# Identifying & Calculating Time Zones

Instructor: Fedora Sutton Butler
Knowing the time zones is very important for communication. To be able to identify and calculate time zones, you must know terms like lines of longitude, prime meridian and the International Date Line.

## Lines of Longitude

What time it is at different places on earth depends on something we call lines of longitude. These are imaginary lines that run from the North Pole to the South Pole. To understand how the lines are chosen, think of looking at a picture of the earth that's been sliced evenly across. What would it look like?

It would look like a circle.

A full circle is 360 degrees around. (By the way, from here on out, we'll write the term 'degree' just as 'deg' because that's how you'll usually see it when people talk about time zones.) So how does that get divided into time zones?

The line of longitude labeled 0 deg is the prime meridian and it passes through a place called Greenwich, which is in England.

Lines to the right of the prime meridian are labeled east; for example, see the line labeled 90 degs E.

Lines to the left of the prime meridian are labeled west; for example, see the line labeled 90 degs W.

If you leave one spot on the earth and you travel all the way around the earth in a circle to return to that spot, you would have traveled 360 degs.

Now, how is this related to time?

When the earth rotates, we experience day and night. We describe it as the sun rising and setting, but what's really happening is that our position on the earth is facing towards and then away from the sun. A full circle rotation of the earth on its axis towards and then away from the sun equals one day, or 24 hours.

If a full rotation in time = 24 hours and a full rotation in degrees = 360, then 24 hours = 360 degs. This also means that one hour = 360 degs ÷ 24, which = 15 degs.

This means that, as the earth rotates, it takes one hour for it to move 15 degs on its axis.

## International Date Line

On the map of the world, the lines of longitude are labeled from 0 deg to 180 degs and then back to 0 deg, for the total of 360 degs .

The 180 deg line of longitude is the International Date Line (IDL). Crossing the International Date Line results in a change of the day.

## Practice Problem

So how do we use all this information to identify what time zone we're in and figure out the difference between our time zone and someone else's time zone?

Your friend who lives in Greenwich, England, plans to call at 8:15 pm your time to wish you happy birthday on Saturday. Imagine that you live in New York City. What time and day will it be in Greenwich when your friend telephones?

### Solving the Problem

First, let's find the lines of longitude for each location. Greenwich, England is located at 0 deg. Your house in New York City is located at 75 deg W.

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