Page 202 - NAVAL SCIENCE 3 TEXTBOOK

II Time and Navigation

You probably ha\'e not gi\'en much thought to the stud)1 of time. ters. All)' error is recorded, and the navigator must take it into con-
The navigator of a vessel, however, needs to know the exact time in sideration during celestial navigation.
order to determine the vessel's position at sea.
If YOli have e\'er taken a long plane trip across the country. or
Military T ime
to Hawaii or Europe, you have felt the effects of time zone changes.
It often takes a day or two to get one's body adjusted to the new We all know how to tell time by watches and clocks. We know that
time schedule-when to get up. when to eat, when to go to bed. the new day begins a fraction of one second after midnight. and
This condition is called jet lng, a term used to describe the effect concludes twenty-four hours later at midnight. The time between
of rapid time zone changes on the body, as for example when you midnight and noon is labeled "A.M."; these letters mean allte meri-
take a long east-west plane flight. It occurs because your body is dielll, or before the middle of the da)' (noon). The time between
used to reckoning time based on the relationship of the Earth and noon and midnight is labeled "P.M.," meaning post meridiem, or
the SUIl. As we will see later in this chapter, this Illeans that your after the middle of the day. We are comfortable with this system
day becomes considerably longer jf you are traveling in a westerly of timekeeping in civilian life, because we can simply look out the
direction, and shorter if you are traveling easterly. window. so to speak, to see if it is morning or afternoon. \,Ve do not
In this chapter we will discllss how time is reckoned, the instru- conh.lse 5:00 in the evening with 5:00 in the morning, to say noth-
ments the Navy uses for timekeeping, and how to deal with time ing of I :00 in the morning and I :00 in the afternoon.
zone changes around the world. But it has long been a custom in the Navy and other military
services to tell time by the twentypfour-hour clock. There are sev-
eral reasons for this. It is done to avoid confusion in message COI11-
Time and Timepieces
munications, all of which are identified by the date and Greenwich
Everyone is familiar with watches and docks. In the Navy, time and Mean Time of transmission. Also. this is a common way to tell
timekeeping are of great importance. both because the routine of time in many European countries. including England, where many
shipboard life is often fast-paced. and because time is essential in of our military customs began. It is also another way to nvoid the
navigation and operation of the ship. As part of their duties in most confusion that might happen if the A.M. or P.M. were accidentally
Navy ships, c\'eq' couple of days the quartermasters check and reset left out of directions or orders.
as necessary all ship's clocks to the correct time, so that everyone So the Navy, and other milital')' services, lIses the twenty-four-
can be sure they are using an accurate time in their log entries. tacti- hour clock. In this way of keeping time, the day begins with a frac-
cal plots, messages, and all other phases of their daily routine. tion of a second after midnight, 0000 (zero hour), and continues
In addition to the usual types of watches and clocks, there are past 0100 (I:OO A.M.) and 0200 (2:00 A.M.) toward noon, 1200.
more specialized timepieces found on board ship. These arc called The time after noon continues with 1300 (I:OO P'M.), 1400 (2:00
chrollol1leters. A chronometer is an extremely accurate timepiece P.M.), and so forth until 2400, midnight. The terms "o'clock;'
used in navigation. It is made to withstand shock, vibration, and A.M., or l~r\'l.. are not lIsed, nor is a colon used to separate hours
temperature variation. Years ago ship chronometers werc mcchan- from minutes. Rather, we speak in terms of "hundred." For exam-
ical clocks, but nowadays they are electronic quartz clocks. They ple, 0100 is "zero one hundred;' 1000 is "ten hundred;' 1800 is
are set to Greenwich ~'Iean Time (GMT), the basic time used in "eighteen hundred," and 2130 is "twenty-one thirty."
fixing position by celestial navigation, and the time llsed as a refer- All NjROTC cadets should learn to use the twenty-four-hour
ence in all message traffic and many other things aboard ship. clock. Mathematically, it is vcry easy to figure out; simply add all
Radio stations in Colorado and Hawaii broadcast time sig- P.l\'i. time numbers to 1200 (noon). For example. 2:25 P.M. becomes
nals every five minutes, twenty-four hours per day. Time signals 1425, and 10:30 P.M. is 2230. You should memorize the twent)'-
are also obtainable from GPS and loran. The ship's chronometer is four-hour clock so it becomes second nature when telling time:
periodically checked against these time signals by the quartermas-

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