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Do you know how Local Time is determined? UPSC geography

You might have heard about time standards like IST (Indian Standard Time ),

GMT ( Greenwich Mean Time ). Ever Wondered How are they defined?

Let's dive deep into how they are defined.

In our last blog, we saw how latitudes and longitudes are defined and how any point on earth is defined. In this blog, we will talk about how time zones are defined.

Image source: Kurdistan planetarium

We know that earth rotates from west to east and the axis of the earth is tilted towards the sun at an angle of 23.5 degrees. This tilt is responsible for different seasons and varying lengths of Day and night.

So at any point in time, a part of the earth receives sunlight and the other half is dark. Consider the above image, we see that the sun is at 90 degrees to the Tropic of Cancer and hence is overhead over it. This results in summers in the northern hemisphere as it receives more sunlight and there is winter in the southern hemisphere as there is darkness.

Also, we notice that more area in the northern hemisphere receives sunlight and less area is dark due to which the northern hemisphere gains more heat and loses less heat hence there is summer.

Now we see that the earth is rotating from West to East and rotation is about 1˚ in 4 minutes. (15 degrees in 1 hour). So now imagine a point on the earth's surface. This point will also rotate 1 degree in 4 minutes from West to East.

Now consider a point on the dark side of the earth in the above figure. Can we say that since the earth is rotating from west to east, that point will travel from the dark side to the brighter one as the earth rotates? Yes, definitely that is why we see days and nights. When It's Night our country is on the dark side and with the rotation of the earth our country shifts from dark to brighter side and hence comes the day.

Now consider a snapshot of the Earth at a particular point in time below :

Let's consider two countries A & B on the dark and bright sides of the Earth respectively. Now there is night in country A and morning in country B. Let's say it's 11:00 Pm in country A and 5:00 Am in the morning in country B. So in other words can I say that country B is 6 hours ahead of country A (11:00 Pm to 5:00 Am is 6 hours) as there is already morning in country B while there is still night in country A.

So can we now say that as we move from West to East there is Gain in Time and therefore opposite would be true as we move from East to West (ie. there would be loss of time as we move from East to West ).

In our last blog we have already studied prime meridian. It passes through Greenwich, Near London, and is defined as 0 degrees longitude. We define East and west with respect to Prime meridian that is also how we name longitudes. East and West of the prime meridian. So in short if we move east to prime meridian we gain time and vice versa.

Next, let's see how much is time gain and loss.

As the earth rotates every 15˚ in 1 hr. So for every 15˚we move from west to east, 1 hr is added. Example: Madras is 80˚E of prime meridian ie we have to move 80˚E east from prime meridian to get to madras and hence the local time would be 1*80/15 = 5.33 hr ahead of Greenwich.

Calculation :

For 15 degrees = 1 hour added

For 1 degree = 1/15 hours added

For 80 degrees East = 80 * (1/15) hours = 5.33 hr added

The reverse happens when we move towards the west (So if we move from madras to Greenwich we would lose 5.33 Hr of time). But each city cannot keep its own time zone, so a country defines one standard time zone w.r.t to GMT (0-degree longitude). GMT ( Greenwich mean time) is a standard w.r.t which every country defines its time.

India has defined 82.5˚E as the standard time zone (5 hr 30min ahead of GMT). Larger countries like Russia have multiple time zones as there is significant loss and gain of time moving across the continent.


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