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Sea Time

Sea Time. Junior Navigation Chapter 3. H1. What time is it?. Navigational Times. Civil (Standard) Time – kept on land (DST & ST) Local Time – time at our location Watch Time – uncorrected time on your watch (WT) Apparent Time – position of the sun

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Sea Time

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  1. Sea Time Junior Navigation Chapter 3 H1

  2. What time is it? Navigational Times Civil (Standard) Time – kept on land (DST & ST) Local Time – time at our location Watch Time – uncorrected time on your watch (WT) Apparent Time – position of the sun Zone Time – within 15° longitude segments (ZT) Mean Time – arbitrary, based on rotation of earth Universal Time – time at 0° longitude (UT) Sidereal Time – relative to Aries (Navigation)

  3. Learning Objectives • Determine watch error and rate • Determine zone description (ZD) from longitude • Determine Universal Time (UT) for sights • Convert • Zone time (ZT) to Universal Time (UT) • Universal Time (UT) to zone time (ZT) • One ZT to a second ZT • Draw time diagrams to visualize Lo, UT, ZT

  4. Keeping Accurate Time • Chronometer – historically, the time reference onboard • Recreational navigators - now use good quality quartz watches • A watch used for timing sights must have: • a sweep second hand that can be set OR • have a digital display showing seconds

  5. ZD+7 ZD+6 C 110 S 7.5 0800 0900 ZD+7 1000 ZD+6 Timekeeping at Sea Standard maritime practice: • Chronometer is set to UT • Ship’s daily routine regulated by ZT • When vessel crosses a time-zone boundary, shipboard clocks/watches (but not chronometer) reset to the new zone time; at the first whole-hour after crossing zone boundary • Captain has discretion on setting ship time

  6. Fundamentals • Accurate time is needed to look up celestial body’s GP in the Nautical Almanac • Celestial bodies – tabulated in the Almanac according to UT • Error of 1 sec in time leads to an error of 0.25nm in LOP

  7. Watch Error and Rate • Watch error (WE): • the inaccuracy with respect to correct time • The rate of a timepiece: • amount timepiece gains or loses per day • Need to know watch error rate • so you can calculate accurate time of a sight • To determine watch error rate: • check time against a radio time signal at 24-hour intervals for at least a week

  8. Watch Error and Rate Chronometer Log Date 23 Sept 24 Sept 25 Sept 26 Sept 27 Sept 28 Sept 29 Sept Signal 12:00:00 12:00:00 12:00:00 12:00:00 12:00:00 12:00:00 12:00:00 WT 12:00:15 12:00:15 12:00:16 12:00:17 12:00:17 12:00:18 12:00:19 WE +00:15 +00:15 +00:16 +00:17 +00:17 +00:18 +00:19 Rate of Change: 4sec/7day = +0.6sec/day 8

  9. Time Signals • U.S. Radio Time Signals • National Institute of Standards and Technology (NIST) • WWV at Fort Collins, CO (1-303-499-7111) • WWVH in Hawaii (1-900-410-TIME)

  10. GPS Time • Check your GPS receiver’s time display with accurate source • “GPS Time” – not identical to UT • If GPS time used for sights: • Include in “Remarks” of your Sight Log the results of checking your GPS time

  11. Green Bay St Paul 12:01:08 12:00:00 Wausau Eau Claire St Cloud 12:00:44 12:00:20 11-59-40 Apparent & Mean Time • Apparent time • The time measured directly by observation of the sun • Mean time • Arbitrary time that averages out irregularities in sun’s apparent motion & creates a constant 24-hour day • UT is Mean Time Sundial indicates apparent time Zone Time is 12 – 08 – 58 (EqT)

  12. Civil vs. Zone Time • Civil time, convenient and easy to use: • On land, determined by law • daylight saving time • Pages 262-265, Nautical Almanac • At sea, use Zone Time – divides earth into zones • each 15º Lo wide

  13. Time Zones 12 +11 +10 +9 +8 +7 +6 +5 +4 +3 +2 +1 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 12 ZD 12 172.5ºW 172.5ºE 180º -12 +12 Follow the sign to Greenwich Follow the sign to Greenwich Reverse the sign from Greenwich Reverse the sign from Greenwich ZM 7.5º 7.5º 165°W 135°W 105°W 75°W 45°W 15°W 15°E 45°E 75°E 105°E 135°E 165°E 180°W 150°W 120°W 90°W 60°W 30°W 0° 30°E 60°E 90°E 120°E 150°E 180°E M ZD -/+ P C E 13

  14. Time & Arc TimeArc • 24 hours 3600 • 1 hour 150 • 4 minutes 10 • 1 minute 15’ • 4 seconds 1’ • 1 second 0.25’ • The GP of the Mean sun moves at a constant rate • 24 hours to go around the earth = 360° • In one hour, the mean sun will cover 15° of longitude • 360°/ 24 hr per day = 15°per hour

  15. Visualizing Time • Earth, as seen from the South Pole

  16. Visualizing Time • Observer’s meridian is shown at the top and drawn as a solid line, labeled “M” and called Upper Branch of Observer’s Meridian M 16

  17. Visualizing Time • Lower branch of a meridian is 180º away, always shown as broken line and labeled “m” M 17 m

  18. Visualizing Time • Upper branch of Greenwich meridian is a solid line labeled “G” • In this example observer is at Lo 60ºW M G 18 m

  19. Visualizing Time • Lower branch of Greenwich meridian is a broken line labeled “g” M G g 19 m

  20. Visualizing Time • Sun’s position plotted as a solid line and labeled with symbol “ ” M G g 20 m

  21. Visualizing Time • Sun moves in a counterclockwise (west) direction • Sun can be located on diagram for any given time • In this example it is 1500 ZT and 1900 UT on the same date M G g 21 m

  22. M West m Time Diagram Circle represents the equator Long tick marks drawn every 15° (hour) Small tick marks placed every 5° (20 minute) Center represents South Pole

  23. Visualizing Time M m What time is it at your position? What time is it at Greenwich? At Greenwich new day begins Mean Sun is at ‘g’ and it’s GMT 0000 Your day begins when Mean Sun is at ‘m’ and it’s ZT 0000 ZT 0000 ZT 1800 UT 0600 92º St Paul – 92º W of Greenwich g UT 0000 G 23

  24. UT for Sights • Nautical Almanac uses Universal Time • Remember: UT is Mean Time • Need to convert ZT of sight to UT • Apply watch error (WE) to watch time (WT) recorded for sight • If the WE is fast - subtract; if slow - add • Determine ZD of your position • Apply ZD to corrected WT to get UT

  25. Sight Reduction Form Date of Observation Watch Time Watch Error Time Diagram Zone Time Zone Description Computed UT Date at Greenwich

  26. Determining ZD Rule: To Determine ZD Divide Lo by 15 Round answer to nearest whole number Use a plus sign if Lo is west Use a minus sign if Lo is east 26

  27. How to find ZD • For longitude 114°15´W, what is the ZD? • To convert minutes of arc to decimal degrees, divide number of arc minutes by 60 • 15’ ÷ 60’ = .25° • 114°15´ = 114.25° • 114.25° ÷ 15° = 7.6, rounded to 8 • Since Lo is West, the ZD is positive • ZD = +8

  28. Find ZD For longitude 79° 30´E, what is the ZD? For longitude 172° 29´ W, what is the ZD? Longitude 79° 30´E = 79.50° 79.50° ÷ 15° = 5.3, rounded to 5 ZD = –5 Longitude 172° 29´W = 172.48° 172.48° ÷ 15° = 11.49 Since remainder < 0.50, rounded to 11 ZD = +11 28

  29. Converting ZT to UT Rule: To obtain UT • Add or subtract the ZD from ZT, using the sign of the ZD. Aid to memory: Follow the sign to Greenwich.

  30. A ship is approaching the entrance to Puget Sound, at 125° W longitude. The ship’s clock shows 1100, Pacific Standard Time. What is UT? 1100 8 + 1900 Converting ZT to UT • Lo is 125°W • ZD is 125 ÷ 15 = 8.33 rounded to 8 • Since West Lo, the sign is (+) • ZD = +8 30

  31. M m Using Time Diagram ZD+8 • Ship is at M, 125° west of Greenwich (G) • Zone meridian is Lo 120°W, ZT measured from lower branch of this meridian • Sun located 11 hours (165°) west of this lower branch - it is 1100 ZT throughout Zone +8 g UT 1900 ZT 1100 G

  32. 29 Oct 3000 0600 9 - 2100 28 Oct ZT to UT Your longitude is 135° E, at ZT 0600, 29 October. What is the UT and date? ZD is 135 ÷ 15 = 9.0 rounded to 9 • Since this is east longitude, sign is (-)

  33. Using Time Diagram Since Lo is East, place “G” 135° west of “M” Since you are on a ZM position sun 6 hours past “m” Since sun has not reached “g” you are one day ahead of Greenwich M m g ZT 0600 G 33

  34. 4 Jul 2000 8 + 0400 2800 5 Jul ZT to UT Your longitude is 126° 30´W at ZT 2000, 4 July. What is the UT and date. ZD is 126.5 ÷ 15 = 8.43 rounded to 8 • Since this is west longitude, sign is (+)

  35. Using Time Diagram Since Lo is West, place “G” 126.5° east of “M” Draw ZM at 120W, position sun 20 hours past “zm” Since sun has passed “g” but not reached “m” you are one day behind Greenwich M ZM g ZT 2000 G 35 zm m

  36. 1 Aug 2019 9 - 1119 1 Aug ZT to UT Your longitude is 128° 45´E at ZT 2019, 1 Aug. What is the UT and date. ZD is 128.75 ÷ 15 = 8.6 rounded to 9 • Since this is east longitude, sign is (-)

  37. Using Time Diagram Since Lo is east, place “G” 128.75° west of “M” Draw ZM at 120E, Position sun 20 hours past “zm” M ZM g ZT 2000 G 37 m zm

  38. International Date Line • IDL - mainly follows the 180th meridian • Add or Subtract a day - depends on direction of travel • Go through Greenwich Why were civil IDL boundaries changed in 1999? French Polynesia (including Tahiti & Bora Bora) Chatham Islands, New Zealand

  39. IDL Westerly • You cross the IDL in a westerly direction at 1200 on 6 November. • Before you cross, you are in Zone +12, & after you cross, you will be in Zone -12. • What is the ZT and date after you cross the IDL? ZT1 1200 6 Nov (before crossing) ZD1+ 12 UT 2400 6 Nov ZD2- 12 (rev) ZT2 3600 6 Nov - 24hr + 1day ZT2 1200 7 Nov (after crossing)

  40. IDL Easterly • You cross the IDL in a easterly direction at 1200 on 6 November. • Before you cross, you are in Zone -12, & after you cross, you will be in Zone +12. • What is the ZT and date after you cross the IDL? ZT1 1200 6 Nov (before crossing) ZD1- 12 UT 0000 6 Nov + 24 hr - 1 day UT 2400 5 Nov ZD2 + 12 (rev) ZT2 1200 5 Nov (after crossing)

  41. Calculate ZT Crossing IDL • Enroute to Guam from Honolulu, Hawaii • Position fix at 1200 on 12 May 16°05’N 178°36’W • Traveled westerly for 8 hrs • New position fix 14°42’N 179°24’E • What is ZT & date at your second fix?

  42. Calculate ZT Crossing IDL Lo 178°36’ W ZD = +12 Lo 179°24’ E ZD = -12 ZT1 1200 12 May (1st fix) ZD1+ 12 UT1 2400 12 May Travel + 8 hr UT2 3200 12 May - 24 hr +1 day UT2 0800 13 May ZD2- 12 (rev) ZT2 2000 13 May (2nd fix) 42

  43. One ZT to Another You want to call your wife in St Paul (ZD +6) at 1000, St Paul time, before she leaves for work on 31 Oct, to wish her a happy birthday. You are at anchor in Sydney, Australia (ZD -10), partying with your Auzzie friends, on your around-the-world cruise. When would you initiate your call from Sydney?

  44. M m One ZT to Another 0200 next day g ZD -10 ZT1 1000 31 Oct ZD1 +6__ St Paul UT 1600 31 Oct ZD2- 10 (rev) Sydney ZT2 2600 31 Oct - 24 hr +1 day ZT2 0200 1 Nov ZD +6 G 1000 44

  45. Sight Log • Data for sights in Sight Folder • Record in Sight Log • Sight Log included in Appendix D • Sight Log sheets have columns for WE, ZD, & WT • “Remarks” - note anything unusual • If ZT not related to Lo or Standard time • If Daylight Savings Time is applicable

  46. Quiz 1. You are at Lo 60°23'W. Ship's time is kept in ZD 0. If you take a sight at 09-10-12 by your watch set to ship's time, what is UT for this sight? a. 05-10-12 b. 09-10-12 c. 13-10-12 d. 09-10-12 the next day

  47. Quiz 2. ZT is 1758, 8 June. What are ZD and UT and date if you are at the following positions? a. Lo 27° 43'W ZD ___ UT __________ b. Lo 154° 38'E ZD ___ UT __________ +21958, 8 June -10 0758, 8 June Determine ZD: 27°43’W = 27.72° 27.72° ÷ 15° = 1.8 Therefore ZD = +2 ZT 1758 8 Jun ZD +2 UT 1958 8 Jun Determine ZD: 154°38’E = 154.63° 154.63° ÷ 15° = 10.3 Therefore ZD = -10 ZT 1758 8 Jun ZD -10 UT 0758 8 Jun

  48. Quiz • 3. UT is 0443, 27 September. What are ZT and date if you are at the following positions? • Lo 156° 14'E ZT _________________ • b. Lo 003° 58'W ZT _________________ 1443, 27 September 0443, 27 September Determine ZD: 156°14’E = 156.23° 156.23° ÷ 15° = 10.4 Therefore ZD = -10 ZT 0443 27 Sep ZD -10 (rev) UT 1443 27 Sep Determine ZD: 3°58’W = 3.97° 3.97° ÷ 15° = 0.3 Therefore ZD = 0 ZT 0443 27 Sep ZD 0 UT 0443 27 Sep

  49. Quiz 4. One day last June, the sun rose at UT 0958 at Cape Fear, NC, and at UT 1238 at Huntington Beach Calif. If both places are at the same latitude, what is their approximate difference in longitude? Solution: Difference in time of Sun rise is 2 hrs 40 min 2 hours at 15°/hour = 30° +40 minutes at 4 minutes/° = 10° Difference in longitude = 40°

  50. Quiz 5. You have your radio set to receive station WWV. You listen to time ticks for a while and then hear a spoken message: “At the tone, 19 hours zero minutes, Coordinated Universal Time“ At the beginning of a short, high-pitch tone, you notice your watch reads 18-59-32. What is your watch error? Watch Error = 28 seconds slow

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