Page 205 - NAVAL SCIENCE 3 TEXTBOOK

~ I ARI N E NAVIGATI O N 211

The standard time zones begin at the Greenwich meridian (0°). \·"hen llsing the formulas, YOllmust be careful to remember to
Since the Earth rotates toward the east, time zones to the west of use the rule of algebra that two minuses together make a plus. For
Greenwich are earlier; to the eaSl, the zones are later. Eveq' merid- example, if we were at a position in a time zone east of Greenwich
ian east and west of Greenwich that is a multiple of IS degrees where the zone description was - 5, and we wanted to cOllvert a
(15°, 30°, 45°, 60°, and so on) is a standard time meridian. Each GrvlT of 0600 to our standard zone time, we would set up the for-
standard time meridian is at the center of its time zone, nnd the mula like this:
zone extends 7'/2 degrees (half of 15 degrees) on either side of the
ZT =GMT-ZD
meridian. Some stnndard time zones ashore vaq' somewhat from
ZT = 0600 - (- 5) = 1100
this, to make life easier for the people lil'ing there.
Local mean time along each standard time meridian is ZOIlC Standard time zones are also described by letter suffixes. as we
tilllC, or standard time, for that entire time zone. Zone time in noted earlier. In writing naval time, it is generally required that a
navigation is abbreviated ZT. Each time zone is identified by an time zone's suffix letter be placed after the numbers. For instance,
alphabetical letter and by a negative or positive number from 1 to eleven o'clock in the morning in Norfolk, Virginia, zone time,
12, east or west of the prime meridian. The number is called the would be written IIOOR; 3:30 P.M. in San Diego would be written
zOlle descriptioll (ZD) of the zone, and the letter is called the suffix. I 530U. This avoids confusion for the persall reading that time.
Time zones to the east of Greenwich have negative zone descrip-
tions, and time zones to the west have positive ones.
DatefTime Group
To sepnrate one dny from the next, the 180th meridian in the
mid- Pacific Ocean has been designated the llltematiolln/ Dnte Another aspect of naval time involves the date, the month, and the
Lillc. On both sides of the line, the time of day is the same, but west year in naval communications.1vlcssages and other data keep com-
of the line it is one day later than it is to the east. ing twenty-four hours a day, every day of the month, and every
The continental United States has four standard time zones. month of the year. Therefore, there must be some way to identify
The East Coast keeps +5 Romeo (R) time, called Eastern Standard exactly when a communication was originated or received. To do
Time (EST). Central Standard Time is +6 Sierra (S), Mountain so, the Nav}'uses what is called the nafe/till1cgrollp.
Standard Time is +7 1bngo (T), and Pacific Standard Time is +8 Let us assume that a message was originated at 1635Z on
Uniform (U). All of Alaska keeps + 9 Victor (V), and Hawaii keeps 15 April 2010. This would be placed in the message heading as
+ 10 \'\~1iske)' (W). 151635Z APR 10. The first six numbers and letter Z show the date,
Daylight savings time. A is simply zone time set ahead one hour the time, and the ZD suffix (usuall), Z for GMT). For further clari-
to extend the time of daylight in the evening, usually in summer, fication, the month and year are also added. All messages are sub-
is done strictly for convenience ashore in some localities. Daylight sequentl), filed and referenced b)' their originator and the datel
savings time is not used in navigation. time group.
Zone time and GMT. Greenwich Mean Time (GMT) is the ZT
at the Greenwich meridian. The Greenwich meridian is the stan-
dard time meridian for the time zone numbered o. It has the zone
STUDY GUIDE QUESTIONS
description suffix letter Zulu (Z). l\,lost information in naviga-
tional tables and naval cOlllmunications uses GMT, so you must
I. Wh), do )'ou sufIer jet lag if )'ou take a long east-west plane
know how to convert the time in any zone to GM1~ and vice versa.
flight?
Remember that the solar day has twenty-four hours, and
2. A. \·"hat is a chronometer?
each time zone represents one hour. Beginning with the 0 zone
(Greenwich), time zones rlln east and west from zone I to zone S. To what time is a ship's chronometer usually set?
12, with zones east of Greenwich being minus, and those west 3. \"hat do A.~1. and P.~1. mean?
of Greenwich being plus. (Note that the + 12 and -12 zones each 4. \·Vhy do the Navy and other armed services use the twenty-
extend over onl), 7'/2 degrees of longitude.) The zone description four-hour clock?
(ZD) tells YOll the difference in hours between your zone time and
5. \·"hat would the following times be 0 11 the twenty-four-hour
GMT 1b convert the time in any time zone to Greenwich Mean
clock?
Time, )'Oll must ann the zone description number algebraically to
A. 8:30 A.'!.
the zone time. To convert from GMT to zolle time. you must sl/b-
fmet the zone description. This procedure can be represented by Il. 5:45 P.,".
the following simple algebraic formulas: C. 11:15p.,1.
GMT = ZT + ZDand ZT = GMT -ZD D. Midnight