# What do Time Measurement and the Sun Have in Common?

## You can count the hours with sundials!

The August sun pours down on mid-summer days as it has for thousands of years. To us, sunshine might mean a trip to our favorite ocean beach, freshwater lake, or hike up to fields of hillside flowers. One of the many crucial things sunshine meant to the ancient world of Greece, Rome, Egypt, and Babylonia was a way to answer the simple question: “What time is it?”

Around 3500 years ago, people within that region changed the course of organized civilization with the invention of the first tool to tell time––the sundial. Have you ever been outside all day and noticed how your shadow moves? If you’re out at 12:00 noon and the sun is right overhead, your shadow is at its shortest. Through the afternoon, it moves and lengthens. The way a sundial works is based on how shadows move as the sun makes its daily journey across the sky.

A sundial is a measurement instrument. They’re usually made of a thin circle of stone, with a short pole called a “gnomon” set at an angle in the center. The stone is marked to designate 12 equal segments. As the Earth orbits the sun and rotates on its axis, sunlight reaches the sundial from different angles. As the day passes, the gnomon casts a shadow in different places that mark each hour.

Want to make your own sundial? Here’s how!

How long does it take sunlight to reach Earth?
Light photons emitted from the Sun’s surface travel to Earth in about 8 minutes and 20 seconds, energizing our planet––and enabling us to tell time with sundials. Mathematicians calculate that the Earth moves in an elliptical orbit around the Sun from an average distance of 93 million miles. Light rockets through the vacuum of space at about 186,000 miles per second. The distance crossed divided by the speed of light equals 500 seconds, which is 8.3 minutes.

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