Home Up

 

Why is the time different around the Globe?

(i.e. Why are there Time Zones?)

First, you know that the Sun rises in the East and sets in the west because the Earth is really turning from the west to the east.  Also remember that lines of longitude go from the north pole to the south pole and that the 24 major lines of longitude are spaced 15 degrees apart around the entire sphere of the earth (24 x 15 degrees = 360 degrees).

Now imagine that in New York City it is 12:00 Noon and the Sun is at its highest point for the day as the clock strikes 12. At that moment, if you had a friend located 15 degrees away in longitude to the west, for example in Chicago, you'd expect that it would take the Sun an extra hour for the Earth to turn another 15 degrees so that the Sun would be at its highest point there because it takes the Earth 1 hour to turn 15 degrees under the Sun. If you had another friend at that moment living in Denver, they'd have to wait two hours for the Sun to rise to its highest point in the sky because Denver is about 30 degrees of longitude west from NYC.

Since the Earth rotates 15 degrees in one hour, it takes 24 hours for it to turn all the way around once, back to the point where the Sun is in the same point in the sky. (One day)

Wouldn't it be crazy if 12:00 Noon meant that that the Sun was at its highest point in New York City, but in Honolulu, Hawaii that the Sun wasn't even up yet? It would be really confusing as to what 12:00 Noon meant.  Obviously, all of the people around the world expect that 12:00 Noon means that the Sun is at its highest point - for them.

Realizing this, the world’s scientists divided yjr Earth up into 24 "Time Zones" APPROXIMATELY 15 degrees across in longitude, starting at Greenwich, England, and going around the world. There is also a line called "The International Date Line" which is the longitude where ‘today’ becomes ‘tomorrow’ (12:00 Midnight).  The International Date Line is located about halfway around the world from Greenwich.  I said approximately, because there are plenty of cases where politicians have drawn the time zone lines artificially to go around some piece of land that juts out east or west into the next longitude, where a particular country wanted to be part of one time zone and not another, or to account for some countries like Iran, Afghanistan, India, and Burma who have picked to be 1/2 hour off of the time that everyone else in the world is following :-)  What is important is that time zone lines are artificial; arbitrarily decided by politicians but trying to match the Earth's major lines of Longitude to a close extent.

The whole point of time zones is so that the Sun rises about 6:00 am local time, is at its highest at about 12:00 Noon local time and sets about 6:00 pm local time (on average), for each location on the Earth.

I have included Wikipedia references that will explain more, but if you want a surprisingly fascinating, well written account of how all of this was first envisioned and implemented, you might want to read Dava Sobel's excellent short book called "Longitude".


http://en.wikipedia.org/wiki/Time_zone

 

11/2011

05/26/2017