World Time Zones: How do we manage time changes when traveling around the world. Who tells us when to change the ship’s time. How does this change in world time zones affect us?
In today’s post I will explain how we manage different time zones. How and when we change the ship’s time. And how we synchronize our clocks with the ports of arrival.
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First of all, there are at least five types of time zones that we need to know about. They are called Greenwich Mean Time (GMT), Zone Time (ZT), Local Time (LT), Ship Mean Time (SMT) and Standard time (ST).
How is time on Earth related to longitude?
Another fun fact is that time is independent of latitude, no matter how north or south of the equator you are, time is dependent on longitude. Our earth is divided into 360 parts just like a circle, the only difference is that these 360 degrees are divided into 180 degrees east and 180 degrees west. We call them east longitude and west longitude.
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What is Greenwich Mean Time, how is it referred to?
The reference point here is the Greenwich Observatory in the UK. The longitude passing through the Greenwich Observatory is referred to as “0” degrees longitude. This meridian line is also called the “prime meridian”. The longitude east of this observatory is referred to as east longitude and west longitude.
What are the prime meridian and time zone “0”
This meridian is called the western longitude. The time that is kept here at the Greenwich Observatory is known as GMT or Greenwich Mean Time.
Our clocks all over the world are based on this division of longitude, there are 360 degrees of the world and there are 24 hours in a day, and if we divide 360 degrees by 24 hours the answer will be 15. This means that for every 15 degrees of change in longitude the clocks were changed by one hour.
To make this division easier, the time zone from 7.5 degrees west of Greenwich to 7.5 degrees east of Greenwich is 0 GMT.
What is zone time ZT, how is it calculated?
After every 15 degree change in longitude, 1 hour is added or subtracted, this one hour zone is called time zone. To find out your time zone, you need to know your longitude, for example, let’s take the longitude of Islamabad, Islamabad is located at 33.68 N, 073.04 degrees East, add 7.5 degrees to it (to compensate for GMT).
The answer is 80.5 degrees Now we already know that every 15 degrees longitude makes one hour, so now divide 80.5 degrees by 15 degrees and your answer will be 5.36. Ignore whatever is in the digits and our answer will be 5. This way we know that the time zone of Islamabad will be GMT+5 This is called ZT or Zone Time.
What is local time (LT)?
To understand LT or local time, just take your longitude and divide it by 15, the answer will be your local time, for example, let’s consider Islamabad if we divide Islamabad’s longitude 073.04 by 15 then the answer will be 4.86 hours or 4 hours 52 minutes. This is called LT or local time.
What is Local Mean Time (GMT)?
If you add Greenwich time to it, i.e. if you add 7.5 degrees to this longitude. Then your answer will be 80.54 degrees or 5.369 hours or 5 hours 22 minutes. Now this time will be called LMT or local mean time.
What is the Standard Time (ST) of any country?
Now let’s understand, Standard Time, it is different from GMT and LT or LMT. For this example, let’s consider India, from our initial examples, if we consider different cities of India, you can see that they are located in different time zones some are in zone time 5, some in 6, and some in 7. But as we all know India does not keep different time zones for different states or parts of the country, they keep one time zone all over India. It is GMT+5.5.
This is called standard time, despite the country expanding into different time zones, the government prefers to keep one standard time, this standard time may or may not correspond to the time of the zone in which this country is located.
Another example is Pakistan and Afghanistan, although most parts of Pakistan and the whole of Afghanistan are located in the same zone time, Afghanistan keeps -4.5 as its standard time and Pakistan keeps -5 as its standard time.
Unlike the above two (GMT and LT), standard time is at the sole decision of the government, but it is not so far off from the zone time. Where that country is located on the map of the earth. If they move too far away from the zone time then all the sunrise and sunset times will be very much off.
What is Daylight Saving Time (DST) and why is it implemented?
Another interesting thing here is Daylight Saving Time or DST which many countries use. In the northern spring season, the clocks advance by 1 hour and in the northern autumn season the clocks go back by one hour.
This is done to take advantage of the early rising sun in the summer season, this gives an extra hour of light a day and helps reduce electricity consumption during the summer.
But not every country does this and the exact dates on which the clocks are advanced or dimmed depends from country to country and usually a Sunday or Saturday is chosen to do so to have the least impact on people’s routine.
Let’s now talk about us, what are the important times for us and how we manage all these time changes on ships.
What is Universal Time Coordinated (UTC), why is it important for ships?
For us, 2 times are most important, first is UTC or Universal Time Coordinated and second is the standard time of countries. Now you guys might be thinking what is this UTC and some might also be saying oh it is the same as GMT captain why would you mention it separately?
No, that is not true, for the most part, GMT and UTC are quite similar. But the main building blocks of UTC and GMT are different, GMT only considers a change of 1 hour every 15 degrees rotation of the earth.
UTC takes into account the actual rotation of the earth around its axis, while GMT considers 365 days as a year. UTC adds a leap second every 19 months to keep the world all coordinated and all the clocks in the world are actually based on UTC, though the difference is small but important and that is why we actually consider only UTC when we synchronize our clocks.
Standard time is important to us because obviously, we will be maintaining the time of the country we are traveling to, for a day or two before arrival there.
What is Ship Mean Time (SMT), how do we convert time on a ship during a voyage?
But the most interesting things actually happen when we are at sea. Until then we keep a time called SMT or Ship Mean Time.
At the beginning of the post I told you guys that when we go east of Greenwich meridian time is added for every degree of longitude that we move east and subtracted for every degree of longitude that we move west.
Now a ship is always or mostly in motion at sea, and it would be very foolish if we kept changing our clocks every degree or minute or second of longitude.
How do I decide to change the time on my ship?
I personally take into account daylight hours, when the sun rises, and sets each day so that we have a full daylight available to work with and then we check what time zone we are in, we usually look up our longitude and calculate that.
The clocks actually dim or go forward an hour unless we are traveling to a country that has half an hour added or subtracted to the time zone.
What happens when a ship crosses the Dateline?
The biggest shock or thrill depending on people is when we cross the Dateline which is close and usually synonymous with the Anti-Meridian – or 180 degrees longitude, when the ship crosses the dateline.
This means either a day disappears from your life completely or a day gets added and you get to live the same day again.
How advanced are all watches really?
This process of advancing and retarding the clocks is done very fast throughout the ship with just a few touches. All the watchman has to do is to specify how many minutes or seconds or even hours he wants to adjust and once this command is executed here, all the ships clocks start advancing or retarding in harmony.
So that is all for today’s post, if you guys found this post useful or learned something from it, don’t forget to share it with your friends.
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