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Zone Time / ETA's

Zone Description - How to correct local time to get to GMT

GMT - Greenwich Mean Time - the starting point for measuring time zones.

The Earth makes one revolution around its axis every 24 hours.  It takes 24 hours from sunrise on day to sunrise the next day.  If the Earth did not also orbit around the Sun we would not have seasons & the time of sunrise at a specific location would not change from day to day.

We want time to the same around the world.  It should be light outside when we wake up & go to work & dark out when its time for bed.  If the Earth had only one time we wouldn't have that.  Sunrise happens when the sun is within the line of sight of that specific location.  When the sun is rising in one part of the world, it is setting some where else.

We break the Earth up into 24 time zones so the the time of sunrise is just about the same time of day throughout the world.

The Sun move 15 degrees of Longitude over every hour in the day.  We break the time zones based on the time at the center point of the time zone or the average time for that zone.

International Date Line - the staring of the day.

How to determine zone description.

Longitude divided by 15 = zone description. 

Zones must be whole numbers so round the number off.

+ zd in the Western Hemesphere

- zd in the Eastern Hemesphere.

Each zone description is 15 degrees of longitude wide.

The Prime Meridian 000 degrees longitude = ZD 0, GMT, UTC (Universal Time Coordinate) or zulu time.

ZD 0 goes from 007.5E to 007.5W

015W / 15 = zd +1 (+ because it is in the West.)  ZD +1 goes from 007.5W to 022.5W, centered on 015W.

The International Date Line = 180 degrees

180 / 15 = 12.  On the East side of the Date Line is zd -12. 

On the West side of the Date Line is zd +12.  It is the same time on either side of the date line but a different day.

Time/Date in each zone:

zd-12     0800     3/18     0800 (3200) - 12 = 2000 GMT 3/17

zd-11     0700     3/18

zd-10     0600     3/18

zd-9      0500      3/18

zd-8      0400     3/18

zd-7      0300     3/18

zd-6      0200     3/18

zd-5      0100     3/18

zd-4      0000     3/18

zd-3      2300     3/17

zd-2      2200     3/17

zd-1      2100     3/17

zd 0      2000     3/17

zd+1     1900     3/17

zd+2      1800     3/17

zd+3      1700     3/17

zd+4      1600     3/17

zd+5      1500     3/17 (NY time - 075W/15= 5)

zd+6      1400     3/17

zd+7      1300     3/17

zd+8      1200     3/17

zd+9      1100     3/17

zd+10     1000     3/17

zd+11     0900     3/17

zd+12     0800     3/17   0800 + 12 = 2000 GMT

 

Long  012E / 15 =  0.8 rounded to 1 =  zd -1 (East is -)

Long 152W / 15 = 10.1333 rounded to 10 = zd +10 (West)

Remember - ZD is how we correct to GMT. If we are going from GMT then the zd sign is reversed! No need to worry about the Date Line Day if you always go to GMT, apply the transit time & then go from GMT.


Start with time of Departure in zone time (local time where ever you are)
Apply the ZD to get TO GMT - if it is +10 then add 10 hours.
Now you have the GMT time of Departure.

Now get the transit time. -
Dist. / Speed = time in hours
hours /24 = days
subtract out the whole # of days & put it off to the side
now 0.xxx days X 24 = hours
subtract out the whole # of hours & put it off to the side
now 0.xxx hours X 60 = minutes (you can round the # here)

go back to the GMT time of departure & add in the transit time of days,
hours & minutes

Now you have the GMT time of arrival
You have to go FROM GMT to ZT so reverse the ZD of the arrival port & apply
it to the GMT time of arrival.
(if the zd of arrival is zd -10, then you add 10 hours to GMT to get the
zone time of arrival)

Remember - there are only 24 hours in a day - if you have more than 24 hours
you have to subract out the 24 hours & add 1 day to the arrival date.
30 days hath September, April, June & November
Feb. has 28 or 29 days
All other months have 31 days.

Time Zone Chart - see "Favorite Links" Page