John Harrison's Chronometer H5
Navigation by ships and planes depends on the ability to measure latitude and longitude
Latitude is fairly easy to determine through celestial navigation, but the measurement of longitude requires accurate measurement of time
This need was a major motivation for the development of accurate mechanical clocks
John Harrison created the first highly accurate marine chronometer in the mid-18th century
The Noon gun in Cape Town still fires an accurate signal to allow ships to check their chronometers.
Use of an atomic clock in radio signal producing satellites is fundamental to the operation of GPS (Global Positioning System) navigation devices.
An alarm clock is a clock that is designed to make a loud sound at a specific time
The primary use of these clocks is to awaken people from their sleep in order to start their days in the mornings, but can also be used for short naps; they are sometimes used for other reminders as well
To stop the sound, a button or handle on the clock needs to be pressed, and some stop automatically after a few minutes if left unattended
A classical analog alarm clock has an extra hand that is used to specify the time at which to activate the alarm.
Traditional mechanical alarm clocks have one or two bells that ring, but digital alarm clocks can make other noises
Simple battery-powered alarm clocks make a loud buzzing sound, or other similar noise to wake a sleeper, while novelty alarm clocks can speak, laugh, or sing
Some alarm clocks have radios that can be set to start playing at specified times, and are known as clock radios
A progressive alarm clock, still new in the market, can have different alarms for different times (see Next-Generation Alarms).
In a mechanical bell-style alarm clock, a mainspring drives a gear that propels a clacker back and forth between two bells or between the sides inside a single bell
In an electric bell-style alarm clock, the bell rings with an electromagnetic circuit and armature that turns the circuit on and off again repeatedly.
The ancient Greek philosopher Plato (428–348 BC) was said to possess a large water clock with an unspecified alarm signal similar to the sound of a water organ; he used it at night, possibly for signalling the beginning of his lectures at dawn (Athenaeus 4.174c). The Hellenistic engineer and inventor Ctesibius (fl
285–222 BC) fitted his clepsydras with dial and pointer for indicating the time, and added elaborate "alarm systems, which could be made to drop pebbles on a gong, or blow trumpets (by forcing bell-jars down into water and taking the compressed air through a beating reed) at pre-set times" (Vitruv 11.11).
The late Roman senator Cassiodorus (c