How is a second defined?

In 1967 a new definition of a second was created based on the radiation from the caesium-133 atom. It has been since known as the SI definition (SI is the abbreviation of the French Le Système International d'Unités, International System of Units in English)

The second became "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom."

We measure time using the Earth's rotation. Universally people expect noon to occur at midday, when the Sun is at its highest point. As accurate clocks were developed, it was found that Earth time has a deviation from mechanical time. The overhead Sun at noon can arrive 16 minutes 18 seconds early or up to 14 minutes 28 seconds late. In fact only four days in a year are precisely 24 hours long as measured by the Sun. These days occur on, or about 25 December, 15 April, 14 June and 31 August. The remaining days are longer or shorter as measured from the overhead midday Sun.

A more precise measure of time was required and the atomic clock arrived on the stage.

Historical timeline

• 1884 - International Meridian Conference (Washington DC): based on a mean solar day at Greenwich, England
• 1956 - Ephemeris Time: Based on lunar observations
• 1958 - Atomic Time: based on clock data from numerous countries
• 1967 - Atomic Time: based on radiation patterns of the caesium element
• 1972 - Coordinated Universal Time (UTC) is introduced
• 1984 - Dynamical time - based on motion of celestial bodies
• 1986 - UTC replaced GMT

Despite time being measured highly accurately by atomic means, Earth time stills rules. Should Earth time and Atomic time get out of step scientists adjust time by subtracting or adding "leap seconds" on the last day of June or December.

Confused? 300 years of history mean that Greenwich Mean Time (GMT) is still viewed by most people as setting the standard for world time and time zones.