An hour as we know it today is made up of 3600 seconds or 60 equal minutes. There are 24 hours in a solar day, which is the one we use today. It is thought that the ancient Egyptians were the first to use a solar day and divide it up in this way. Until the 18th century and the widespread use of mechanical clocks to measure time, an hour was usually measured as the 12 th part of the period of time between sunrise and sunset (daytime) or between sunset and sunrise (nighttime). Because days are shorter in winter and longer in summer, and the length of an hour changed throughout the year.

There are also 24 invisible lines of longitude that go round the earth from the North Pole to the South Pole which indicate different time zones. If you cross one of these when you are travelling you usually put your clock one hour forward if you are travelling East or one hour back if you are travelling West. We talk about the time in hours and minutes, for example, 'It's 20 minutes past 3'. The 24 hour clock is also used, often in airports - 3.20 p.m. becomes 15.20. In astronomy, an hour is a unit used for measuring angles in the co-ordinate system.