<font size="1" color="#FF0000">LAST EDITED ON 18-Feb-02 AT 00:21AM (EDT)</font>
This is a very good question. I find it interesting that no one has ask this question soon.
As best we know, 5000 to 6000 years ago great civilizations in the Middle East and North Africa initiated clock making as opposed to calender making. After the Sumerian culture was lost without passing on its knowledge, the Egyptians were the next to formally divide their day into parts something like our hours. Obelisks (slender, tapering, four-sided monuments) were built as early as 3500 B.C. Their moving shadows formed a kind of sundial, enabling citizens to partition the day into two parts by indicating noon.
Another Egyptian shadow clock or sundial, possibly the first portable timepiece, came into use around 1500 B.C. to measure the passage of "hours." The device divided a sunlit day into 10 parts plus two "twilight hours" in the morning and evening.
The merkhet, the oldest known astronomical tool, was an Egyptian development of around 600 B.C. A pair of "merkhets" were used to establish a north-south line by lineing them up with the Pole Star. They could then be used to mark off nighttime hours by determinimg when certain other star crossed the meridian.
The earliest Egypian calendar was based on the moon's cycles, but later the Egypian realized that the "Dog Star" in Canis Major, which we call Sirius, rose next to the sun every 365 days, about when the annual inundation of the Nile began. Based on this knowledge, they devised a 365-day calender that seens to have begun in 4236 B.C., the earliest recorded year in history.
We know little about the details of timekeeping in prehistoric eras, but wherever we turn up records and artifacts, we usually discover that in every culture, some people were preoccupied with measuring and recording the passage of time. Ice-age hunters in Europe over 20,000 years ago scratched lines and gouged holes in sticks and bones, possibly counting the days between phases of the moon.
The familiar subdivision of the day into 24 hours, the hour into 60 minutes, and the minute into 60 seconds is of ancient origins but has come into general use since about 1600 A.D. The system of consecutively numbering the years of the Christen Era was devised by Dionysius Exiguus in about 525; it included the reckoning of dates as either A.D. or B.C. (the year before 1 A.D. was 1 B.C.) The Julian calendar, introduced by Julius Caesar in the 1st century B.C., was then in use, and any year whose number was exactly divisible by four was desingated a leap year. In the Gregorian calendar, introduced in 1582 and now in general use, the canturial years are common years unless their numbers are exactly divisible by 400; thus, 1600 was a leap year, but 1700 was not.
In 1869 Charles F. Dowd, principal of a school in Saratoga Springs, N.Y., proposed the use of time zones. time zones were adopted by U.S. and Canadian railroads in 1883.
Numerous time scales have been formed, here is a list with a brief description.
Universal Time:
The mean solar time of the meridian of Greenwich, England.
Coordinated Universal Time:
The basis of legal, civil time.
Rotational Time:
Based on the Earth's rotation.
Standard Time:
Local mean solar time depends upon longitude.
Ephemeris Time:
Based on the Earth's orbital motion.
Dynamical Time:
Defined descriptively as the independent variable, T, in the differenial equations of motion of the celestial bodies.
Barycentric Dynamical Time:
The independent variable in the equations, including terms for relativity, of motion of the celestial bodies.
Terrestrial Dynamical Time:
An auxiliay scale defined by the equation TDT = TAI + 32.184 s.
Radiometric Time:
Based on radioactive decay.
