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12. Calculating Time of Meridian Passage

12. Calculating Time of Meridian Passage. Presented By: Mate O. Course Outline. Navigational Astronomy Lecture 1 & 2 Mean Time / Apparent Time Lecture 3 Time Zones, Zone Description, Chronometer Time Lecture 4 The Earth, Celestial, And Horizon Coordinate System Lecture 5 & 6

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12. Calculating Time of Meridian Passage

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  1. 12. Calculating Time of Meridian Passage Presented By: Mate O

  2. Course Outline • Navigational Astronomy Lecture 1 & 2 • Mean Time / Apparent Time Lecture 3 • Time Zones, Zone Description, Chronometer Time Lecture 4 • The Earth, Celestial, And Horizon Coordinate System Lecture 5 & 6 • The Spherical Triangle Lecture 7 • Circle of Equal Altitude Intercept Lecture 8 • Time Diagram, Right Ascension Lecture 9 • The Nautical Almanac, Finding GHA, LHA, and Dec. Lecture 10 & 11 • Instruments For Celestial Navigation Lecture 12 • Sight Reduction of the Sun, Stars, Planets, Moon Lecture 13 - 15 • Plotting and Advancing / Retarding the Assumed Position Lecture 16 • Calculating Time of Meridian Passage (LAN) Lecture 17 • Calculating Latitude at Meridian Passage Lecture 18 • Calculating Time of Sunrise/ Sunset/ Twilight Lecture 19 • Star Shooting Schedule / Pub. 249 Lecture 20 • Azimuth and Amplitudes Lecture 21 – 24 • Polaris – Azimuth and Latitude By Lecture 25 • Star – Finder – Stars, Planets, Selecting the 3 best Stars Lecture 26 • The Complete Day’s Work Lecture 27 & 28

  3. XII: Local Apparent Noon LAN • “LAN” Or Local Apparent Noon (Or “LAM” Midnight) Is Useful To The Navigator. It Is A Time When An Observation Of The Sun Will Obtain A Latitude Line Of Position. • Local Apparent Noon Occurs When The Visible (Apparent) Sun Is Over The Upper Branch Of The Observer’s Meridian. The Local Hour Angle Of The Sun Is 000°. • Local Apparent Midnight Occurs When The Visible (Apparent) Sun Is Over The Lower Branch Of The Observer’s Meridian. The Local Hour Angle Of The Sun Is 180°. • Usually The Sun Is Only Visible During Local Apparent Noon. In High Latitudes And Depending On The Time Of Year , The Sun Can Be Observed At Noon And Midnight.

  4. XII: Meridian Transit Meridian Transit • The Meridian Transit Of A Celestial Body Is When The Body Is Directly Passing Over The Upper Branch Or The Lower Branch Of The Observer’s Local Meridian • Upper Transit Is When The Body Passes Over The Longitude Of The Observer. • Lower Transit Is When The Body Passes Over The Longitude On The Other Side Of The Earth (180° From His Longitude). The Lower Branch Of His Meridian Is The Other Half Of A Meridian (Great Circle). It Is The Other Half Of The Great Circle That Passes Through His Position On The Other Side Of The Poles.

  5. XII: Local Apparent Noon Time of LAN • In Order To Accurately Observe The Celestial Body As It Passes At Meridian Transit It Is Important To Calculate The Exact Time Of Passage. • There Are Different Methods Of Determining Time Of Meridian Transit (LAN). • One Is The GHA Interpolation Method. • Another Method Often Used Is The DLO Method.

  6. XII: Local Apparent Noon DLO Method of Determining Time of LAN • DLO Method Of Determining LAN • Although not as accurate as the GHA Method Some People Prefer To Use The DLO Method Of Determining LAN Because It Is Easier To Visualize The Passage Of The Body Over Their Meridian In Relation To The Body’s Passage Over The Central Meridian. • To Use The DLO Method Of Determining Time Of LAN – The Time Of Meridian Passage (Mer. Pass) Must Be Determined From The Lower Right Corner Of The Almanac. This Is The Time Of Passage Of The Apparent Sun Over The Central Meridian. • All That Is Required Is Finding Your DLO From The Central Meridian At The Time Of Meridian Passage And Converting This DLO Arc To Time. • Subtract the time from the time of Mer. Pass. If You Are East Of The Central Meridian. • Add the time to the time of Mer. Pass if you are West of the Central Meridian.

  7. XII: Local Apparent Noon DLO Method of Determining Time of LAN Example: On 31 January 1981, your 0920 zone time DR position is LAT 24° 16.0’ S, λ 151° 33.0’ E. Your vessel is on course 258° T at a speed of 18.5 kts. What is the zone time of local apparent noon (LAN)? First find your DR position at the time of Mer. Pass at 1214: 1214 -0920 2h54m @ 18.5kts= D: 53.65nm L=sin c.a x D [sin12 x 53.65] L= 11.2’ S (bigger) • 1214 Position: • L 24° 27.2’S • λ 150° 40.5’ E Dlo=cos c.a. x D [cos12 x 53.65 DLO= 52.5’ W (smaller)

  8. XII: Local Apparent Noon DLO Method of Determining Time of LAN • DLO Method Of Determining LAN Example: On 31 January 1981, your 0920 zone time DR position is LAT 24° 16.0’ S, λ 151° 33.0’ E. Your vessel is on course 258° T at a speed of 18.5 kts. What is the zone time of local apparent noon (LAN)? First find your DR position at the time of Mer. Pass. 1214 L 24° 27.0’S λ 150° 40.5’ E Central Meridian 150° E DR λ 150° 40.5’ E 1214 – 00s - 3 - 00s Ans. 1211-00 ApparentSun 1214 DLO = 0° 40.5’ of Arc to the East or 3 minutes of time The Apparent Sun will be over the Central Meridian at 1214, So It Will Cross our DR λ 3 minutes before 1214

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