How to Tell Time by the Sun (page 2)

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Author: Beth Touchette
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Dog Days of Summer Science


Your results will vary depending on where you live, and particularly on your location’s latitude (if you live in the Southern hemisphere, your results over the months will be the opposite what we describe next)! The shadow of the nail will move in a semi-circular pattern throughout the course of the day. The height of the nail’s shadow will be longest in the early and late part of the day, and shortest in the middle of the day. As you observe the sundial over the course of the school year, the nail’s shadow will get longer until December, and the sun might set before you can complete some readings! If your state observes Daylight Saving Time, your sundial will be an hour off during the winter. As spring approaches, the shadows on your sundial will shorten, and your results should begin to resemble your observations in August.


You made your sundial out of wood and a nail to ensure that the wind wouldn’t blow it over and make your readings inaccurate. You can only use a sundial if the sun is shining.

Your sundial works as a clock because the Earth rotates. As the Earth spins, the Sun seems to move across the sky. When the sun appears over the horizon at dawn, its light strikes the nail from the side, making a long shadow. As your part of the Earth continues to rotate, the Sun gets higher in the sky. The Sun’s light strikes the nail from above, making shorter shadows from 11 to 1. As the part of the Earth where you live rotates away from the Sun, it seems to sink in the sky, and the nail shadows again get longer. 

When you looked at the sundial in December, the nail shadows were longer and the day was shorter because the Earth had reached a certain position in its path around the Sun. In this position, the Northern hemisphere is tilted away from the sun, making its light less direct. Because you turned the clocks back at the end of Daylight Saving Time in late October, your sundial was ahead by an hour for the winter: You created the sundial using a clock on daylight savings in August. In spring, when clocks are returned to daylight savings, your sundial will again match your watch. By May, the day will be longer and the nail’s shadow shorter, because the Earth continued to revolve around the Sun, and the Northern hemisphere returns to a position where the Sun’s light is more direct.

Going Further

Research what ancient sundials looked like. You might also investigate how different civilizations determined how to tell time by the sun. The Mayan pyramid at Chichen Itza is particularly interesting.

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