Sundials

A sundial is a device used to display time. The position of the sun in the sky produces a shadow on a flat surface corresponding to a specific time of day. Although only functional during daylight hours and in direct sunlight, sundials can be very accurate. The majority of sundials measure apparent solar time, time determined by the angle of the [Page 1210]sun's shadow. As the earth rotates, causing the sun to appear to move across the sky from east to west, the sun's shadow, in the northern hemisphere, moves in a clockwise direction around the object. This phenomenon is responsible for the direction that the arms of clocks spin. The longitudinal position of the sundial must also be known to accurately measure solar time because the earth tilts on its axis. As the earth revolves around the sun, it tilts either the northern or southern hemisphere toward the sun at approximately 6-month intervals, giving the earth its seasons. Because of this tilt and the curved shape of the earth, the speed at which the shadow rotates around an object is not uniform. To solve this, the gnomon, the part of the sundial that casts the shadow, is aimed north. As a result, the hour marks on the sundial do not fluctuate throughout the year; this makes sundials remarkably accurate.

The earliest sundials appeared in ancient Egypt around 3500 BCE. These tall, narrow, four-sided stone monuments were known as obelisks. (The Washington Monument in Washington, D.C., is an example of a modern obelisk.) Its moving shadow allowed the Egyptians to determine the time of the day. In addition, the length of the shadow at midday allowed the Egyptians to roughly determine the seasonal date. The pyramidal top of the obelisk gave the end of its shadow a point. The position of this point at midday showed the degree of the axial tilt of the earth. Although the ancient Egyptians likely did not understand this concept, they did recognize the significance of the length and angle of the shadow from the obelisk. It is from the Egyptians that the modern divisions of the day into 12 hours, the hour into 60 minutes, and the minute into 60 seconds are traced.

Over time, advances in science and mathematics led to a plethora of variations of the sundial. The simplest sundial is the equatorial sundial, which consists of a disk with a bar through its center. The bar is positioned so that it is parallel with earth's axis of rotation, making the disk parallel with the earth's equatorial axis. This style of sundial indicates both the time of day, with the gnomon, and the date, with the nodus. The nodus is the tip of the gnomon. Another common type of sundial is the garden sundial. Although similar to the equatorial sundial, it instead uses a disk that is parallel to the ground. This allows the garden sundial to display time all year long. The gnomon is positioned so that it is parallel with the earth's axis of rotation and therefore will cast its shadow on the hour-marked flat disk. Many forms of sundials exist, including the vertical sundial, the anal-emmatic sundial, the portable diptych sundial invented during the Middle Ages, the ring sundial, and the equatorial bow sundial. Sundials have even entered the modern age with the digital sundial. The two types of digital sundials are the fiberoptic sundial and the fractal sundial. Using no electricity and having no moving parts, the digital sundial displays the time with numerals formed by sunlight hitting them. As technology continues to develop, the future for sundials seems bleak, but their impact on human history is remarkable. Their simplicity affords them the ability to display time from the surface of any planet in the universe where an angle exists between it and its sun. So even though it may seem that time has passed the sundial by, its universal application may be called on again in the future.

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