It's about time

Once upon a time, there were no such thing as hours, minutes, and seconds - only days, nights, and seasons. The sun and moon marked the passage of day and night. Changing weather, falling leaves, and blooming flowers told of the turn in the seasons.

The first "precision" timepiece wasn't something you could put in your pocket. It wasn't very precise, either. The massive stones, pits, and mounds of England's Stonehenge served as pointers to the setting of the sun and moon at various times of the year.

Some say Stonehenge is a sun temple. Others claim it's an early astronomical computer. Whatever it is, every June 21 the sun rises directly over the great stone called the Friar's Heel when viewed from the central Altar Stone. June 21 is the summer solstice, the first day of summer in the Northern Hemisphere. It is the "longest" day, meaning it has more daylight hours than any other day of the year.

On the "shortest" day of the year, the winter solstice (Dec. 21), a similar event occurs. Computer analysis shows that certain of the stones may be lined up to forecast eclipses of the sun and the moon. Obviously, the stones were precisely placed by keen observers of the sky.

The Saxons found Stonehenge when they landed in Britain 1,600 years ago. Even then, Stonehenge was ancient. Scientists say it dates back about 4,000 years.

Dawn of the sundial

Sundials ushered in the hour.

The earliest sundial was a stick pushed straight into the ground. The stick cast a shadow that moved across the ground as the sun moved across the sky. The shadow touched certain points at certain times.

Ancient Chinese and Egyptians kept time with sundials. The Greeks even had portable sundials - early pocket watches, perhaps. The first literary reference to a sundial is the "dial of Ahaz" in the Bible, in the 8th century BC. (See II Kings 20: 8-11 and Isaiah 38: 4-8.)

Early sundials did not keep good time. As the days grew longer in summer and shorter in winter, so did the length of the "hours" the sundials recorded.

By AD 100, sundials were much more accurate. Two design improvements were responsible:

First, the sundial's pointer, or gnomon (NO-mun), was aimed due north. Second, the gnomon was tilted so that its angle matched the sundial's latitude.

How did ancients know true north and their latitude?

Easy: When the sun is highest in the sky (noon), the shadow of a stick stuck straight in the ground will point due north. Now tilt the stick so that it points directly at the sun (you'll know when this happens because the stick's shadow will disappear). The angle of the stick will equal your geographical latitude. That's what latitude is: your location's angle relative to the sun.

Now sundials were accurate almost to the minute. In fact, through the 1800s, French railroaders used precision sundials to set their pocket watches. Town clocks often had sundials, too.

Sundials were a good solution to timekeeping. But they recorded only the sunny hours. How could you tell time at night or on cloudy days?

The oldest known water clock originated in Egypt around 1400 BC. It had a cone-shaped container with a hole in the bottom. The container held water that dripped into a vessel with a scale that marked the passage of time. About 100 BC, the Greeks made a water clock with a piston that moved an hour hand.

But water clocks weren't very accurate. Water draining from a container does not flow at a constant rate. And water can freeze.

Sometimes a burning candle or slow-burning rope was marked to show the passage of time. But until the Middle Ages, most people were farmers. And farmers didn't need to know the precise time of day.

Religion gives clocks a boost

The rise of religion and commerce in the Middle Ages called for more precise timekeepers. Devout Muslims needed to know the hours of prayer, so did Christian monks and nuns in monasteries and convents. Beginning in the 1100s, banks and other businesses needed to observe time reliably and accurately.

Mechanical clocks that relied on weights and gravity arrived in the 1200s. One of the earliest clock towers, built in 1354 in Strasbourg, France, still stands today. Clocks began to miniaturize after 1510, when Peter Henlein of Nuremberg, Germany, invented the spring-powered clock.

But it wasn't until 1656 that the pendulum clock was born. A pendulum moves at a fairly constant rate. Dutch scientist Christiaan Huygens's clock was accurate to less than one minute a day. Later, Huygens made clocks accurate to less than 10 seconds a day. (How did they know? They checked it with a sundial.)

Variations in temperature caused the length of a clock's pendulum to expand and contract, making the clock run fast or slow. When George Graham, a Briton, found a way to compensate for this, clocks became accurate to one second a day.

Who could want more? Sailors.

Timekeepers at sea

Pendulum clocks were fine on land, where it was stable. But sailors needed a clock that kept accurate time despite a ship's side-to-side and up-and-down motion.

They needed reliable, accurate clocks to navigate accurately at sea. A captain required only the North Star and a sextant (a device for measuring angles) to find his latitude - how far north or south he was. But longitude - how far east or west - was harder. Longitude could be determined by knowing the difference between what time it was at sea and what time it was at home.

It was easy to tell time at sea. When the sun reached its highest point in the sky, it was noon. So if it was noon at sea, and the clock you brought from home said 10 a.m., you knew that your ship had traveled two hours west. Knowing the circumference of the earth, you could translate hours (plus or minus) into miles traveled (west or east).

Determining longitude at sea was such an urgent problem that in 1714 Queen Anne of England offered the phenomenal sum of 20,000 (about $1.5 million today) to anyone who built a device to measure longitude at sea to within 30 miles. Decades later, John Harrison of England qualified for the reward with his marine chronometer No. 4. Upon its return from a two-month journey to Jamaica in 1761, his timepiece was found to be only 5 seconds slow.

More constant than the earth

Today's quartz watches are probably about as accurate as some of the precious chronometers used by 18th- and 19th-century navigators. In a quartz timepiece, electricity from a battery makes a quartz crystal vibrate at a constant rate, like the constant rate of a pendulum's swing.

The ultraprecise clocks of today are atomic clocks. First developed in the 1950s, the clocks are not atomic-powered, as the name suggests. They run on electricity and keep time based on the vibration of the cesium atom or other very stable atoms.

Atomic clocks are accurate to one second every 300,000 years. That's even more constant than the spinning of the earth! To maintain the accuracy of atomic clocks, timekeepers add a "leap second" every year or so.

THE WORLD'S MOST FAMOUS FACE

The Clock Tower, the most recognized clock in the world, has loomed over Britain's houses of Parliament since 1859. Tourists mistakenly call it 'Big Ben,' but that's the name of the 13-1/2 ton bell that tolls the hour.

The clock is regulated the old-fashioned way: with pennies. Old British coppers are placed on its pendulum or removed to adjust the time. Each penny taken off slows the pendulum by 2/5ths of a second every 24 hours.