B-r-r-r-ring! Your alarm clock is telling you that it's time to get out of bed and go to school. Sometimes we wish that clock would just disappear!

alarm clocks graphicHave you ever wondered how you would keep track of time if all the clocks in the world really did disappear? Long ago our ancient ancestors were faced with just such a problem. Clocks, as we know them today, hadn't been invented yet.

How did man first learn to measure time? The difference between the dark nights and the daylight was probably the first division of time recognized by early peoples. They would also have noticed that the sun came up over the eastern horizon and went down again below the western horizon bringing darkness to their world.

During the day they saw that the shadow cast by a tree, a rock, or even their own body was long early in the morning and grew shorter and shorter until it disappeared when the sun was overhead in the middle of the day. They also would have noticed that the shadow grew longer again, on the other side of the tree, as night came.
After awhile they were able to tell how much of the day was over by looking at the shadows. The first timepiece was probably invented by a person who put a stick in the ground and made marks in the dirt to show where the stick's shadow was every hour.
The shadow stick is the earliest form of sundial. People judged the time of day by the length and position of the stick's shadow. The technical name for a shadow stick is gnomon,(NO mon) which is Greek for "the one that knows".

The ancient Egyptians built tall stone towers called obelisks. Everybody could tell the time by looking at the obelisk's shadow. Obelisks were sometimes called "Cleopatra's Needles".

How Do Sundials Work?

As the earth turns on its axis, the sun appears to move across the sky. The shadows the sun casts move in a clockwise direction for objects in the northern hemisphere. If the sun rose and set at the same time and spot on the horizon each day shadow sticks would have been accurate clocks. However, the earth is always spinning like a top. It spins around an imaginary line called its axis. The axis runs through the center of the earth from the North Pole to the South Pole. The earth's axis is always tilted at the same angle.

Every 24 hours the earth makes one complete turn, or rotation. The earth rotates on its axis from west to east. The earth's rotation causes day and night. As the earth rotates, the night side will move into the sunlight, and the day side will move into the dark.

On the earth's yearly trip around the sun the North Pole is tilted toward the sun for six months and away from the sun for six months. This means the shadows cast by the sun change from day to day.

Because the earth is round, or curved, the ground at the base of a shadow stick will not be at the same angle to the sun's rays as at the equator. Because of this the shadow of the shadow stick will not move at a uniform rate during the day.

Eventually man discovered that angling the gnomon and aiming it north made a more accurate sundial. Because its angle makes up for the tilt of the Earth, the hour marks remained the same all year long. This type of gnomon is called a style. After this discovery, people were able to construct sundials that were much better at keeping accurate time.

The History of Sun Clocks

Many types of sundials evolved as man worked towards creating accurate timepieces.

Egyptians began using a T-shaped "time stick". It consisted of one vertical stick and one crossbar The names of five hours were written on the stick in hieroglyphics. In the morning the stick was placed so that it faced east. The shadow of the crossbar would then fall across the stick and move toward the crossbar until noon. In the afternoon the stick was turned to face west.

Around 1500 BC smaller Egyptian timepieces were created. The sundial was a smaller version of the obelisk. The discovery of small, portable versions of sundials tells us that taking time with them became important to the Egyptian people.

In the Middle Ages peasants in northern Europe used sundials carved into the bottom of their wooden clogs. To tell time, the peasant would take off his shoe and stand it up facing the sun. The hour was told by the shadow the heel cast on the dial.

Another medieval European device for telling time was the hand dial. The gnomon on this sundial was just a stick. It was held at angle by a person's thumb. The gnomon was held in his left hand in the morning and the hand was held horizontal to the ground, pointing west. In the afternoon the gnomon was held in the right hand and the hand was pointed east.

In the quest for better accuracy throughout the shifting position of the Earth during the year, sundials changed from flat plates to more detailed forms.

During the Renaissance period sundials changed rapidly and many various designs were created. In addition to having hour and minute marks for telling time, other features were sometimes added. Some sundials had markings to indicate the seasons, the calendar date, the times of sunrise and sunset, Zodiac signs, and the points of the compass.

People continued to make sundials long after clocks were invented. Sundials could be counted on to keep accurate time, and they were often used to set the time on clocks that had stopped! The sundial was still the most popular way of keeping time until watches were invented making it easier for people to tell time wherever they were.

Today most people who have sundials in their gardens use them for decoration rather than as a way of keeping time. Thanks to the invention of the digital wristwatch, people no longer need to carry around a portable sundial. But sundials should not be forgotten by modern man. The sun will always rise in the morning and set at night, making sundials remain one of the most reliable methods of telling time.