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

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