The history of clocks is very long, and there have been many different types of clocks over the centuries. Not all historians agree on the history of the clock. The word clock was first used in the 14th century (about 700 years ago). It comes from the word for bell in Latin ("clocca").
Using the Sun
The first way that people could tell the time was by looking at the sun as it crossed the sky. When the sun was directly overhead in the sky, it was the middle of the day, or noon. When the sun was close to the horizon, it was either early morning (sunrise) or early evening (sunset). Telling the time was not very accurate.
Sundial Clocks
The oldest type of clock is a sundial clock, also called a sun clock. They were first used around 3,500 B.C. (about 5,500 years ago). Sundials use the sun to tell the time. The shadow of the sun points to a number on a circular disk that shows you the time. In the big picture below on the right, the shadow created by the sun points to 9, so it is nine o'clock.
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Since sundials depend on the sun, they can only be used to tell the time during the day.
To learn more about sundials, you can go here.
Water Clocks
Around 1400 B.C. (about 3,400 years ago), water clocks were invented in Egypt. The name for a water clock is clepsydra (pronounced KLEP-suh-druh). A water clock was made of two containers of water, one higher than the other. Water traveled from the higher container to the lower container through a tube connecting the containers. The containers had marks showing the water level, and the marks told the time.
Water clocks were very popular in Greece, where they were improved many times over the years. Look at the picture below. Water drips from the higher container to the lower container. As the water level rises in the lower container, it raises the float on the surface of the water. The float is connected to a stick with notches, and as the stick rises, the notches turn a gear, which moves the hand that points to the time.
Water clocks worked better than sundials because they told the time at night as well as during the day. They were also more accurate than sundials.
Dividing the Year into Months and Days
The Greeks divided the year into twelve parts that are called months. They divided each month into thirty parts that are called days. Their year had a total of 360 days, or 12 times 30 (12 x 30 = 360). Since the Earth goes around the Sun in one year and follows an almost circular path, the Greeks decided to divide the circle into 360 degrees.
Dividing the Day into Hours, Minutes, and Seconds
The Egyptians and Babylonians decided to divide the day from sunrise to sunset into twelve parts that are called hours. They also divided the night, the time from sunset to sunrise, into twelve hours. But the day and the night are not the same length, and the length of the day and night also changes through the year. This system of measuring the time was not very accurate because the length of an hour changed depending on the time of year. This meant that water clocks had to be adjusted every day.
Somebody finally figured out that by dividing the whole day into 24 hours of equal length (12 hours of the day plus 12 hours of the night), the time could be measured much more accurately.
Why was the day and night divided into 12 parts? Twelve is about the number of moon cycles in a year, so it is a special number in many cultures.
The hour is divided into 60 minutes, and each minute is divided into 60 seconds. The idea of dividing the hour and minute into 60 parts comes from the Sumerian sexagesimal system, which is based on the number 60. This system was developed about 4,000 years ago.
Pendulum Clocks
 Before pendulum clocks were invented, Peter Henlein of Germany invented a spring-powered clock around 1510. It was not very precise. The first clock with a minute hand was invented by Jost Burgi in 1577. It also had problems. The first practical clock was driven by a pendulum. It was developed by Christian Huygens around 1656. By 1600, the pendulum clock also had a minute hand.
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As you already learned, a clock only shows 12 hours at a time, and the hour hand must go around the clock twice to measure 24 hours, or a complete day. To tell the first 12 hours of the day (from midnight to noon) apart from the second 12 hours of the day (from noon to midnight), we use these terms:
- A.M.--Ante meridiem, from the Latin for "before noon"
- P.M.-- Post meridiem, from the Latin for "after noon"
Quartz Crystal Clocks
Quartz is a type of crystal that looks like glass. When you apply voltage, or electricity, and pressure, the quartz crystal vibrates or oscillates at a very constant frequency or rate. The vibration moves the clock's hands very precisely. Quartz crystal clocks were invented in 1920.
Time Zones
Because the Earth turns, it is daytime in part of the world when it is nighttime on the other side of the world. In 1884, delegates from 25 countries met and agreed to divide the world into time zones. If you draw a line around the middle of the Earth, it is a circle (equator). The delegates divided the 360 degrees of the circle into 24 zones, each 15 degrees (24 x 15 = 360). They decided to start counting from Greenwich (pronounced GREN-ich), England, which is 0 degrees longitude. To see a larger picture of the standard time zones of the world, click the picture below.
In the continental United States, there are four time zones: Eastern, Central, Mountain, and Pacific. Each time zone varies by one hour, so when it is 7 p.m. in the Eastern time zone, it is 6 p.m. in the Central time zone, 5 p.m. in the Mountain time zone, and 4 p.m. in the Pacific time zone.
