A day is the amount of time it takes the Earth to turn in a full circle. But how did people decide how long an hour, a minute, and a second are? The first people to break up a day into smaller parts were the ancient Egyptians. More than 3000 years ago, they began using sundials, which were the first types of clocks. Have you ever noticed how your shadow grows bigger or smaller depending on what time of day it is? The Egyptians told time by putting stakes in the ground and measuring the shadows they made.

Eventually sundials got bigger and fancier. One sundial shaped like the letter “T” was used to divide the half of the day when the Sun is shining into 12 parts. While no one knows for sure why they chose the number 12, some people think it’s because it can be divided evenly by two, three, four, and six. Ten is easy to count—you have 10 fingers and 10 toes—but 10 can only be divided by two and five.

Yet sundials weren’t very useful after the Sun went down. To tell time at night, the Egyptians looked to the stars. Like the Sun, the stars move across the sky as time passes. By choosing a handful of stars to follow, the Egyptians could tell what time of night it was by looking up to check where they were in the sky. They chose 12 stars to track to help them measure the time when it was completely dark outside. Add it all together: 12 hours of darkness + 12 hours of light = 24 hours.

Another ancient people called the Babylonians liked to use the number 60. Lots of civilizations borrowed from this number system, including the ancient Egyptians. That's why we now divide circles into 360 parts, or degrees: 60 goes into 360 six times. (Did you know you can cut a circle into six triangles too?) It's also why people eventually decided to break up the face of a clock, which is also a circle, into 60 minutes … and then to break down each minute into 60 seconds. It took a long time before this happened. So next time you try to measure a minute by counting to 60, remember that people who lived thousands of years ago counted to 60 too!

For further fun reading, check out Scientific American's article on how and why we measure the day.