"Who decided to have a 12-hour clock? How did they choose 12?"asked a student in Karen Bromley's third-grade class at Prairie Trail School in Gurnee.
It's about time to learn about time.
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The Warren-Newport Public Library District in Gurnee suggests these books on time:
• "The Greenwich Guide to Time and the Millennium" by Graham Dolan
• "The Story of Time and Clocks" by Anita Ganeri
• "The Time Book: A Brief History From Lunar Calendars to Atomic Clocks" by Martin Jenkins
• "About Time: A First Look at Time and Clocks" by Bruce Koscielniak
• "What Time is It?" by A.G. Smith
There's a word -- chronometry -- that sums up the science of measuring time. Inventions like time and the various ways to measure it all came about when people needed to do something at a particular time.
In this case, people needed to know when to plant crops, so they invented calendars. When they needed to define smaller fragments of time, they invented clocks.
Archaeologists and historians have learned that since the beginning of time people have wanted to measure time.
The calendar was invented by astronomers who followed the days that make up the lunar cycle. More than 4,000 years ago, the ancient Sumerians, who lived in what is now the Middle Eastern country of Iraq, used a 12-month lunar calendar that included 29 and 30 days in each month.
Like today's Gregorian calendar, the Sumerians even added extra days every four years to balance out the cycle of the Earth's 365-day orbit around the sun.
Farming played a key role in Sumerian society. Abundant crops meant people could try their hands at other activities. Over time, they developed a complex alphabet using cuneiform letters and kept records on clay tablets about everything from stored grain to business transactions.
They wrote stories like the Epic of Gilgamesh, a legend crafted on 12 tablets. They created laws called the Code of Hammurabi.
The need to divide time in smaller segments fueled the invention of the sundial, the clepsydra or water clock, and the gnomon -- a stick planted in the ground that casts a shadow to indicate the time of day.
Some sources say these time inventions were the very first inventions developed by mankind.
Both sundials and wristwatches mark off 12 hours. How did inventors come up with the number 12 for their timepieces?
Sumerians used a 60-base counting system. Sumerian mathematicians thought up 360-degree circles and used algebra, geometry and square roots to add, subtract, divide and multiply.
Units of 12 fit into 60 five times, and historians believe the importance of the number 12 is based on the number of finger joints on one hand (excluding the thumb). Sundial faces were crafted with 12 marks that separated the day into 12 segments. Later civilizations doubled that to create two 12-hour shifts to mark off one day.
The Sumerian interest in the number 12 was passed through the generations and continues today. We use 12 to count one dozen -- one dozen eggs are nestled in a carton; one dozen donuts are packed into a take-home box. Our ruler measures 12 inches. Clocks tick off 60 seconds for one minute and 60 minutes to complete an hour.