9781422283363

Another way of measuring the length of a day is to call it the time between each arrival of the Sun at its highest point in the sky. This is known as a solar day .

You need to remember that it is because the Earth spins that the Sun appears to cross the sky. A day is really the time it takes for the Earth to spin completely round its axis once. THE HOURS OF DARKNESS It was the Egyptians who decided to divide up a day into 24 hours. They didn’t start by dividing up the daylight hours, however. Instead they started with the night. By carefully studying the movements of the starts in the night sky they realized that they could divide up the night.

The regular motion of the stars across the night sky makes them excellent natural clocks. Just like the movement of the Sun during the day, this apparent movement of the stars across the night sky is also caused by the Earth spinning. A day can be measured as the length of time it takes for the stars to make one complete turn. A day measured in this way is called a sidereal day. This word comes from a Latin word meaning “star.” A sidereal day is actually about four minutes shorter than a solar day .

The Egyptians decided to divide the right into 12 parts or hours. The start of each part was marked by the rising of a particular star or group of stars above the eastern horizon. For the sake of balance they thought that there should also be the same number of daylight hours. This is why we now have a day that is 24 hours long. The Egyptians needed a way to divide up the daylight hours. They used the Sun to do this by using a sundial—the oldest one that has been found is more than 3,500 years old. As the Sun moves across the sky, the shadows it casts move For thousands of years people thought that the Sun went round the Earth, but the Polish astronomer Copernicus proved this to be wrong.

1 1 .