The telling of time originated in watching the sun cross the sky. Morning was demonstrated self-evidently by sunrise, night by dusk, and noon by having the sun directly overhead.
The Sundial (3400-3600BC):
However, as more complex civilizations developed, so grew the need to organize time more efficiently. Thus the sundial was invented, characterized as a flat marked disc with a 45-degree-angled pole rising from the center, whose shadow demonstrated the time of day. It isn’t known which civilization used the sundial first, as it was used by all major ancient civilizations across India, Asia and Europe.
The Water Clock (1400 BC):
The sundial, which could only be used during sunny days, was improved upon in concept by the invention of the water clock. Along with 24-hour functionality, the water clock also had the advantage of being more accurate.
The water clock was invented in Egypt and became popular in Greece, where it underwent frequent improvement. Using a system of gravity-powered water exchanges, the time was measured with gears and a notched stick.
The Greeks are also responsible for dividing our measurement of time into our current system of months, days, weeks, and hours (minutes and seconds are derived from the 60-based Sumerian mathematical system). Since the Greek system wasn’t perfectly accurate (day and night, for example, are not the same length), water clocks had to be reset every day to maintain their accuracy.
The Pendulum Clock (1656):
The pendulum clock, which proved to be accurate and useful (while other attempts at mechanical clocks were not), can be credited to Christian Huygens, who had the bright idea in 1656. Further development led to the addition of the minute hand.
As the pendulum swings, it turns a wheel inside the clock, which in turn turns the hands around the face of the clock. The most serious problem with the pendulum clock was that it would eventually lose momentum and required regular restarting. The addition of a battery, in the latter half of the 19th century, repaired this inconvenience.
Because the 12-hour clock represents a 24-hour day, requiring it to cycle around twice a day, the terms “AM” and “PM,” acronyms for Latin phrases, were employed for the purpose of differentiation.
The Quartz Crystal Clock (1920):
This kind of clock is used frequently in our modern day, in wall clocks, wristwatches, etcetera. It is purely mechanical, using quartz (a very common mineral) to keep time very precisely in its ability to bend an electric field. It keeps time to the second, as is shown by the (usually red) second hand.
The Digital Clock (1956):
The digital clock originally employed mechanisms similar to those of the round-faced quartz crystal clock, but displayed a row of digits to show the exact minute. After the 1970s, as LEDs grew cheaper, the method of showing numbers on a screen in LED (think glowing alarm clocks), VFD (12:00-flashing VCRs), and LCD (black numbers on a green or grey background) became very widely popular.