Atomic Clocks and NTP Servers
In conventional electronic clocks time is kept by running an electrical current through an oscillator which produces a repetitive electrical signal this is then governed by a quartz crystal to keep precision. These crystal oscillators are far more accurate than mechanical clocks but will still drift, perhaps over a second a week.
For day-to-day use crystal oscillators are a fine way to keep track of time; in the everyday running of our lives, a second makes very little difference, however, as light or radio waves can travel 300,000 miles in a second, some high technologies such as satellite navigation or global communication, require far more accuracy to be possible.
Atomic clocks are a timekeeping device that uses the known atomic resonance frequency of an atom to keep time. The first truly accurate atomic clock was built in 1955 at the National Physical Laboratory in the UK and was based on the caesium atom -133 which oscillates at exactly 9,192,631,770 every second.
This oscillation is actually a repetitive signal from the microwave radiation emitted by electrons in an atom when they change energy levels. Much of an atomic clock is designed to create the correct state to cause and augment oscillations.
Although other atoms can be used, the oscillation (9,192,631,770 a second) of the caesium -133 atom is now accepted by the International System of Units (SI) as being the definition of one second.
Atomic clocks are generally very large and constitute many highly technical apparatus such as vacuums and require whole teams of scientists to maintain and monitor the clocks. Much of which goes into compensating for unwanted side-effects such as frequencies of other atoms in the clock and even gravitational dilation (where according to Einstein's theory clocks at different heights run differently because of the differences in the gravitational field) This makes atomic clocks highly expensive.
Fortunately many large scale national physical laboratories transmit radio time signals from their atomic clocks which can be used to synchronise standard crystal oscillators too.
Atomic clocks are also the basis of GPS (Global Positioning System) as each satellite contains an atomic clock as accurate time is integral for positioning (a position anywhere is made up of a direction, a velocity and time).
GPS signals can also be used to capture a time signal. This is now the most common way computer networks retain accurate time which is also essential in many communications and applications.Most computer networks use a NTP server (Network Time Protocol) to synchonise their devices to an atomic time signal received via the GPS network.
A universal timescale, UTC (Coordinated universal Time), has been developed based on the time told by atomic clocks, TAI (International Atomic Time). UTC accounts for the slowing of the Earths rotation by adding leap seconds to TAI so as to prevent the gradual drift of night into day (although that would take 40,000 years or so) and allows the whole world to communicate using the same timescale.