10 Its Time To Learn About Time

CHAPTER 1 What Time Is It? TODAY, THE RIGHT TIME IS IMPORTANT FOR ALL OF US. BUT WAS IT ALWAYS LIKE THAT? What is time? Is it always the same? It goes slowly when we're bored and fast when we're having fun. Many older people feel that years go very quickly for them. Time is different for each of us. Centuries ago, it wasn't so important to know the right time. Many people were farmers. Farmers don't need to know the time to the minute. They must know the season and if the weather will be rainy or sunny, cold or hot. But when people began to travel a lot at sea, sailors needed to know the right time. So clockmakers made new, better clocks. Then came trains. Now minutes were important. If the clock in one station showed 10:15 and the clock in another station showed 10:00, people were late for their train. So clocks had to show the same time in every station. Today, we need to know the right time to catch planes, meet friends, and get to school on time. We look at time differently now.

CHAPTER 2 Calendars A LONG TIME AGO, PEOPLE IN DIFFERENT PLACES MADE DIFFERENT CALENDARS. Calendars are things we use to measure months, weeks, and years. A year is the time the Earth takes to go around the Sun. —A lunar month is the time the Moon takes to go around the Earth. —A calendar month is part of a year. But the Moon goes around the Earth 12.4 times each year, not 12 times. So to make a year, we have different numbers of days in our 12 calendar months. In Roman times, more than 2,000 years ago, Julius Caesar saw that the year was 365.25 days long, not 365. So he said that every four years there must be one more day, to make a leap year of 366 days. This became the Julian calendar. Now our leap day is February 29. But now we use the Gregorian calendar. About 1,600 years after Julius Caesar, Pope Gregory XIII saw that the year was really a little shorter than 365.25 days. So he made a new calendar to make the dates right. He said that the day after October 4, 1582, was October 15! Many people were unhappy. They thought this made their lives 10 days shorter! But now all the dates on our calendar are right.

In most of the world today, we use the Gregorian calendar. But there are many others. The Gregorian year 2014 is the year 4711, 4651, or 4712, in different Chinese calendars. For Muslims, it is 1435. The Muslim calendar is lunar: it uses the Moon and not the Sun. So seasons start on different dates each year. But after 32.5 years, the seasons come back to the same date. India, one of the biggest countries in the world, uses the Gregorian calendar. But it also has a Hindu calendar. The year 2014 is also the year 1936 in the Hindu calendar! In 1792, France became a republic: a country with no king. It also chose a new calendar. Weeks were ten days long. People didn't like this. Before, they worked six days and had one day off, one day when they didn't work. Now they had only one day off in ten! So the old calendar came back in 1806. The Mayans, a group of people in Central America, made a calendar a long time ago. Some people thought their calendar showed the end of the world on 12/ 21/ 12— December 21, 2012. They were wrong!

CHAPTER 3 Clocks HOW DID WE BEGIN TO MEASURE TIME? The first clock was the sundial. As the Sun moved in the sky, the dark shadow on the sundial showed the change of time. Of course, sundials didn't work when there was no sun at night or on a cloudy day! Later, people used other clocks to tell the time. In a water clock, water went slowly down from one part to the other. When there was more water in the bottom, people knew it was later. With candle clocks, the candle got shorter, and the line on it showed the time. People on boats at sea needed clocks. If they could see the sun and knew the right time, they knew where they were. But pendulum clocks didn't work on boats because a boat moves too much. In 1714, Britain asked clockmakers to make a clock for boats. There was a prize of 20,000 that's like $4,500,000 today! Clockmakers worked on this for many years. Then, an Englishman called John Harrison made a new watch with a spring, not a pendulum. It showed the right time at sea. At first, people didn't think it was true. They thought it was impossible. But the watch worked, and Harrison got his prize money in the end!

Most people use the 12-hour clock. They say "three o'clock in the afternoon" or "3 p.m." But soldiers really have to get the time right! So they use the 24-hour clock. They don't say "three o'clock." They say "fifteen hundred hours" (15:00). For 3:15 p.m., they say "fifteen-fifteen hours" (15:15). At twelve o'clock at night, the day starts again. So they say "zero zero hundred hours" (00:00), not "twenty-four hundred." Many countries use the 24-hour clock for things like trains and planes. For hundreds of years, the shortest time was a second. But today, we use shorter times. When athletes run, we need to measure the time to a hundredth (0.01) of a second. That can be the difference between best and second best. And scientists use even shorter times. Light travels one kilometer in 0.0000033 seconds! That really is a short time! So how short can time get? Is there a shortest possible time? Scientists say yes: 1043 seconds!

CHAPTER 4 Time Zones TIME CHANGES AROUND THE WORLD. WHY AND HOW? When the sun comes up in Japan, it goes down in the USA. So we cannot have the same time in every country. Around the world, there are 24 different time zones. Each time you go from one time zone to another, the time is different. Big countries have more than one time zone. Russia has nine! So when people in different parts of Russia look at their watches, they see different times. Sometimes people who live very close together are in different time zones. Florida, USA, has two time zones. There's a river between them. You live in Bristol, and you have a friend in Blountstown on the other side of the river. You want to meet. But at what time? Bristol's 4 p.m. is Blountstown's 3 p.m. It's easy to get it wrong. You must say, "I'll see you at three o'clock your time." What about phoning people in other countries? If you are in Sydney, Australia, at 11 a.m., don't phone your friend in Johannesburg, South Africa. It's 3 a.m. there!