That the lines of longitude were semi-circular joined at the North and South poles? We also said that all places on the same meridian had noon at the same time. There re altogether 180 pairs of meridians. As the earth turns round it makes a complete circle of 360degrees once in twenty-four hours –that is, it passes 15 degrees in one hour (306 degrees divided by 24 hours) or 1 degree in 4 minutes (60 minutes divided by 15 degrees).

The place from where time calculated is the prime meridian, o degrees (Greenwich). This is called Greenwich Mean Time or GMT. Places east of Greenwich see the sun earlier and gain time, while west of Greenwich see the sun later and lose time. For every 15 degrees that we go eastwards from the prime Meridian, the local time of that places will be advanced by 1 hour from the Greenwich Mean Time and for every 15 degrees that we go west words, the local time will lag behind by I hour from the Greenwich mean time.

If we know the GMT we can calculated the time of any place whose longitude is known. For example, if it is 12 o’ clock noon. GMT and the people on London are having their lunch, let us see what the people are doing in a place that is 102 degrees E 102 degrees 4= 408 minutes. = 6 hours 48 minutes. Since the place is east, the time is ahead, 6.48 P.M. so people here are almost ready for dinner while those London are having their lunch.

The place 74 degrees west (New York) is 74 degrees x4 =296 minutes before 12 noon. So the local time, there is 7.04 A.M. and it is time for breakfast.

So, all three people are having different meals at the same time because they live in different longitudes. Remember the `Kon Tiki’? As the men traveled in a westerly direction, they found their days were longer as they were traveling with the sun.

India is a very large country and many meridians pass through India. If very place had its own local time- can you imagine the confusion there would be? So we have one time for the whole of India. This is known as the Indian standard time or IST. I.S.T is calculated from 84 degree E, the longitude of Allahabad, and is 5 ½ hours ahead of GMT.