8.03 International time zones and time differences

Longitude lines and time

The Prime Meridian divides the world effectively into an eastern hemisphere and a western hemisphere. 

Looking at the map above, identify the Prime Meridian (in the centre), and how moving west (left) we have North and South America, Greenland, a spattering of African countries like Mauritania and Morocco and a few European countries like Portugal, the UK and Iceland. The rest of the world is pretty much in the east. Where the eastern hemisphere meets the western hemisphere on the opposite side of the world, (not the Prime Meridian), this is called the International Date Line (IDL). 

The International Date Line, is a special meridian. The western side is always $24$24 hours ahead of the eastern side of the line. It passes through the middle of the Pacific Ocean at about $180°$180° line of longitude but it has been drawn around some territories and island groups, so that the date change won't happen through the middle of a town.

 

 

The world is spinning on its axis as it rotates around the sun. Each full spin on its axis is $24$24 hrs ($1$1 day). We often focus on $24$24 lines of longitude, all spaced $15$15° apart, making a full $360$360° rotation. This means that roughly speaking, it takes $1$1 hour for the earth to spin $15$15° on its axis.

 

Practice questions

Question 1

 

Time zones

The world has $24$24 standard time zones, from $-11$−11 to $+12$+12, plus non-standard time zones, including $+13$+13 and $+14$+14, as well as about $15$15 different half-hour and $45$45-minute offset time zones. A time zone is a region of the Earth that has adopted the same standard time, and those regions are roughly divided up by the lines of longitude. These time zones are based on Greenwich Mean Time (GMT) or Coordinated Universal Time (UTC).

In the following map, the colour of the region is used to indicate the time zone.  

 

If you look at the coloured bar along the bottom of the map, you will see how the time zones should be broken up, according to the lines of longitude. If you compare this to the colours of the map, you will notice that the countries that follow that time zone aren't always perfectly within that zone. There are various reasons for this:

• some countries want to stay in sync with their neighbours' time zones, like many of the countries in western Europe
• many larger countries, like the USA, break themselves into multiple time zones, so that everyone experiences a similar day
• some countries, even though they are wide and cross many time zones, choose to stay with the same time zone, like China
• some countries and states, like India and Nepal, and South Australia and the Northern Territory, use half hourly or quarter hourly time zones, which are indicated with stripes

 

Greenwich Mean Time (GMT) and Coordinated Universal Time (UTC)

Greenwich Mean Time (GMT) is a time zone based upon the Prime Meridian, which goes through the Royal Observatory in Greenwich, England. Coordinated Universal Time (UTC), is a time standard, or a system of time-keeping, and is often used interchangeably, as the actual times are the same. 

All other time zones are based on how far they are behind or ahead of GMT/UTC. The zone of Greenwich Mean Time is considered to be $0$0. Countries to the west of the Prime Meridian are behind GMT and use a negative ($-$−) GMT/UTC. Countries to the east of the Prime Meridian are ahead of GMT and use a positive ($+$+) GMT/UTC.

For example, Argentina and Paraguay are $-3$−3 GMT, which means these countries are $3$3 hours behind Greenwich Mean time, so if was midday in England, it would be $9$9 am in Paraguay.  Western Australia is $+8$+8 GMT, which means it is $8$8hours ahead of GMT. If it was midday in England it would be $8$8 pm in WA. 

 

 

Calculating time differences

Using maps like the one above, tables like the one below, or websites like www.timeanddate.com, we can calculate the time difference between two cities or counties

GMT offset	Location
$-5$−5	New York, Cuba, Peru
$-4$−4	Chile, Barbados, Brazil (west), Bolivia
$-3$−3	Argentina, Brazil (east), Uruguay, Greenland
$-2$−2	south Sandwich Islands
$-1$−1	Azores
$0$0	UK, Ireland, Iceland, Portugal, Ghana, Liberia, Mali
$+1$+1	Algeria, Angola, Chad, Bosnia, Croatia
$+2$+2	Finland, Greece, Lebanon, Egypt, south Africa
$+3$+3	Russia (west), Saudi Arabia, Kenya, Madagascar, Iraq
$+3.5$+3.5	Iran
$+4$+4	Mauritius, United Arab Emirates, Armenia
$+4.5$+4.5	Afghanistan
$+5$+5	Pakistan, Maldives, Kazakhstan
$+5.5$+5.5	India, Sri Lanka

From the map above, we can see that Johannesburg is behind Tokyo. Johannesburg is $+2$+2 GMT, and Tokyo is $+9$+9 GMT, which means the difference is $+9-+2=7$+9−+2=7 hours. This means Johannesburg is $7$7 hours behind Tokyo, or alternatively, Tokyo is $7$7 hours ahead of Johannesburg. 

From the table, Chile is $-4$−4 GMT and Iran is $+3.5$+3.5. The difference is $+3.5--4=7.5$+3.5−−4=7.5 hours. As $+3.5>-4$+3.5>−4, this means that Iran is ahead of Chile. We can then say that Iran is $7.5$7.5 hours ahead of Chile, or alternatively, Chile is $7.5$7.5 hours behind Iran.

 

Practice questions 

Question 2

question 3

Question 4

Question 5