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Why Do Days Start At 12 O'Clock?

A sundial
In ancient Egypt, the day was measured using sundials. The shadow on the face of a sundial tells the time, and the shadow depends on where the sun is in the sky.

How was time created? How did one minute become 60 seconds and one hour became 60 minutes? Why is time segmented into 12-hour periods? How do clocks work? Why is a year 365 days? Why is there an extra day in February every four years? Does time have a beginning or an end? Is time travel possible? Answers to all of your time questions with Andrew Novick of NIST, the National Institute of Standards and Technology.


Time is such a big topic! In science, understanding time is essential to the study of physics, which is a branch of science that studies the nature and properties of matter and energy.

But even if you don't understand those things, your day might be governed by the ticking of that clock. For example: what it's time for now, what day of the week it is, what year it is even!

"Why do the days start at 12 o'clock in the morning?" - Charlotte, 6, Longwood, FL

The reason a new day starts at 12:00 goes back to ancient Egypt when the day was measured using sundials. The shadow on the face of a sundial tells the time, and the shadow depends on where the sun is in the sky.

"When the sun is highest overhead and the shadow goes straight up to the top of the sundial, that's noon. And as the sun goes overhead, the shadow moves until sunset when the shadow disappears," Novick explained.  "Then we have 12 more hours when the sundial doesn't work and that's at night."

When the Egyptians were developing their time system they had to account for the time when there was no shadow. Since the highest point of the day was noon, the opposite has to be midnight that was when the 12 started over again, so that's why the day starts at midnight.

"Why do clocks have to go clockwise?" - Bentley, 8, Greeley, CO

The reason that the hands on a clock travel the way they do, or "clockwise,also goes back to the invention of the sundial in ancient Egypt. Egypt is in the northern hemisphere.

"In the northern hemisphere, the sun rises in the east and sets in the west. So when the sun goes from the east to the west, the shadow goes from the left to the right," Novick said. "The idea of the shadow going from the left to top and then to the right derived the definition of clockwise." 

When clocks were invented, the mechanism's hands followed the same direction.

Read the full transcript

Melody is the Contributing Editor for But Why: A Podcast For Curious Kids and the co-author of two But Why books with Jane Lindholm.
Jane Lindholm is the host, executive producer and creator of But Why: A Podcast For Curious Kids. In addition to her work on our international kids show, she produces special projects for Vermont Public. Until March 2021, she was host and editor of the award-winning Vermont Public program Vermont Edition.
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