Why you have different time in every country?

Answer 1

If you think about it 'mid day' happens when the Sun is high in the sky directly above your head. But you know that the Sun rises in the East and then travels across the sky to set in the west and therefore it is only 'mid day' for that instant when the Sun is above your head.

The movement of the Sun across the sky each day is not caused by the Sun moving but because the Earth is spinning. As the Earth is a ball, as it spins a different part of the balls surface comes to be directly opposite the Sun as it turns. Thus while it is morning for you it is 'mid day' 90 degrees (a quarter of the way) eastwards of you. Thus every part of the Earth has a different time as measured by the Sun.

This is all very confusing so human beings have invented Universal Time (UT) which is based on one point on earth, the time on the Greenwich meridian. Using this we can agree when things happen in an absolute frame.

However this does not help people that live in very large countries (e.g. the US, Russia or China) where one side of the country can be in daylight while the other is still in the dark.

Thus as you move round the planet, depending on where you are relative to the Greenwich meridian you have different local time.

To help here the countries frequently apply time 'zones' to areas of the country on an east west extent. In China the whole country uses the time that is correct in Beijing, the US and Russia use time zones.

In general as you move round the planet in an eastally direction (starting form the Grenwich merridian, you add on an hour for each 15° of longitude you cross, up to 180°, after which a corresponding one hour decrease is made for every 15° heading westward.