The need to measure period arose with the development of agriculture. Farmers used timekeeping to ascertain the best planting intervals and crude lunar calendars were created.
The Egyptians were the first people to create broadly a way of telling time with clocks and calendars. By roughly 2800 BC they'd established that a 365-day calendar, based on their observations of the rising and setting of glowing stars such as Sirius and also the periodic inundations of the Nile, upon which their agriculture relied. By 2100 BC the Egyptians had invented a way to divide the day into 24 hours. Around the exact same time, they made the very first sundials, or darkness clocks, to measure period throughout the day. A sundial indicates the time of day by the position of the shadow of some object where the sun's rays fall.
From 1500 BC Egyptians had devised another, more accurate, way of telling time-the water clock along with a clepsydra, which uses the steady dripping of water by a vessel to drive a mechanical device that signals the hour.
Due to this effect, noon could be as much as a half hour before or after the time when the sun is highest in the sky.
Around 270 BC the Alexandrian engineer Ctesibios designed water clocks that rang bells, moved puppets, and caused mechanical birds to sing.
Measurement of short time intervals, however, was possible with the hourglass. But these were not portable.
The first watches
The invention of springs and escapement mechanism ushered in the era of portable watches.
Another mechanical method is the balance wheel mechanism. The balance wheel together with the balance spring (also known as Hairspring) - these form a simple harmonic oscillator, which controls the motion of the gear system of the watch in a manner analogous to the pendulum of a pendulum clock. These watches produce a ticking sound.
Purely mechanical watches are still popular. The high level of craftsmanship of purely mechanical watches accounts for much of their attraction. Compared to electronic movements, mechanical watches are inaccurate, often with errors of seconds per day. They are frequently sensitive to position and temperature, they are costly to produce, they require regular maintenance and adjustment, and they are more prone to failure.
Further accuracy was achieved in the sixties by Tuning fork watches, which use a tuning fork at a precise frequency (most often 360 hertz) to drive a mechanical watch. Since the fork is used in place of a typical balance wheel, these watches naturally hum instead of tick. Tuning fork movements are electromechanical. The task of converting electronically pulsed fork vibration into rotary movement is done via two tiny jeweled fingers, called pawls, one of which is connected to one of the tuning fork's tines. As the fork vibrates, the pawls precisely ratchet a tiny index wheel. This index wheel has over 300 barely visible teeth and spins more than 38 million times per year. The tiny electric coils that drive the tuning fork have 8000 turns of insulated copper wire with a diameter of 0.015 mm and a length of 90 meters. This amazing feat of engineering was prototyped in the 1950s and the early 60's.
Advent of the electronic quartz watch in 1969
In 1948, Max Hetzel used an electronic device, a transistor to create the first electronic watch. This development became obsolete with the use of a quartz crystal which brought in the quartz watches, which use the piezoelectric effect in a tiny quartz crystal to provide a stable time base for a mostly electronic movement: the crystal forms a quartz oscillator which resonates at a specific and highly stable frequency, and which can be used to accurately pace a timekeeping mechanism. These primarily electronic movements are geared to drive mechanical hands on the face of the watch. Quartz movements are ten times better than a mechanical movement.
Further developments introduced the following types of watches:
In manual watches the spring must be rewound by the user periodically by turning the watch crown.
Self-winding or automatic watches
A self-winding or automatic mechanism is one that rewinds the mainspring of a mechanical movement by the natural motions of the wearer's body.
Kinetic power or automatic quartz
Some electronic watches are also powered by the movement of the wearer of the watch. Kinetic powered quartz watches make use of the motion of the wearer's arm turning a rotating weight, which turns a generator to supply power to charge a rechargeable battery that runs the watch. The concept is similar to that of self-winding spring movements, except that the electrical power is generated instead of mechanical spring tension.
Battery powered watches in 1957
Electronic watches require electricity as a power source. Some mechanical movements and hybrid electronic-mechanical movements also require electricity. Usually the electricity is provided by a replaceable battery. Watch batteries (strictly speaking cells) are specially designed for their purpose. They are very small and provide tiny amounts of power continuously for very long periods (several years or more). Environment unfriendly mercury batteries gave way to Silver-oxide and lithium batteries. Cheap batteries may be alkaline, of the same size as silver-oxide but providing shorter life. Rechargeable batteries are used in some solar powered watches.
Some electronic watches are powered by light. A photovoltaic cell on the face of the watch converts light to electricity, which in turn is used to charge a rechargeable battery. The movement of the watch draws its power from the rechargeable battery. As long as the watch is regularly exposed to fairly strong light (such as sunlight), it never needs battery replacement, and some models need only a few minutes of sunlight to provide weeks of energy.
Some of the early solar watches of the 1970s had innovative and unique designs to accommodate the array of solar cells needed to power them (Nepro, Sicura and some models by Cristalonic, Alba, Seiko and Citizen). As the decades progressed and the efficiency of the solar cells increased while the power requirements of the movement and display decreased, solar watches began to be designed to look like other conventional watches.
Some electronic quartz watches are able to synchronize themselves with an external time source. These sources include radio time signals directly driven by atomic clocks, time signals from GPS navigation satellites, the German DCF77 signal in Europe, WWVB in the US, and others. These watches are free-running most of the time, but periodically align themselves with the chosen external time source automatically, typically once a day.
Because these watches are regulated by an external time source of extraordinarily high accuracy, they are never off by more than a small fraction of a second a day (depending on the quality of their quartz movements), as long as they can receive the external time signals that they expect. Additionally, their long-term accuracy is comparable to that of the external time signals they receive, which in most cases (such as GPS signals and special radio transmissions of time based on atomic clocks) is better than one second in three million years. For all practical purposes, then, radio-controlled wristwatches keep near perfect time.
Movements of this type synchronize not only the time of day but also the date, the leap-year status of the current year, and the current state of daylight saving time (on or off). They obtain all of this information from the external signals that they receive. Because of this continual automatic updating, they never require manual setting or resetting.
A disadvantage of radio-controlled movements is that they cannot synchronize if radio reception conditions are poor. Even in this case, however, they will simply run autonomously with the same accuracy as a normal quartz watch until they are next able to synchronize.
In the seventies two types of displays were developed.
A numbered dial upon which are mounted at least a rotating hour hand and a longer, rotating minute hand. Many watches also incorporate a third hand that shows the current second of the current minute. Watches powered by quartz have second hands that snap every second to the next marker. Watches powered by a mechanical movement have a "sweep second hand", the name deriving from its uninterrupted smooth (sweeping) movement across the markers, the hand merely moves in smaller steps, typically 1/6 of a second, corresponding to the beat of the balance wheel. All of the hands are normally mechanical, physically rotating on the dial, although a few watches have been produced with "hands" which are mimicked by a liquid-crystal display.
A digital display simply shows the time as a number, e.g., 12:40 AM rather than a brief hand pointing towards the number 12 and a long hand pointing towards the number 8 on a dial.