Taking TIME.

Got time on your hands? Read on.

How many times do you check the clock in an hour? How many times in a day do you forget time even exists? What is time anyway? It's an ongoing mystery, but scientists think of time as a measurable--though invisible--quantity.

From the dawn of human history, people have devised infinite ways to track time. Early hunters recorded the phases of the moon by carving notches on animal bones. The Druids in prehistoric England arranged truck-size stones at Stonehenge to calculate solstices (longest and shortest days of the year) based on the sun's position in the sky.

Whether the moon, the sun, or hands on a watch--if something moves at a constant rate, humans have learned to tell time by it. Read on to learn some of these "timely" ways.

1500 B.C.

Direct Dial

Ancient Egyptians may have invented the sundial. The instrument consists of two parts: a gnomon (pointer) which sits on a dial plane (plate marked with times of day). As the sun moves across the sky, the shadow cast by the gnomon on the dial plane changes throughout the day and marks time.

When the Romans defeated the Egyptians in a war around 30 B.C., soldiers were so intrigued with the ingenious device they hauled some home as souvenirs. But once in Rome they were shocked to find sundials made lousy timekeepers. How come?

Sundials demand a different shape depending on their location. In the ninth century, Iranian astronomer al-Battani discovered that for a sundial to function properly, the needle must point toward Polaris (North star).

1000 A.D.

Gong Clock

Ancient China used fire to power this early alarm clock (below). A tar-covered rod sits in a metal-dragon boat tray--tar prolongs the rod's burn, like wax to a candlewick. The rod burns for about two hours before its flame reaches a string with two dangling copper balls. When the string singes, the balls drop and strike a brass plate to sound a "gong." The clock kept track of prayer or study sessions.

FACT:

In 325 B.C. water clocks in Greek courts timed lawyers' speeches.

1800s

Flower Hour

Thanks to the work of Swedish taxonomist (expert in plant and animal classification) Carl von Linnaeus (1707-1778), folks in the 19th century were treated to one of history's prettiest timepieces: the floral clock.

Linnaeus kept extensive records of Aequinoctales, or flowers that open and close at fixed hours of the day--the times vary from species to species. He envisioned a clock that could be made if the plants were arranged by their flowering sequence. Decades later, gardeners adopted his concept to map out time-telling flowerbeds to decorate formal European gardens.

How do Aequinoctales know when to open or close their flowers? "Basically, light controls the process," says botanist (plant scientist) Doug Walker at the University of California at Davis. When light hits outer flower petals it triggers a hormone (function-controlling chemical) called auxin that causes cells to grow and expand. The flower opens, but since its inner petals are less exposed to light, the cells remain the same and cause the flower to close once light is gone.

1961

May the quartz be with you

The quartz watch, the most common type of watch today, debuted in Switzerland. It makes use of the piezoelectric property of crystals: when electricity is applied to a crystal, it puts stress or pressure on the crystal's shape. Conversely, when stress is applied to a crystal, it generates electricity.

When quartz is placed in an electronic circuit (contained electrical path), the interaction between stress and electricity causes the crystal to vibrate at a constant 100,000 times per second. Since quartz vibrates at a uniform rate, watchmakers captured the "buzz" to make extraordinarily accurate timepieces.

How does a quartz watch work?

1. A battery emits an electrical current.

2. The current flows through quartz packed inside a small metal case. The interaction makes the crystal vibrate.

3. A tiny computer comics the vibrations and sends one bit of current, or pulse, per second to the stepping motor.

4. The motor turns a tiny gear at a steady rate. (You can't see the gear in this photo.)

5. The gear moves the secondhand at one click per second. The "tick" you hear is the sound of the gear moving the second hand. (This gear also moves other gears that control the minute and hour hands.)

FACT:

In 1914, England invented daylight saving time. The extra hour of daylight saved fuel for the war effort.

2001

Y10K 

Want a clock that will last and last? Computer scientist Danny Hillis has created a Millennium Clock for the Long Now Foundation in San Francisco, Calif. The clock is designed to tell time for the next 100 centuries. Why aim for such longevity? Says Hillis: "The reason I aimed for ten thousand years is that's about how long we've had technology." The slow-moving clock ticks once a year--its "century hand" advances once every 100 years, and it chimes every thousand years. A working prototype sits in the Science Museum in London, U.K., today, but the foundation hopes that one day Hillis' clock will loom above Nevada's Great Basin National Park.

For more on the clock: www.longnow.org

How does it work?

1. It takes one year for gravity (force that pulls all things to Earth) to drag this weight down. (Every year the clock must be wound manually: someone must lift the weight back up.) As the weight falls, it turns a corkscrew that powers a pendulum (suspended weight that moves in a constant back-and-forth motion).

2. The pendulum, made of three giant metal balls, rotates back and forth once a minute. Its movement spins gears that rotate the display-altering wheels of the clock face.

3. The face can tell when it's half past June, but not half past noon--it shows time in months, seasons, and years and years and years.

FACT:

In 1883, an American schoolteacher invented standardized time zones to synchronize train schedules.

Build a Dripping Clock

Are water clocks accurate? Build one to find out.

YOU NEED:

2-5 paper cups * jar with straight sides (an olive jar works well--the opening to the jar must be smaller than the bottom of your paper cups) * masking tape * water * clock with a second hand * straw * ruler * 2-3 colors of pens or markers

TO DO:

1 Make a small hole in the center of the bottom of a paper cup.

2 Tear off a piece of masking tape about as long as the jar is tall. Line up the bottom of the tape with the bottom edge of the jar, and stick it on the jar.

3 Tape a cup on top of the jar.

4 Cover the hole in the cup with the end of a straw, and hold it there. Do not push the straw through the hole.

5 Fill a new cup nearly to the top with water. Pour the water into the cup on the jar.

6 Lift the straw. Have a partner start timing when the first drop of water hits the jar.

7 Every 15 seconds, the timer should say "Mark!" Use a pen to mark the water level in the jar every time you hear "Mark!"

8 Record the number of 15-second units that your clock measures.

9 Pour the water from the jar into the cup without the hole. Use the same water to test your clock again, making marks with a different color pen.

CONCLUSION:

How many IS-second units did the clock measure? Were the 15-second units equal in size? What makes this a good or bad clock?

The Story of Timekeeping

[GRAPH OMITTED]

Cross-Curricular Connection

History: Research other ways that humans have tracked time throughout history.

Did You Know?

* In 3000 B.C., the Sumerians (who lived where Iraq is located today) divided each day into 12 units of time.

* In 1504, German locksmith Peter Henlein built the first watch.

* In 1803, Eli Terry began using machines to mass-produce parts for inexpensive clocks. He opened his first factory in Connecticut.

* In 1914, wristwatches became popular in the U. S., and inventor Henry Warren designed the first electric clock.

National Science Education Standards 

Grades 5-8: transfer of energy * abilities of technological design * science and technology in society * history of science

Grades 9-12: understanding about science and technology * abilities of technological design * science as a human endeavor

Resources

Clocks: Chronicling Time by A.J. Bracken, The Encyclopedia of Discovery and Invention, Lucent Books, 1991

For information on ancient calendar, go to: physical.nist.gov/GenInt/ancient.html

"The Magic Clock," New Scientist: www.newscientist.com

Taking Time

Directions: Read "Taking Time" on p. 14 and then fill in the blanks.

1. The longest and shortest days of the year are called --.

2. Cave people followed the passage of time by recording the -- of the -- on animal bones.

3. Ancient Egyptians are credited with the invention of the --.

4. Today's most common type of watch is a -- watch.

5. Carl von Linnaeus studied --, flowers that open and close at certain times of the day.

TAKING TIME

1. solstices

2. phases of the moon

3. sundial

4. quartz

5. Aequinoctales