Kids: Why are days 24 hours long?
Photo: Flickr user simpologist
You might have wondered, who came up with the idea of the twenty-four hour day?
Credit for the concept of a twenty-four hour day goes to the early Egyptians. Around the 30th century B.C., they used a yearly calendar of 365 days. It had 12 months of 30 days, plus five holidays at the end of the year. Each month was divided into three ten-day periods, and each day into twenty-four hours.
Egyptian astronomers used observations of the stars to designate each of the ten-day periods over the course of the year. All stars move in a yearly cycle, due to Earth’s motion around the sun. The first appearance of a star before sunrise – at the start of its new season of visibility – is known as its heliacal rising. The Egyptians came to associate the heliacal rising of a particular bright star with each ten-day periods.
Meanwhile, the Egyptians divided the period from sunrise to sunset into 10 parts. And they added an hour before sunrise and after sunset for twilight periods – for a total of 24 hours in a day/night period. The length of these original “hours” varied with the seasons – they were called “seasonal” hours. Hours of equal length were introduced by Greek astronomers. But as late as the Middle Ages, when mechanical clocks were coming into use, seasonal hours were still used in everyday life.
