|
|
|
COVER STORY: What time is it anyway?
Do you know what time is it in London right now? Or in New York? Or how about Tokyo? Believe it or not, with only a watch on your wrist and a world map in your hand, you can tell the time at any place in the world, and it is all thanks to our Time Zones.
A long time ago, long before mechanical clocks were invented, people used to tell solar time via sundials. These primitive instruments used the position of the sun in the sky to tell the time. As the earth turned around on its axis, the sun appeared to travel across the sky, casting a moving shadow along the sun dial. This demarcated the time of the day. But this “solar” time as it is called, was not at all as uniform as the time in our mechanical clocks today. Today, our clocks follow the time specified by the respective time zone in which we are located.
Latitudes and longitudes
Any location on Earth is described by two numbers — its latitude and its longitude. These are imaginary lines across the globe (horizontal and vertical, respectively). On the map, these lines represent the “coordinates” along which any place can be located.
Longitudes are imaginary vertical lines all over the globe. Any place on the earth can be located east or west of a north-south line called the Prime Meridian, which represents 00 longitude. Longitudes go up to +180° eastward and -180° westward.
Latitudes are imaginary horizontal circles of different sizes. The biggest is the equator, at zero degrees (00). At the poles, the latitude rises to +90° (north) and -90° (south). Here, the circles shrink to a point. A specific longitude may be combined with a specific latitude to locate a precise position on the Earth’s surface on the map.
Determination
Determination of location is not all that longitudes do. They also determine time. The world is divided into a number of standard time zones, and these time zones have been developed according to their respective longitudes. Today, we measure the time at any place on earth via its location on a pre-specified time zone.
We all know that there are 24 hours in a day, and 360 degrees in a circle. Since the earth is circular as well, it is divided into 24 time zones and 360 degrees of longitudes. For every hour that the earth rotates on its axis, the sun appears to move by 15 degrees across the sky (360/24 = 15). Thus each time zone is spaced at 15 degrees in longitude. Within each time zone, the time of the day is defined to be the same.
But why do we need time zones anyway? Why don’t we measure time according to solar movements any more? Well, solar noon time differs, even for small distances such as nearby towns, by a few minutes. This leads to problems in making timetables for long distance travel e.g. via train or planes. By defining time zones, watches need only be adjusted by one hour across each successive time zone travelled. This eliminates the hassle of continuous time adjustment along any east-west journey.
GMT
In England there is a place called Greenwich. In 1884, the International Meridian Conference adopted the Greenwich meridian as the Universal Prime Meridian or zero point of longitude. Thus the longitude passing through Greenwich is the Prime Meridian at 00. All time zones around the world are specified by the number of hours they differ from Greenwich Mean Time (GMT), also called Universal Time (UT).
AM and PM
We often used the terms AM to indicate morning and PM to indicate noon and afternoon hours. What do these terms really mean? AM stands for “ante meridian”, meaning “before the sun crosses the meridian” or “before noon”. PM, post meridian means “after the sun crosses the meridian”, and it distinguishes morning hours from afternoon hours.
How about Tokyo?
So finally, if it is 12 o’clock noon at your place right now, 15° to your east, the time is 1:00pm, because this is the meridian which faced the Sun an hour ago. On the other hand, 15° to your location’s west, the time is 11:00am, and in an hour’s time, that place will face the Sun and experience noon. Kappish? Now dig up your atlas, grab your watch and answer me: what time is it in London right now? Or in New York? Or how about Tokyo?
Kinky time zones
Take a look at the picture on the left showing time zones. Count them and you’ll see there really are 24 time zones across the world map. But why are they so irregular? Notice how the lines are zigzagging down the map? Why do you think that is so? The fact is that these time zones are allocating a uniform time to common territories so that there isn’t a shockingly different time in the same political region. Imagine this: if a perfectly vertical time zone passing through Karachi, then one part of Karachi would have one time, while the other part would be a whole one hour ahead or behind of that time! That sounds funny, but in truth, it would be really inconvenient and impractical. To avoid such wacky problems, time zones have been deliberately kinked along state borders. The regions with dark colour adopt a time different from their time zone.
The International Date Line
So we’re all clear on the fact that there are 24 time zones around the world, and each time zone is one hour ahead of the time zone to its left. This brings us to another interesting issue: at 12 o’clock in the morning, i.e. midnight, a new day is officially starting, with a new date all together. Where exactly on the globe do we place this day-break? Obviously there must be a place where there is a difference in days … where the day actually “starts” on the planet. Here, we come to the International Date Line, which lies on the 180º line of longitude in the middle of the Pacific Ocean. It is the imaginary line that separates two consecutive calendar days. The IDL is not a perfectly straight line, and over the years, it has been kinked to accommodate the needs of various countries in the Pacific Ocean. Take a look at the picture of the IDL: you’ll see that it bends itself quite a bit to include all of Kiribati in the Eastern Hemisphere, which keeps all the Kiribati within the same day. Because of this kink, Hawaii and Kiribati are a whole day apart, although they are located in the same area of longitude.
We witness an interesting phenomenon across this Line: In the Eastern Hemisphere, left of the International Date Line, the date is always one day ahead of the date in the Western Hemisphere. If you cross the line from the east to the west, a day is added. Cross from west to the east and a day is subtracted. Interesting, eh? People travelling to and from the United States and Japan actually have to adjust to calendar date changes, rather than just hourly changes in time.
|
|
|
|
