11

Sea TimeSea Time

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Chapter 3Chapter 3H1

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)

Navigational TimesNavigational TimesWhat time is it?

33

Learning ObjectivesLearning Objectives Determine watch error and rateDetermine watch error and rate Determine zone description (Determine zone description (ZDZD) from ) from

longitudelongitude Determine Universal Time (Determine Universal Time (UTUT) for sights) for sights Convert Convert

Zone time (Zone time (ZTZT) to Universal Time () to Universal Time (UTUT)) Universal Time (Universal Time (UTUT) to zone time () to zone time (ZTZT)) One One ZTZT to a second to a second ZTZT

Draw time diagrams to visualize Draw time diagrams to visualize LoLo, , UTUT, , ZTZT

44

Keeping Accurate TimeKeeping Accurate Time Chronometer – historically, the time Chronometer – historically, the time

reference onboardreference 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

55

Timekeeping at SeaTimekeeping 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 timeCaptain has discretion on setting ship timeZD+7ZD+6C

110 S 7.5 080

0 0900

ZD+7

1000

ZD+6

66

FundamentalsFundamentals 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

77

Watch Error and RateWatch Error and Rate Watch error (Watch error (WEWE): ):

the the inaccuracy with respect to correct time

The rate of a timepiece: amount timepiece gains or loses per day

Need to know watch error rateNeed to know watch error rate so you can calculate accurate time of a sightso you can calculate accurate time of a sight

To determine watch error rateTo determine watch error rate:: check time against a radio time signal at check time against a radio time signal at

24-hour intervals for at least a week24-hour intervals for at least a week

88

Watch Error and RateWatch Error and Rate

Chronometer Log

Date 23 Sept 24 Sept 25 Sept 26 Sept 27 Sept 28 Sept 29 Sept

WT 12:00:15 12:00:15 12:00:16 12:00:17 12:00:17 12:00:18 12:00:19

Signal 12:00:00 12:00:00 12:00:00 12:00:00 12:00:00 12:00:00 12:00:00

WE +00:15 +00:15 +00:16 +00:17 +00:17 +00:18 +00:19

Rate of Change: 4sec/7day = +0.6sec/day

99

Time SignalsTime Signals U.S. Radio Time SignalsU.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)

Second

WWV WWVH

0-1 0.8s Minute beep

1-45 Standard tone

45-52.5

Ticking Voice Announcement

52.5-60

Voice Announcement Ticking

1010

GPS TimeGPS 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

1111

Apparent & Mean TimeApparent & Mean Time Apparent timeApparent time

The The time measured directly by observation of the sun

Mean time Arbitrary time that Arbitrary time that averages out

irregularities in sun’s apparent motion & creates a constant 24-hour day

UT is Mean Time

12:01:08

Green Bay

12:00:44

Wausau

12:00:20

Eau Claire

12:00:00

St Paul

11-59-40

St Cloud

Sundial indicates apparent time

Zone Time is 12 – 08 – 58 (EqT)

1212

Civil vs. Zone TimeCivil vs. Zone Time Civil time, convenient and easy to use:Civil time, convenient and easy to use:

On land, determined by lawOn land, determined by law daylight saving timedaylight saving time Pages 262-265, Pages 262-265, Nautical AlmanacNautical Almanac

At sea, use At sea, use Zone TimeZone Time – divides earth into – divides earth into zoneszones each 15each 15ºº Lo wide Lo wide

1313

Time Zones Time Zones

0°15°W

30°W45°W

60°W75°W

90°W105°W

120°W135°W

150°W165°W

180°W15°E

30°E45°E

60°E75°E

90°E105°E

120°E135°E

150°E165°E

180°E

0+112 +11+10+9 +8 +7 +6 +5 +4 +3 +2 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 12

ZD 12

172

.5ºE

-12 +12 172

.5ºW

180

º

ECMP

ZM7.5º 7.5º

ZD -/+

Follow the sign to GreenwichFollow the sign to Greenwich

Follow the sign to GreenwichFollow the sign to Greenwich

Reverse the sign from GreenwichReverse the sign from Greenwich

Reverse the sign from GreenwichReverse the sign from Greenwich

1414

Time & ArcTime & Arc The GP of the The GP of the

MeanMean sun moves sun moves at a at a constantconstant rate rate 24 hours to go 24 hours to go

around the earth around the earth = 360= 360°°

In one hour, the In one hour, the mean sun will cover mean sun will cover 1515°° of longitude of longitude 360360°°/ / 24 hr per hr per

day = 15°per day = 15°per hourhour

TimeTime ArcArc 24 hours24 hours 360 36000

1 hour1 hour 15 1500

4 minutes 14 minutes 100

1 minute 15’1 minute 15’ 4 seconds4 seconds 1’ 1’ 1 second 1 second 0.25’ 0.25’

1515

Visualizing TimeVisualizing Time Earth, as Earth, as

seen from seen from the South the South PolePole

1616

Visualizing TimeVisualizing Time Observer’s Observer’s

meridian is meridian is shown at the shown at the top and top and drawn as a drawn as a solid line, solid line, labeled “M” labeled “M” and called and called Upper Branch Upper Branch of Observer’s of Observer’s MeridianMeridian

M

1717

Visualizing TimeVisualizing Time Lower Lower

branch of a branch of a meridian is meridian is 180180ºº away, away, always always shown as shown as broken line broken line and labeled and labeled “m”“m”

m

M

1818

Visualizing TimeVisualizing Time Upper Upper

branch of branch of Greenwich Greenwich meridian is a meridian is a solid line solid line labeled “G”labeled “G”

In this In this example example observer is observer is at Lo 60at Lo 60ººWW

G

m

M

1919

Visualizing TimeVisualizing Time Lower Lower

branch of branch of Greenwich Greenwich meridian is a meridian is a broken line broken line labeled “g”labeled “g”

m

M

G

g

2020

Visualizing TimeVisualizing Time Sun’s Sun’s

position position plotted as a plotted as a solid line and solid line and labeled with labeled with symbol “ ”symbol “ ”

M

G

g

m

2121

Visualizing TimeVisualizing Time Sun moves in a Sun moves in a

counterclockwicounterclockwise (west) se (west) directiondirection

Sun can be Sun can be located on located on diagram for diagram for any given timeany given time

In this example In this example it is 1500 ZT it is 1500 ZT and 1900 UT and 1900 UT on the same on the same datedate

M

G

g

m

2222

Time DiagramTime DiagramM

m

Circle represents the Circle represents the equator equator

Long tick marks drawn Long tick marks drawn every 15° (hour)every 15° (hour)

Small tick marks Small tick marks placed every 5° placed every 5° (20 minute)(20 minute)

Center represents South Center represents South PolePole

At Greenwich new day

begins Mean Sun is at ‘g’ and it’s GMT

0000

2323

Visualizing TimeVisualizing TimeM

m

gG

St Paul – 92º W of Greenwich

92º

Your day begins when

Mean Sun is at ‘m’ and it’s ZT

0000

ZT 0000

UT 0000

What time is it at your

position? ZT 1800

What time is it at Greenwich?

UT 0600

2424

UT for SightsUT 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

2525

Sight Reduction FormSight Reduction Form

Date of ObservationWatch Time

Watch Error

Zone Description

Zone Time

Computed UT

Date at Greenwich

Time Diagram

2626

Determining ZDDetermining 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

2727

How to find ZDHow to find ZD For longitude 114°15´W, what is the ZD?

To convert minutes of arc to decimal To convert minutes of arc to decimal degrees, divide number of arc minutes by degrees, divide number of arc minutes by 6060 15’ ÷ 60’ = .25°15’ ÷ 60’ = .25° 114°15´ = 114.25°114°15´ = 114.25°

114.25° ÷ 15° = 7.6, rounded to 8114.25° ÷ 15° = 7.6, rounded to 8 Since Lo is West, the Since Lo is West, the ZDZD is positive is positive

ZDZD = +8 = +8

2828

Find ZDFind 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 5ZD = –5

Longitude 172° 29´W = 172.48°172.48° ÷ 15° = 11.49Since remainder < 0.50, rounded

to 11ZD = +11

2929

Converting ZT to UTConverting 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.

3030

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?

Converting ZT to UTConverting ZT to UT

Lo is 125°W ZD is 125 ÷ 15 = 8.33 rounded to 8 Since West Lo, the sign is (+)

ZD = +81100

8+1900

3131

Using Time DiagramUsing Time DiagramM

m

Ship is at M, 125° Ship is at M, 125° west of Greenwich (G)west of Greenwich (G)

Zone meridian is Lo Zone meridian is Lo 120°W, 120°W, ZTZT measured measured from lower branch from lower branch of this meridianof this meridian

Sun located 11 Sun located 11 hours (165°) west hours (165°) west of this lower branch of this lower branch - it is 1100 ZT - it is 1100 ZT throughout Zone +8throughout Zone +8 G

g

ZD+8U

T 1900

ZT 1100

3232

ZT to UTZT 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 (-)

0600

9-2100

29 Oct

3000

28 Oct

3333

Using Time DiagramUsing Time DiagramM

m

Since Lo is East, Since Lo is East, place “G” 135° place “G” 135° west of “M”west of “M”

Since you are on a Since you are on a ZMZM position sun position sun 6 hours past “m” 6 hours past “m”

Since sun has not Since sun has not reached “g” you reached “g” you are one day are one day ahead of ahead of GreenwichGreenwich G

g

ZT 060

0

3434

ZT to UTZT to UTYour 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 (+)

2000

8+2800

4 Jul

5 Jul

0400

3535

Using Time DiagramUsing Time DiagramM

m

Since Lo is West, Since Lo is West, place “G” 126.5° place “G” 126.5° east of “M”east of “M”

Draw ZM at 120W, Draw ZM at 120W, position sun 20 position sun 20 hours past “zm”hours past “zm”

Since sun has Since sun has passed “g” but passed “g” but not reached “m” not reached “m” you are one day you are one day behind Greenwich behind Greenwich

G

g

zm

ZM

ZT 2

000

3636

ZT to UTZT to UTYour 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 (-)

2019

9-1119

1 Aug

1 Aug

3737

Using Time DiagramUsing Time DiagramM

m

Since Lo is east, Since Lo is east, place “G” 128.75° place “G” 128.75° west of “M”west of “M”

Draw ZM at 120E, Draw ZM at 120E, Position sun 20 Position sun 20 hours past “zm”hours past “zm”

G

g

zm

ZM

ZT 200

0

3838

International Date LineInternational 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?

Why were civil IDL boundaries changed in

1999?

Chatham Islands, New Zealand

French Polynesia (including Tahiti & Bora Bora)

3939

IDL WesterlyIDL 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 + 12UT 2400 6 NovZD2 - 12 (rev)

ZT2 3600 6 Nov - 24hr + 1dayZT2 1200 7 Nov (after

crossing)

4040

IDL EasterlyIDL 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 - 12UT 0000 6 Nov + 24 hr - 1 dayUT 2400 5 NovZD2 + 12 (rev)ZT2 1200 5 Nov (after

crossing)

4141

Calculate ZT Crossing IDLCalculate ZT Crossing IDL

Enroute to Guam from Honolulu, Hawaii Enroute to Guam from Honolulu, Hawaii Position fix at 1200 on 12 May 16°05’N Position fix at 1200 on 12 May 16°05’N

178°36’W 178°36’W Traveled westerly for 8 hrsTraveled westerly for 8 hrs New position fix 14°42’N 179°24’ENew position fix 14°42’N 179°24’E What is What is ZTZT & date at your second fix? & date at your second fix?

4242

Calculate ZT Crossing IDLCalculate ZT Crossing IDL

Lo 178°36’ W ZD = +12Lo 179°24’ E ZD = -12

ZT1 1200 12 May (1st fix)

ZD1 + 12UT1 2400 12 MayTravel + 8 hrUT2 3200 12 May

- 24 hr +1 dayUT2 0800 13 MayZD2 - 12 (rev)ZT2 2000 13 May (2nd fix)

4343

You are at anchor in Sydney, Australia (ZD -10), partying with your Auzzie friends, on your around-the-world cruise.

One ZT to AnotherOne ZT to AnotherYou 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.

When would you initiate your call from Sydney?

4444

ZT1 1000 31 OctZD1 +6__ St PaulUT 1600 31 Oct

ZD2 - 10 (rev) SydneyZT2 2600 31 Oct - 24 hr +1 dayZT2 0200 1 Nov

One ZT to AnotherOne ZT to Another

M

m

g

G

ZD -10

1000

ZD +6

0200 next day

4545

Sight LogSight 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

QuizQuiz1. 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

QuizQuiz2. 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 __________

+2 1958, 8 June

Determine ZD:27°43’W = 27.72° 27.72° ÷ 15° = 1.8

Therefore ZD = +2ZT 1758 8 JunZD +2 UT 1958 8 Jun

Determine ZD:154°38’E =

154.63° 154.63° ÷ 15° =

10.3Therefore ZD = -10

ZT 1758 8 JunZD -10 UT 0758 8 Jun

-10 0758, 8 June

QuizQuiz3. UT is 0443, 27 September. What are ZT

and date if you are at the following positions?

a. Lo 156° 14'E ZT _________________b. Lo 003° 58'W ZT _________________

1443, 27 September0443, 27 September

Determine ZD:156°14’E =

156.23° 156.23° ÷ 15° =

10.4Therefore 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 = 0ZT 0443 27

SepZD 0 UT 0443 27

Sep

QuizQuiz4. 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°

QuizQuiz5. 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

5151

Sea TimeSea Time

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