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Watches Back In Time: Sundials

Long ago, in the past, knowing night and day was important for people to survive. Daylight was easy to see in, which was the best time for foraging for food and hunting; however, night was dangerous because most predators liked to hunt when it was dark. People wanted to be safe and would watch the sun in the sky to make sure they went back home before dark. But staring at the sun hurts and sometimes it was hard for people to really know how much time a person had before dark. So sundials were made to help tell time and helped people figure out things like hours so that they could estimate how much time they had before night. 

History of Sundials

The history of sundials begins in Egypt, where they made a sundial using a vertical stick with five hours written on it. They would measure time by looking at the length of the shadow it made throughout the time. In the morning this stick was placed to face east with the rising sun, and then in the afternoon it faced west with the setting sun. They also built something called obelisks which where stone pillars that also measured time in the same way. In Greece they made a sundial called a pelekinon which measured time better than the sundials before had. Using their knowledge of geometry and understanding the tilt of the earth's axis, they made a very accurate sundial. Since then, almost all sundials have been based off of their design.

How Sundials Work

The part of the sundial that casts a shadow is called the gnomon. As the sun rises, the gnomon casts a different length shadow. For example, at noon the gnomon's shadow is the shortest. When the sun is rising and setting, the shadow is the longest. The length of the shadow is also affected by the seasons. The shadows in summer are long than the shadows in winter because the sun is higher in the sky. How fast the shadow moves also depends on the length of its gnomon. However, because time is measured differently in every place, sundials have to be carefully marked and placed in order to measure time the most accurately.

Different Types of Sundials

There are seven types of sundials: horizontal, vertical, vertical declining, meridional reclining, declining reclining, equatorial, and polar. Horizontal sundials lie on a horizontal surface. Vertical sundials are placed on a vertical surface and face either North or South. Vertical declining sundials are placed on a vertical surface but do not necessarily face North or South. Meridional reclining sundials face either North or South, and are tilted to the horizon at an angle. Declining reclining sundials do not face North of South exactly and are neither horizontal nor vertical. Equatorial sundials are placed on parallel (side by side) to the equatorial plane and the gnomon is actually perpendicular (making a perfect "+" shape) to the dial. Polar sundials face either East or West.

How To Make a Sundial

There are several ways and several types of sundials that can be made. The easiest is to place a stick into the earth and gather some small rocks or pebbles. Then using a watch, place a rock where the shadow hits on every hour during daylight. Once every daylight hour has been marked, the sundial is complete and can be used any time. The time that each of the pebbles represents can be marked with a piece of chalk. Other sundials can be made using templates from online. By following instructions, the template is cut out using scissors or a craft knife (with parental supervision!), and then the piece of paper that is folded to make the gnomon is placed on the template. A compass is then used to place the sundial in the right position facing north.

Additional Resources

For additional resources about sundials, consult these links: