This document discusses various concepts relating to time and navigation. It begins by describing jet lag and how time is based on the relationship between Earth and the Sun. It then discusses chronometers, which are precision timekeeping devices used to determine longitude at sea. It also discusses ship's bells, which are rung to indicate half-hour and full-hour intervals, and various timekeeping systems used in the military such as the 24-hour clock. Finally, it discusses time zones and how to convert between zone time and Greenwich Mean Time.

1. CHAPTER 3
TIME AND NAVIGATION

2. Jet Lag

3. Jet Lag
Temporary disruptions of the body’s
normal biological rhythms after
high-speed air travel through several
time zones

4. Time is based on
the relationship
of Earth and the
Sun.

5. Ship’s Clock

6. Ship’s
Chronometer
in its case
Key For Winding
Gimbal Ring
Gimbal Lock

7. Chronometer
A timepiece or timing device with a
special mechanism for ensuring and
adjusting its accuracy, for use in
determining longitude at sea or for
any purpose where very exact
measurement of time is required

8. Chronometer
• Once started, it is
never allowed to
stop.
• A record is kept
of whether it is
running fast
or slow.
• Time checks are
made against
radio time signals.

9. Radio stations in Washington, D.C.,
and Honolulu, Hawaii, broadcast time
signals every 5 minutes, 24 hours a
day.

10. Radio station announcement might be:
―When the tone returns, the time will
be 8:50 a.m. Eastern Standard Time;
8:50 a.m.‖

11. The ship’s chronometer is then checked
against the time signal by the quartermaster.
Any error is recorded, and the navigator
must take it into consideration when finding
position.

12. a.m. Ante
Meridiem,
p.m. Post
or before the
Meridiem
middle of the
day (noon)

13. 24–Hour Clock

14. Why does the military use a 24–hour
clock?

15. NAVY AIR FORCE
MARINES
ARMY
All military services use the 24–hour
clock.

16. Why does the military use a 24–hour
clock?
• To avoid the confusion that would
result if ―a.m.‖ or ―p.m.‖ were left out
of a message
• To be consistent with military time in
Europe

17. The day begins a fraction of a second after
midnight, 0000 and continues past 0100 to
noon, 1200. The afternoon continues with 1300
to 2400 (midnight).
While 0000 and 2400 are the same time, it is
common practice to start each day at 0001 and
end it at 2400.

18. As a cadet, you should memorize the
24–hour clock so it becomes second
nature when telling time.

19. What time would it be on the 24–hour
clock for the following times?
A. 7:00 a.m.
B. 12:30 p.m.
C. 6:50 p.m.
D. 9:15 p.m.
E. 12:01 a.m.

20. What time would it be on the 24–hour
clock for the following times?
A. 7:00 a.m. 0700
B. 12:30 p.m. 1230
C. 6:50 p.m. 1850
D. 9:15 p.m. 2115
E. 12:01 a.m. 0001

21. Ship’s Bell Time
• Bells are rung every
half-hour.
• One bell is struck for
the first half hour, and
one bell is added for
each half hour up to
eight bells.
• After eight bells (four
hours), the sequence
begins again.

22. Ship’s Bell Time
• An odd number of
bells marks a half-hour.
• An even number of
bells marks an hour.

23. Bells are rung only from reveille to taps,
but not during divine services or when
fog requires that the bell be used as a
fog signal.

24. Morning Watch
Time Bells Time Bells
* 0430 1 0630 5
* 0500 2 0700 6
* 0530 3 0730 7
0600 4 0800 8
* Not normally sounded

25. Forenoon Watch
Time Bells Time Bells
0830 1 1030 5
0900 2 1100 6
0930 3 1130 7
1000 4 1200 8

26. Afternoon Watch
Time Bells Time Bells
1230 1 1430 5
1300 2 1500 6
1330 3 1530 7
1400 4 1600 8

27. Dog Watch or First Watch
Time Bells Time Bells
1630 1 1830 5
1700 2 1900 6
1730 3 1930 7
1800 4 2000 8

28. Dog Watches
1600 — 1800 First dog watch
1800 — 2000 Second dog watch
The first and second dog watches
straddle the time when the evening
meal is traditionally served. Those
with the first dog eat the evening
meal after being relieved and those
with the second dog eat before
assuming the watch.

29. Evening Watch
Time Bells Time Bells
2030 1
2100 2
2130 3
2200 4
* Bells are not sounded after taps

30. How many bells would be struck
at the following times?
A. 0600
B. 0930
C. 1600
D. 1830
E. 2000

31. How many bells would be struck
at the following times?
A. 0600 4 bells
B. 0930 3 bells
C. 1600 8 bells
D. 1830 5 bells
E. 2000 8 bells

32. Time is reckoned according to the
travel of the Sun once around the
Earth each day.

33. In the sixteenth
century, the
astronomer
Copernicus
established
that the Earth’s
Rotation makes
the Sun seem to
move.

34. The Earth rotates 360° in 24 hours.

35. In 1 hour, the Earth rotates 15°.
(360° ÷ 24 hours = 15° per Hour)

36. If the Earth rotates 15° in 1 hour, it will
take 4 minutes to rotate 1°:
60 minutes (1 hr) ÷
15° = 4 minutes
Therefore...
1° longitude = 4 minutes of time

37. Time-Arc Relationship
Time Arc
24 hours 360°
1 hour 15°
4 minutes 1°
1 minute 15' (minutes)
4 seconds 1'
1 second 15" (seconds)

38. Apparent Motion of the Sun
DIRECTION OF TRAVEL
Sun Travels at 15° per Hour

39. The Sun is the
most convenient
reference for
reckoning time.

40. Solar Time
Time based on the apparent motion
of the Sun around the Earth

41. Apparent Time
Time based on the apparent position
of the Sun from your position.
If the Sun is directly over the
meridian we are on, we say that it
is noon, local apparent time.

42. Sun
23.5°
The Earth’s axis is inclined with respect
to the plane of its orbit around the Sun.

43. Sun
23.5°
Remember, the time required for a complete
rotation of the Earth on its axis, relative to the
Sun, varies according to the position of the
Earth in its orbit.

44. Mean Solar Time
It would be confusing if some days had more
hours and others fewer because of the Earth’s
revolution; therefore mean solar time is used.

45. Mean Solar Time
Average solar time, based on the
Sun’s apparent journey around the
Earth (360°) in exactly 24 hours

46. Local Mean Time (LMT)
The time at your location based on
the mean position of the Sun — that
is 1200, the mean Sun is over your
meridian, not the actual Sun

47. The Equation of Time
The difference between apparent
time and the mean time

48. NAUTICAL ALMANAC
2003 Almanac
DEC 31 2002
THRU
JAN 1 2004

49. Almanac
An annual publication containing a
calendar for the coming year, the times
of such events and phenomena as
anniversaries, sunrises and sunsets,
phases of the moon, tides, etc., and other
astronomical statistical information and
related topics

50. Cesium Atomic Clock
Atomic Time or Universal Coordinated
Time (UTC)

51. Atomic Time or Universal
Coordinated Time (UTC)
An internationally agreed-upon time
scale based on the frequency of
vibrations of the radioactive cesium
atom

52. Greenwich Mean Time (GMT)
Time based on the relationship of the
mean Sun to the prime meridian

53. Zone Time (ZT)
Local mean time along each standard
time meridian

54. Since one hour is 15°, each time zone is
based on a division of the globe into 24
zones of 15° each. A standard time-zone
system is fixed by international agreement
and by law in each country.

55. Standard Time Zone
15°-wide zone of the Earth
that has the same time

56. Zone Description (ZD)
The difference in hours between your
Zone Time (ZT) and Greenwich Mean Time
(GMT) (Use ―+‖ for longitude west, and
―-‖ for longitude east, for example +8)

57. Zone Description (ZD)
Letter and a negative or positive
number for 1 to 12, east or west of
the prime meridian

58. Daylight Savings Time
Zone time set ahead 1 hour
The practice of advancing standard
time by 1 hour in the spring of
each year and of setting it back by
1 hour in the fall in order to gain
an extra period of daylight during
the early evening

59. Daylight savings time is not used in
navigation.

60. International Date Line
• The 180th meridian
• Separates one day from the next

61. Pacific Mountain Central Eastern
The continental United States has four
standard time zones.

62. Pacific Mountain Central Eastern
Eastern Standard Time (EST) is
identified as +5 Romeo (R).

63. Pacific Mountain Central Eastern
Central Standard Time (CST) is
identified as +6 Sierra (S).

64. Pacific Mountain Central Eastern
Mountain Standard Time (MST) is
identified as +7 Tango (T).

65. Pacific Mountain Central Eastern
Pacific Standard Time (PST) is
identified as +8 Uniform (U).

66. R 111420Z9 NOV 03
291936Z NOV 03 GMT
FM USNS SATURN
TO CNO WASH D.C.
Most information in navigational tables
and naval communications uses GMT,
so you must know how to convert the
time in any zone to GMT.

67. How Time Zones are numbered
• +1 to +12 west of the Prime Meridian
• -1 to -12 east of the Prime Meridian

68. The algebraic procedure to go from
zone time (ZT) to Greenwich time (GMT),
or to go from GMT to ZT, is:
GMT = ZT + ZD
or
ZT = GMT – ZD

69. Example of using ZD to find Greenwich
Time (GMT):
1830 (ZT) - 9h 00m (ZD) =
0930 Greenwich Mean Time

70. Example of Computing GMT
Your ship is off the coast of Somalia
(ZD = –3). The local time is 1600.
What is the GMT?
GMT = ZT + ZD

71. Example of Computing GMT
Your ship is off the coast of Somalia
(ZD = –3). The local time is 1600.
What is the GMT?
GMT = ZT + ZD
GMT = 1600 – 3h = 1300

72. Converting GMT greater than 24
Note: If your calculated GMT is greater
than 24, subtract 24 from your answer
to determine the correct GMT, which
will be one day later than your ZT.
Example: At 1600 on 4 July in Adak, AL
(ZD = +11), GMT is 0300 on 5 July.

73. Change the following zone times (ZT)
to Greenwich mean time (GMT):
A. 1230 at Pensacola, FL (ZD = +6)
B. 1800 at Tokyo, Japan (ZD = –9)
C. 2200 at San Diego, CA (ZD = +8)

74. Change the following zone times (ZT)
to Greenwich mean time (GMT):
A. 1230 at Pensacola, FL (ZD = +6)
12h 30m + 6h = 18h 30m, or 1830 GMT
B. 1800 at Tokyo, Japan (ZD = –9)
18h – 9h = 9h, or 0900 GMT
C. 2200 at San Diego, CA (ZD = +8)
22h + 8h = 30h , or 0600 GMT on the
next day(30h – 24h = 6h)

75. Computing ZT
To find ZT, subtract the zone description
(ZD) from GMT:
GMT – ZD = ZT

76. Norfolk
Naval Station Example of
Computing ZT
You are stationed
at Norfolk, VA
(ZD = +5). GMT is
1800. What is the
local time?
GMT – ZD = ZT

77. Norfolk
Naval Station Example of
Computing ZT
You are stationed
at Norfolk, VA
(ZD = +5). GMT is
1800. What is the
local time?
GMT – ZD = ZT
18h – 5h = 13h, or 1300

78. Fairbanks, AL
Computation of ZD
Note: If your calculated zone time is a
negative number, subtract your answer
from 2400 or 24 hours, to determine your
ZT, which will be 1 day earlier than GMT.
Example: At 0500 GMT on 28 May, the ZT
at Fairbanks, AL (ZD = +10), is 1900 on
27 May.

79. Determine the (ZT) for the following
locations at the given GMT:
A. Guantanamo, Cuba (ZD = +5; GMT = 1615)
B. Singapore (ZD = -8; GMT = 0600)
C. Honolulu, HI (ZD = +10; GMT = 0800)

80. Determine the (ZT) for the following
locations at the given GMT:
A. Guantanamo, Cuba (ZD = +5; GMT = 1615)
16h 15m – 5h = 11h 15m, or 1115
B. Singapore (ZD = -8; GMT = 0600)
6h – (–8h) = 14h, or 1400
C. Honolulu, HI (ZD = +10; GMT = 0800)
8h – 10h = –2h, or 2200 of the previous day
(14h – 2h = 22)

81. In writing naval time, it is generally
required to place a time zone’s letter
after the numbers.
Example: 1100 a.m. in Norfolk would
be written 1100R

82. San Diego Naval Base
If it was 6:30 p.m. in San Diego how
would you write it in naval time?

83. San Diego Naval Base
If it was 6:30 p.m. in San Diego how
would you write it in naval time?
1830U

84. 291936Z JUN 03
DTG
Date/Time Group (DTG)
• Used in naval communications
• Is placed in the message heading
• Identifies when a communication
was originated

85. Date Time Group (DTG)
The date-time group is assigned for
identification and file purposes only.
The DTG consists of six digits. The
first two digits represent the date, the
second two digits represent the hour,
and the third two digits represent the
minutes.
Example 221327Z AUG 03 means the
22nd day of August plus the time in
Greenwich mean time (GMT).

86. 291936Z JUN 03
DTG
Date/Time Group (DTG)
A message is originated at 1936Z on
29 June 2003. What is the DTG?

87. 291936Z JUN 03
DTG
Date/Time Group (DTG)
A message is originated at 1936Z on
29 June 2003.
The DTG is 291936Z JUN 03.

88. +5
A ship cruising off the northern coast
of Haiti sent a message at 9:00 p.m.,
August 22, 2003.
What is the DTG of the message?

89. +5
A ship cruising off the northern coast
of Haiti sent a message at 9:00 p.m.,
August 22, 2003.
What is the DTG of the message?
230200Z AUG 03

90. TIME AND NAVIGATION
THE END

91. Q.1. What is the time used in fixing
position by celestial
navigation?

92. Q.1. What is the time used in fixing
position by celestial
navigation?
A.1. Greenwich mean time

93. Q.2. What is celestial navigation?

94. Q.2. What is celestial navigation?
A.2. Finding position by sightings of
the Sun, stars, planets, and
Moon

95. Q.3. Why is a chronometer used in
celestial navigation?

96. Q.3. Why is a chronometer used in
celestial navigation?
A.3. Because it is a very accurate
timepiece

97. Q.4. Why is the ship’s bell time
struck every half hour?

98. Q.4. Why is the ship’s bell time
struck every half hour?
A.4. Bell time originated when a
ship’s timepiece was an hour
glass which ran out of sand
every half hour and had to be
turned.

99. Q.5. How do ships at sea ensure
their clocks are correct?

100. Q.5. How do ships at sea ensure
their clocks are correct?
A.5. Time checks broadcast on radio
from Washington, D.C., and
Honolulu, Hawaii

101. Q.6. Why does the Navy use the
24-hour clock while civilians
use the 12-hour clock?

102. Q.6. Why does the Navy use the
24-hour clock while civilians
use the 12-hour clock?
A.6. 1. Old maritime custom
2. Avoid confusion in message
communications
3. Avoid possible confusion
because of A.M. and P.M.
4. To be consistent with military
time in Europe

103. Q.7. How would you write and say
the time 3:30 P.M. and 4:55 A.M.
in Navy time?

104. Q.7. How would you write and say
the time 3:30 P.M. and 4:55 A.M.
in Navy time?
A.7. 1530, fifteen thirty hours;
0455, zero four fifty-five hours

105. Q.8. How many bells should be rung
at 0730, 1430, and 2000?

106. Q.8. How many bells should be rung
at 0730, 1430, and 2000?
A.8. 7, 5, 8

107. Q.9. How often and why is the series
of bells on a ship repeated?

108. Q.9. How often and why is the series
of bells on a ship repeated?
A.9. Every 4 hours to coincide with
the watch schedule

109. Q.10. When it is noon at our location
(longitude), what is the local
apparent time at the meridian
180 degrees away from ours?

110. Q.10. When it is noon at our location
(longitude), what is the local
apparent time at the meridian
180 degrees away from ours?
A.10. Midnight

111. Q.11. Why do our calendars have 366
days in a year every four years.

112. Q.11. Why do our calendars have 366
days in a year every four years.
A.11. To account for the error in the
Earth's revolution in an
elliptical (not perfectly circular)
path around the Sun

113. Q.12. What is the relationship
between time and the arc of the
Sun around the Earth?

114. Q.12. What is the relationship
between time and the arc of the
Sun around the Earth?
A.12. The Sun travels an arc of 15
degrees each hour or one
degree of arc every four
minutes.

115. Q.13. What is solar time?

116. Q.13. What is solar time?
A.13. Time measured by using the
Sun as a reference point

117. Q.14. What is apparent time?

118. Q.14. What is apparent time?
A.14. Time based on the apparent
position of the Sun at any
given position on the Earth

119. Q.15. What is mean solar time?

120. Q.15. What is mean solar time?
A.15. It is the average solar time that
is a method to prevent
confusion due to the different
lengths of days.

121. Q.16. What is local mean time?

122. Q.16. What is local mean time?
A.16. The average solar time over a
given position for a given time
throughout the year - not the
true Sun position

123. Q.17. What is Greenwich Mean Time
(GMT)?

124. Q.17. What is Greenwich Mean Time
(GMT)?
A.17. The local mean time at the
Royal Observatory in
Greenwich, England

125. Q.18. Why is GMT important to
remember?

126. Q.18. Why is GMT important to
remember?
A.18. By international agreement,
it is the reference time used
for navigation and
communications.

127. Q.19. How many time zones are there
worldwide?

128. Q.19. How many time zones are there
worldwide?
A.19. 24

129. Q.20. How many degrees of arc are
there in a time zone?

130. Q.20. How many degrees of arc are
there in a time zone?
A.20. 15 degrees

131. Q.21. What does a date/time group
(DTG) include?

132. Q.21. What does a date/time group
(DTG) include?
A.21. The date, the time, ZD (zone
description, usually Z for
GMT), month, and last 2 digits
of the year
Example: 151635Z APR 03

133. Q.22. How many time zones cover
the contiguous U.S.?

134. Q.22. How many time zones cover
the contiguous U.S.?
A.22. Four, EST, CST, MST, PST

135. Q.23. What are zone descriptions for
the time zones around the
world?

136. Q.23. What are zone descriptions for
the time zones around the
world?
A.23. Zone descriptions are the
numbers 1 through 12,
prefixed by + west of
Greenwich and - east of
Greenwich.

137. Q.24. What is the equation for
converting zone time to GMT?

138. Q.24. What is the equation for
converting zone time to GMT?
A.24. GMT = Zone time (ZT) + Zone
description (ZD)

139. Q.25. What is Daylight Savings
Time?

140. Q.25. What is Daylight Savings
Time?
A.25. Zone time set ahead one hour
(usually in the summer) to
extend the time of daylight in
the evening. It is NOT used in
navigation.

141. Gimbals
A contrivance, consisting of a ring
or base on an axis, that permits an
object, as a ship's compass,
mounted in or on it to tilt freely in
any direction, in effect suspending
the object so that it will remain
horizontal even when its support is
tipped

142. Chronometer used
for celestial navigation.

143. Arc
Any unbroken part of the
circumference of a circle or
other curved line

144. Longitude
Measurement of position east or
west from the prime meridian

145. To better understand
navigation principles,
imagine the Earth
as standing still at
the center of the
universe, with the
Sun, as well as all
the other celestial
bodies, moving
around the Earth.

146. Rotation (of Earth)
Produces the apparent motion of the
Sun
Revolution (of Earth)
Path the Earth takes around the Sun

147. Time Zone Chart of
the World

148. Letter Designations
Each time zone has a letter designation
and zone description.

149. Time Zones
Time zones extend 7 ½° from either side
of a standard meridian, which are exactly
divisible by 15.

150. USS Dwight D. Eisenhower (CVN 69)
Remember to use the rule of algebra that
two minuses together make a plus. If we
were at a position in a time zone east of
Greenwich where the zone description
was –5, and we wanted to change a GMT
of 0600 to our standard zone time, the
formula looks like this:
GMT 0600 – ZD – (–5) = ZT 1100