Who decided that a circle would have 360 degrees, and why? asks a reader.Look around you (say, by turning in a circle) and you'll notice you're surrounded by circles: the clock on a classroom wall, the lid on a jar, the rim of a coffee mug, the ring on your finger, the face of the sun. Circles all.

Circles are simple, closed curves, each point along the curve equally distant from the center. So to make a circle, attach a short piece of string to a pencil, tape the string's end to a sheet of paper, and pull it taut. Your moving pencil should trace out a closed curve -- a circular ring. The length of the string, like the length of a spoke on a bike wheel, is the circle's radius.

Travel in a circle -- or just arrive back where you started -- and it's said that you've "done a 360." Jog once around a track. Start out down the road and then go home to retrieve your purse. Or just change your mind and then change it back.

But why 360, and not, say, 100?

No one knows the whole story behind the idea of dividing a circle into 360 equal parts along its perimeter. But more than 5,000 years ago, people living around the Mediterranean Sea were already doing it. Our "base 10" decimal system is based on the numbers 1, 10, 100 (10 x 10), and 1,000 (10 x 100). But the Sumerians and, later, the Babylonians, based their math on the number 60.

These ancient mathematicians used 1, 60, 3,600 (60 x 60), and 216,000 (60 x 3,600). And calculations based on the number 60 are still mixed in with our base 10 counting to this day. For example, an hour is divided into 60 minutes, a minute into 60 seconds.

The Sumerians and Babylonians knew that our planet's actual year (which we know today to be one complete trip around the sun) was 365 days. But they decided to simplify the calendar, counting a year as 360 days. This included 12 months, neatly divided into 30 days each.

What about the leftover days? Between the end of one year and the start of the next, there were five "special days," which the Egyptians later celebrated as public holidays.

According to math historians, the 360-day year probably gave rise to the 360-degree circle. After all, when it comes to circles, the number 360 just makes sense. Unlike, say, the number 100, it can be evenly divided by many other numbers -- from 2, 3, 4, 5, 6, 8, 9, 10, and 12 to 30, 36, 40, etc.

So carving a circle into equal triangular slices, like cutting a pie, can be done in many different ways.

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