We all know what it means to say that time is passing by, but it is very hard to say exactly what time is. We notice natural things around us change with time - plants and animals, the seasons and weather, the Sun, Moon and stars. Time is the interval between one instant and another. Anything that changes regularly can be used as a 'clock' to keep track of time. Now that people all round the world are in contact through telephones, the Internet, television and fast air travel, it is important that everyone agrees to keep to the same standard time and measures it in the same way.
History of Timekeeping
People originally measured time by the position of the Sun in the sky. Later, they began to rely on machines for timekeeping, with mechanical motions that were repeated over and over again. Today, people use clocks and watches that can measure time in fractions of a second.
Days and Hours
The day is the most important period of time for us. Our daily patterns of sleeping, eating and working is almost built-in as a kind of natural clock. We divide days up into 24 equal hours. Hours have not always been equal. Before mechanical clocks came into use in the 14th century, an hour was onetwelfth of the period of daylight, so in winter hours were shorter than in summer. The number 24 has been handed down from the distant past. It might have been any number. The ancient Babylonians based their numbers on units of 60, rather than units of 10 like us, and it is from their example that we use 60 minutes in an hour and 60 seconds in a minute.
Days start at midnight and we use a.m. and p.m. to show whether a time is before or after the middle of the day. They are short for the Latin words ante meridiem and post meridiem. The 24-hour clock is often used instead to avoid confusion - 1.00 p.m. is called 13.00, 2.00 p.m. becomes 14.00 and so on.
Measuring and Keeping Time
Anything that happens at regular intervals or at a steady rate can be used as a clock to measure time. It could be the burning of a candle, the swing of a pendulum or the natural vibrations of tiny atoms and molecules. Quartz crystals vibrate regularly, and clocks based on them can accurately keep track of time.
The spin of the Earth gives us our day. Astronomers keep a check on the Earth’s rotation speed by watching the stars travel across the sky at night. It used to be the main way of keeping standard time accurate. Today, though, ‘atomic clocks’ have taken over as the best time-keepers.
International atomic time is kept by special clocks in laboratories round the world. These are the clocks that are used to give the time pips broadcast by radio services and the speaking clock service on the telephone, so that anyone can set a clock accurately.
Standard Time
The time measured at Greenwich, London, is the standard time for the whole world. For every 15 degrees of longitude east or west of Greenwich, the time is one hour ahead of or behind Greenwich time.
Gregorian Calendar
Most Western countries use the Gregorian calendar, which is based on the time it takes for the Earth to circle the Sun. At first, the Romans began their year in March, which is why September to December are named after the Latin numbers seven to ten. Later, in about 150 B.C., January became the first month of the year.
Days and Years
The Earth orbits the Sun in one year. At the same time, the Earth spins on its own axis. It completes one spin in a single day and 365¼ spins in a year. The quarter day is impractical, so after four years they are added together, and every fourth year has 366 days; a leap year. The Earth is slowing down: a few million years from now, leap years will not be needed.
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