Image: wikipedia.org

Radhika Bhandari 

B.E. 4th Year Geomatics Engineering, Kathmandu University, Nepal

Recently, a spacecraft Cassini that was rotating Saturn for thirteen years was destroyed after receiving its final signals at time 7:55 am ET on 15th September 2017. We might wonder what the acronyms ET and likewise GMT, EDT, etc may refer to. They all denotes the time zones around the world. Prime Meridian (PM) which is 0˚ longitude, passes through the Royal Observatory in Greenwich village, is the reference meridian used in distinguishing time zones. And the time at this point is called Universal Coordinated Time (UTC) which was prior known as Greenwich Mean Time (GMT). If we are to the East of PM we are ahead of GMT in terms of time and vice versa for being to west of PM.

Why time zones?

Differentiation of time zones was done to simply understand the point of day occurring in any part of world. If we are following same time system throughout the world it will occur that while it is midnight in Ottawa, it would be midday in Jamaica. This will only result in confusion. This is the main reason behind creating time zones. We have total 24 time zones covering whole world each with roughly 15˚ longitude extent.

Nepal’s context

In Nepal we see time being allotted as +05:45 UTC. The ‘+’ sign denotes that we are to the East of Prime Meridian and 05:45 is the difference in longitudes expressed as time. Time is calculated according to the standard meridian used by that country and Nepal uses standard meridian of 86˚20’ minute passing through Mount Gaurishankar. Since earth rotates completing 360˚ in 24 hours that gives each degree longitude equal to four minutes. Then we multiply the value 86˚20’ (86.333˚) by 4 which gives 345.3333 minutes. This is then divided by 60 to convert it into hours (5.7555) after which the value is consecutively converted to minutes and seconds. And finally we get value 5 hours 45 minutes 20 seconds.

Now with this basic concept we can easily calculate time differences between two places given that we have values of meridian of those places. Let’s take example of Sydney and West Virginia whose longitudes are 151.2093˚E and 78.6569˚W.
Difference in longitudes = 151.2093˚ + 78.6569˚ = 229.8662˚
We have added the longitudes because they lie to different sides of PM(time differences increases when two longitudes are at two sides). If both of the longitudes are either to the east or to the west, we subtract the longitude values.
229.8662˚ = (229.8662 * 4)= 919.4648’ = 15 hrs 19 mins 28 seconds
This means that Sydney time is 15 hrs 19 mins 28 seconds earlier than that of West Virginia.

Time Zones (Image:www.timetemperature.com)

International Date Line (IDL)

The IDL is the longitude exactly opposite to the PM, that is 180˚ longitude. It is passed in such a way that won’t be passing through any country’s physical boundary because if we pass across this line, we need to change our date and no country would want two dates within its boundary. While going from East to West of IDL, we increase one day in calendar and decrease a day while moving from West to East. But not every two places we are comparing lying on the either sides of IDL will have a 24 hour difference. It depends on which time zones they fall and differences in their longitudes.

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