The measurement of time began when people started counting repeating events, like the rising and setting of the sun and the alternating of heat and.
Units of measuring time
In early years, the only changes that were truly regular, were the motions of objects in the sky. The most obvious and first noticed, were the changing between day and night, caused by the rising and setting of the sun. These cycles of the sun came to be called a day.
The moons changing was also a highly visible sight in the night skies. Each of these cycles, which takes approximately 29,5 days, were called a month.
The cycle of the seasons defined even a longer unit of time, namely a year.
There is no change in the sky that lasts seven days or a week. This cycle comes from the Jewish custom of reserving the Sabbath as a holy, every seventh day.
The division of the day into 24 hours, an hour into 60 minutes and a minute into 60 seconds, probably came from the Babylonians. They divided the circular path of the sun across the day sky, into 12 equal parts, awarded the night cycle 12 hours and concluded a 24 hour day.
The circular path of the sun, was divided into 360 degrees, awarding each degree 60 minutes. Later, when clocks became sophisticated enough to need smaller intervals than hours, an hour was divided into 60 minutes and a minute into 60 seconds.
Measuring time by the sun
An imaginary curved line passes through sky, directly above every spot on the earth. This line is called a celestial meridian. The sun crosses every line once during each cycle. When the sun crosses the meridian, the time there is noon and twelve hours later it will be midnight. The period from one midnight to the next is called a solar day. The length of a solar day varies, because of the earths tilt on its axis, its elliptical shape of the orbit and its changing speed along the orbit.
Measuring time by the stars
The earths rotations can be measured according to the stars. This is called sidereal time. Each day an imaginary point among the stars, called the vernal equinox, crosses the celestial meridian. When this happens, the time is sidereal noon. A sidereal day is measured from one sidereal noon to the next.
Devices that measure time
The sundial was one of the earliest devices for measuring time. It can only work in sunlight, as the sun causes the shadow of the indicator to move along an intervaled surface. The most common sundial measures the hour-angle of the sun.
A water clock or clepsydra was used to measure time during night-time in the absence of sunlight. It allowed water to drip from one marked container into another.
Hourglass or Sand glass
A certain length in time can be measured by letting sand tickle through the narrow opening of a sand glass or hourglass.Ornate sandglasses like this one were once used to mark the passage of minutes and hours. Flipping the glass causes the fixed amount of sand to pass though its narrow central hole in a consistent length of time, creating a relatively accurate measure. Generally called hourglasses because an hour was their standard setting, sandglasses measuring almost any period of time could be made by altering the amount of sand or the size of the central opening.
Clocks and watches
In the quest for more accurate ways to measure time than sand glasses and sundials, humans have progressed from watching the Sun, Moon, and stars move across the sky to constructing ingenious mechanical devices and measuring small intervals in time.
The Ancient Babylonians noticed a circular path of the sun in the day sky and divided it into 360 degrees, awarding each degree 60 minutes. Later, when clocks became sophisticated enough to need smaller intervals than hours, an hour was divided into 60 minutes and a minute into 60 seconds.
Galileo Galilee started constructing the first of the Law of the Isochronism, after noticing those Pendulums of the same length, the same amount of time used for one oscillation. Once the rules of the Pendulum were completed, the first sophisticated and accurate clock was made by Christian Huygens during the 17th century.
A clock face has 12 divisions. Each of these divisions equals one hour for the hour hand, five minutes for the minute hand and five seconds for the second hand. This is called a twelve-hour clock, working with an am/PM alternation.
To avoid confusion, twenty four hour clocks are also in use, though not widely. On such a clock, 9 oclock in the morning would be given as 9:00 hours and 9 oclock in the evening would be given as 21:00 hours.
Some watches use a 12 hour face, while a twenty four hour system would be more practical for an electronic watch.
For more information on the history of measuring devices, visit http://www.wpi.edu/~igel/time.html.