Time zones and converting time
We are all familiar with time zones, when flying around Europe and further afield. The earth takes 24 hours to make one full rotation of 360°. This means that in each hour the earth rotates 15°. Each 15° segment of longitude is equivalent to a individual time zone (Fig 5.17)
The Greenwich Meridian, where longitude equals 000°, was adopted by the Washington Meridian Conference in 1884 as the standard for world time. However GMT is no longer the basis for civil time but is loosely interchanged with UTC (Coordinated Universal Time which is shortened to UT) and should be aware that it differs ever so slightly from GMT, but the difference has no practical implications at the Day
Skipper level of navigation. So we can assume that GMT and UTC are one and the same standard for measuring time. Just for the record, GMT is an out-of-date abbreviation as it has been internationally replaced by GMAT (Greenwich Mean Astronomical Time) but is still used in UK, much like other units of measurement such as the mile, fathom, furlong and pint. Just to confuse things, you may also hear UTC/GMT time being called Zulu time by military personnel.
Countries that overlap time zones or have social economic associations sometimes unify their time zones to make trade easier. For instance, the Cape Verde Islands are 25° West of the the Greenwich meridian and still carry GMT time. Belarus is another oddity and although carrying the correct time for there zone are out of measure with the other countries around it. Time zones around the world are expressed as positive or negative offsets from UTC. That is countries to the east of the Greenwich meridian are ahead of GMT and those to the west of the prime meridian are before GMT.
UT 0 in now written as Zone 0
The Greenwich meridian, is an imaginary line running through East London, which has been accepted as the Prime Meridian. The timezone straddles Zone 0, with half an hour on one side and half an hour on the other of the prime meridian. All countries that are Zone 0 share the same time. Other countries that have Zone 0 time include Iceland, Portugal, countries in West Africa and Ascension and St. Helena Islands in the south Atlantic.
UT+1 is now written as Zone -0100
Most of Europe, from Spain to as far east as the Balkans and from Italy to Scandinavia are in this time zone. It also includes a lot of countries running down the middle of Africa. So if its 11 o’clock in UK it will be 12 o’clock in France. If we are in France (zone -0100) and its 4.30pm, what time will it be in UK (zone 0)? All we have to do is convert it into 24 hour clock, 1630 – 0100 = 1530hrs in zone 0 or UTC. When we cross the channel you probably familiar with having to put your watch forward an hour?
UT+2 is now written as Zone -0200
This zone covers Greece, Turkey and most of Eastern Europe. If we were in Athens and the local time on your watch was 5pm (1700hrs), all we need to do to when we are in zone -0200 is subtract 2 hours to get UTC time. 1700 – 0200 = 1500hrs in zone 0 or UTC
UT-8 is now written as Zone +0800
If we were on the Pacific coast in San Francisco we would be in 8 hours behind the time in the UK. So if we were in San Francisco and it was 3am on a Sunday morning, what time would it be UTC time, which is equivalent to GMT, the time we know it UK? All we have to do is convert 3am to 24hour clock, 0300 + 0800 = 1100hrs or 11am in zone 0 or UTC.