Changing Times
| Early people didn't need to know the exact time of day. They slept when it was dark and worked when it was light. |
33,000 - 28,000 B.C..
People notice that changes in the weather (the seasons) are connected to changes in the sun, moon, and stars.
8000 B.C..
Chinese, the Maya, and Sumerians develo calendars based on the cycles of the sun (a year) and the moon (a month). Most people are farmers. Clendars tell them when to plant crops and when their animals will have babies.
3500 B.C.
Sundials come into use.The simplest sundial is a stick in the ground.When the sun shines, the stick casts a shadow. As the sun "moves" from east to west, the shadow moves from west to east. You can tell the time of day by where the shadow is pointing.
1500 B.C.
People in Egypt and Sumer develop some of the first water clocks.Unlike sundials, water clocks measure time even when it's cloudy.
A.D.1280
Around this time, Europeans build the first mechanical clock using a verge escapement. A special gear makes the clock tick.It's attached to more gears that move a hand around the clock's face to count hours. The ticking isn't perfect, so the clock can be too fast or too slow by as much as an hour every day!
Most clocks are machines with an oscillator (something that ticks to mark out equal moments of time) and a counter (something that counts the ticks).
1370-1410
European cities build towers with mechanical clocks inside. Every hour, a bell is rung for all to hear.
1656
Dutch scientist Christiaan Huygens invents the pendulum clock. A swinging pendulum moves the clock's gears evenly.
1850
Most Americans now have clocks at home. Trains keep exact schedules. Railroad workers carry pocket watches that don't gain or lose more than 30 seconds in a week.
1927
Warren Marrison and J.W.Horton build the first quartz clock at Bell Laboratories in New Jersey. When an electrical current passes through a quartz crystal, the quartz vibrates at a certain frequency. Quartz clocks tick more evenly than pendulums.
1955
An atomic clock is built in England. It measures oscillations (vibrations) of light from cesium atoms. The light oscillates even faster than quartz crystals.Atomic clocks are the most accurate clocks yet.
1990s
The Global Positioning System, or GPS is a group of 24 satellites orbiting Earth. They carry atomic clocks and can tell the exact time.
Today
People rely on internet and cell-phone networks , which need super-precise timing. Atomic clocks keep getting better.
