Apparent time uses the sun’s position as the basis of determining the time – when the sun is directly overhead, the apparent time at that position is 1200 midday.
A fixed pole will cast a shadow that can be used to get a rough indication of the current apparent time (similar to a sundial).
However, if everyone used Local Apparent Time then everyone would have a different reference for the current time. Two people on opposite sides of the UK would have about a 15 minute difference in the current time.
So the idea of Standard Time was created: a region (such as a country or part of a country) uses the same reference for the current time.
The reference chosen for the whole of the UK was the Apparent Time along the Greenwich Meridian (the 0° E/W meridian of longitude).
A time zone is a defined region where the same standard time is used.
Times zones are described by their offset from UTC (more on UTC time in the next lesson). For example, Hawaii in the United States has the time zone of UTC -10.
Some countries are divided into several time zones (Australia has 3) and some use only one time zone for the whole country (China has 1 time zone).
Travelling east from the Greenwich Meridian, time increases from UTC. Travelling west from the Greenwich Meridian, time decreases from UTC. When we reach 180° east/west, there is a 24-hour change in the time. This point is called the International Dateline.
Crossing the International Dateline from west to east, you subtract a day.
Crossing the International Dateline from east to west, you add a day.