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Our place on Earth
Latitude and longitude
You can describe your location on Earth using latitude and longitude.
Latitude states how far north or south you are - it is zero at the equator, 90º N at the North Pole and 90ºS at the South Pole. Lines of latitude run parallel to the equator.
Longitude tells you your east-west position, measured from the 0º line of the prime meridian of Greenwich. Lines of longitude run from the North Pole to the South Pole. Moving eastwards, longitude increases to a maximum value of 180ºE (also 180ºW of Greenwich as you move around the sphere of the surface of the Earth). Moving further east, you start to move back towards Greenwich. Points west of Greenwich are given in units of ºW.
The rotation of the Earth from west to east causes the apparent motion of the Sun across the sky from east to west.
Apparent solar time (or apparent local time) is measured by the changing position of the Sun in the sky and is the time shown on a sundial. In practice, the length of the solar day varies slightly, so a standardised mean Sun is used which assumes that the day is a constant length of 24 hours. Over a year, the average position of the Sun at a given time defines mean solar time.
In the UK standard time is co-ordinated Universal Time (UTC), very close to and commonly referred to as Greenwich Mean Time (GMT). This is mean solar time along the Greenwich meridian. East of this line mean solar time is ahead of GMT - the Sun reaches noon earlier than at Greenwich. To the west of this line mean solar time is behind GMT.
In a country as small as the UK, it is not practical to have many local times in use, so clocks are synchronised to a standard or civil time - GMT - across our local time zone. This zone is shared with the Republic of Ireland.Moving east or west away from the UK, countries have different time zones where clocks run ahead of or behind time in the UK. In general, neighbouring time zones have a time difference of 1 hour. In 24 hours the Earth rotates through 360° with respect to the Sun. Each time zone is then theoretically 15° wide, corresponding to a 1 hour difference in mean solar time. In practice the shape of time zones is changed to match internal and international borders.
Calculating zone time
For every 15° east or west of the Greenwich meridian civil time changes by 1 hour forwards and backwards respectively. To find the appropriate time zone, in hours, divide the longitude, in degrees, by 15.
If you travel around the world, changing standard time by 1 hour each time you enter a new time zone, then a complete circuit would mean that you adjusted your clock or watch time by 24 hours. This would lead to a difference of 1 day between the date on your clock and the real calendar date.
To avoid this, countries are on either side of the International Date Line which runs down the middle of the Pacific Ocean. If you cross the date line moving east, you subtract a day, whereas if you are moving west you add on a day.
Questions to think about
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