
Return to Air Navigation Time and Time Zones 
The modern calendar is based on the orbit of the Earth around the sun, and the Earth's rotation about its
axis. The time it takes the earth to complete one rotation about its axis
with respect to the sun is defined as a day. It takes the Earth approximately
365 1/4 days to complete one orbital turn. Each day is divided into 24
hours.
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A need for a standardization of time keeping evolved with the increase in international
commerce and the development of communication systems. The Earth's Coordinates System became
the natural choice as a prime reference.
The current coordinate system was agreed upon in 1884 by the International
Meridian Conference in Washington, DC. The conference established that
the Prime Meridian will be the line of longitude that passes through Greenwich,
England. The prime meridian (0 degrees) and the 180 degrees; meridian form
a great circle. The 180 degrees meridian was designated as the international
Date Line. It was agreed that at noon in Greenwich the date is changing
at the 180 degrees longitude. Greenwich Mean Time (GMT), became the international
global time reference. GMT is reported in a 24 hours system suffixed by
the letter Z (or Zulu) beginning at midnight (0000Z). GMT was later changed
into Universal Coordinated Time (UTC). The UTC time is measured with astronomical
techniques at the Greenwich astronomical observatory in England. It corresponds
with a good approximation to the mean solar time at the observatory.
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Because the Earth is constantly rotating, the time is also constantly changing. This change can be easily
calculated. Since there are 24 hours in a day (or 1440 minutes), the rate
of change is 4 minutes for each degree of longitude (1440/360). It follows
that the rate of change for each minute of longitude is 4 seconds (4/60
of a minute). However this computation is merely academic. For practical
reasons the Earth was divided into 24 time zones, each representing one
hour. Each zone is 15 degrees wide. Zone zero is the zone which extends
from 7° 29'59'' W to 7° 29'59' E. To express the time in UTC the
zone number should be added to the local time when west of the prime meridian
and subtracted when east of it.
The time zone can be calculated by dividing the longitude by 15, (i.e. the time zone on the 150 degrees east longitude is 150/15=10E), which means that the time on the 150 degrees east meridian is 10 hours later than in Greenwich. It is important to indicate that each time zone is approximately 15 degrees wide, representing one hour intervals. For example, the center of time zone 5W is the 75 degrees west meridian and it extends from the 67° 30'00''W meridian to the 83° 29'59"W meridian.
To convert the local time to UTC, it should first be converted to a 24 hour system. The time zone should be subtracted when east of the prime meridian and added when west of it. For example, 10:45 P.M. in New York is 22:45 in a 24 hour system. Since New York is in the 5 degrees west time zone, 5 hours must be added to the New York local time (22:45 + 05:00 = 27:45). A result greater than 24 hours indicates a change of the date. When that is the case, 24:00 hours should be subtracted from the result to get the correct time. By subtracting 24:00 from 27:45 we get the time in UTC as 3:45Z on the next day. Respectively, if subtracting the time zone from the local time results in a negative number, it indicates that the previous date is still valid in UTC. To get the time in UTC the negative number should be added to 24:00, i.e. if the time in Cairo, Egypt, which is located in time zone 2 dgrees east is 0100, the subtraction of 0200 hours from 0100 results in 1. This indicates that the UTC date has not changed yet and that the time is 24:00 + (1) or 2300Z on the previous day.
Economic and convenience reasons forced irregular time zone boundaries. The dividing
lines are often following international borders or topographical features.
World's Time Zones Map
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Determining a position by means of direction and distance from a known position is known as Dead Reckoning. One of the basic problems in dead reckoning is the calculation of the time that is required to travel a given distance. After determining the true heading and ground speed, the calculation of the time that is required to travel a given distance is quite simple:
Maintaining a calculated heading for a computed time, should bring the aircraft over a preselected point, providing that there is no change in the flight conditions.
Timing the legs of a flight is not only important for successful navigation but is also required for determining the fuel that is required to complete the flight. The endurance of an airplane depends largely on the fuel quantity which it carries. The time that takes an airplane to complete a flight is also important for scheduling purposes. There are several ways to calculate the arrival time of an airplane on a flight that departs from another time zone. A practical way to calculate the arrival time and date is by adding the duration of the flight to the departure time converted to UTC. To obtain the destination local time a simple conversion from UTC is required.
For example, a flight that takes 12 hours is departing from time zone 2E at 2300 local
time. The destination airport is located at time zone 5W. What will be the local arrival
time and date?
The departure time in UTC will be 2300  2 = 2100 UTC on the same day. The arrival time in
UTC is 2100 + 1200 = 3300. A result greater than 2400 hours indicates that the arrival is on
the next day and the time in UTC is 3300  2400 = 0900 UTC. Five hours should be subtracted
from 0900 to get the local time so 0900  0500 = 0400 local time on the next day.
Similarly, if a flight departs from time zone 5W at 2100 on a 9 hour flight, at what
local time will it arrive at a location in time zone 2E ?
The first step is to convert the departure time and date to UTC. 2100 + 5 = 2600. This
indicates that there is a date change. The departure time is 2600  2400 = 0200 UTC on the
next day. The time upon arrival will be 0200 + 0900 = 1100 UTC on the next day. Since the
time in zone 2E is two hours later than in UTC, the local time at the destination will be
1100 + 0200 = 1300 local time on the next day.
Another practical method is to convert the departure time to the destination time. This
is accomplished by determining the difference in time zones between the departure and arrival
point. The difference in time zones is either added or subtracted from the local time depending
on the direction traveled. The same rules apply to date changes as in the above mentioned
method.
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