND GOT A  CORRECTED SEXTANT READING (Ho) OF 25° WHEN YOU WORKED OUT THE SIGHT REDUCTION IN 229 USING AN ASSUMED POSITION (POSITION B), YOU FOUND A COMPUTED ANGLE (Hc) OF 37°.  BECAUSE Hc IS GREATER, YOU MUST BE FARTHER AWAY FROM THE BODY AND ITS G.P. IF THE COMPUTED ANGLE IS SMALLER, YOU MUST BE CLOSER TOWARD THE BODY. WHEN YOU WORK THE PROBLEMS, THE DIFFERENCE BETWEEN Hc AND Ho WILL PROBABLY COME OUT IN MINUTES AND TENTHS OF MINUTES. THESE CAN BE CONVERTED DIRECTLY TO MILES, JUST AS ONE MINUTE OF LATITUDE EQUALS ONE MILE.

   LET'S SAY THAT YOUR CORRECTED SEXTANT READING (Ho) WAS 52°-27’ AND THE Hc YOU OBTAINED FROM 229 WAS 52°-24’. BECAUSE Hc IS SMALLER THAN Ho, YOU MUST BE 3 MILES CLOSER TO THE STAR THAN THE ASSUMED POSITION YOU CHOSE IT IS CRITICALLY IMPORTANT TO REMEMBER THESE RULES:

   IF Hc IS GREATER, THE ALTITUDE INTERCEPT (a) IS “AWAY.”

   IF Ho IS GREATER, THE ALTITUDE INTERCEPT (a) IS “TOWARD.”

   THE WORKFORM FOR FINDING "A" IS SIMPLE. IF Hc IS 52°-13.3' AND Ho IS 52°-10.0’ WHAT IS "a"?

 SOLUTION: Hc   52° -13.3’

                        Ho   52° -10.3’

                       a             3.0 MILES AWAY (BECAUSE Hc IS GREATER)

 IF Hc IS 47°-48.5' AND Ho IS 48°-01.2’ , WHAT IS "a"?     WORK THE  PROBLEM OUT

    YOU SHOULD HAVE FOUND THAT "a" WAS 12.7 TOWARD BECAUSE Ho IS GREATER. IT'S ALL RIGHT TO PUT THE LARGER NUMBER ON TOP IF YOU FIND IT AWKWARD TO SUBTRACT UPSIDE DOWN. REMEMBER TO LABEL Hc AND Ho EVERY TIME, HOWEVER. IF YOU SKIP THE LABELS YOU'LL GET THEM MIXED UP SOONER OR LATER.

    HERE ARE SOME PRACTICE PROBLEMS: FIND THE ALTITUDE INTERCEPT AND INDICATE WHETHER IT IS TOWARD OR AWAY.  

	Hc	Ho	A
1	33° -26.5’	33° -15.2’
2	42° -33.6	42° -51.8’
3	64° -12.8’	63° -56.3’
4	78° -47.0’	79° -03.4’
5	25° -36.3’	25° -44.1’
6	19° -00.8’	18° -49.6’
7	56° -40.7’	57° -04.5’
8	82° -51.3’	82° -51.3’

 

	ANSWERS
1	11.3 A
2	18.2 T
3	16.5 A
4	16.4 T
5	7.8 T
6	11.2 A
7	23.8 T
8	0.0 -

 

  USING POSITION PLOTTING SHEETS