Ancient ways of measuring time.
The measure by the sun.(30 BC)
* Ancient people turned to nature for the first timekeeping. People began to track the movement of the sun across the sky, and then began to use objects to measure its progress.
* The Egyptians developed the first sundials that were divided into 10 parts with two twilight hours
* A sundial works by tracking the sun's movement.
* A blade casts a shadow that points to a marked number on the sundial's face, kind of like a clock with only an hour hand.
* At mid-day, someone has to turn these sundials 180 degrees to measure the afternoon hours.
* Of course, an ancient sundial could not tell the time on a cloudy day or at night. It was also inaccurate because the sun is at different angles at different times of the year; hours were longer or shorter, depending on the season.
* Still, a sundial was better than nothing, and by 30 B.C. more than 30 different types were in use in Greece, Italy and Asia Minor
The measure by the Stars. (600 BC)
* Our time-obsessed friends, the ancient Egyptians, seem to have developed the first method of keeping time at night by inventing the first astronomical tool, the merkhet, about 600 B.C.
* A merkhet is a string with a weight on the end used to measure a straight line, much like a carpenter of today uses a plumb bob.
* Egyptian astronomers would align two merkhets with the North Star and use that to mark a north-south line, or celestial meridian, in the night sky.
* Time was counted off as certain stars crossed that line.
* Stars also can be used to mark the passage of not just hours, but days. This measurement of the Earth's rotation is called sidereal time.
* When a certain imaginary point among the stars crosses a celestial meridian, this is called sidereal noon.
* The time from one sidereal noon to another is called a sidereal day.