How is a second defined?
Earth Time and Atomic Time have to be reconciled
Current and past definitions of a second
Time started by being measured by 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.
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."
Did you know?
- The first name mentioned in relation to the word 'second' used as a unit of time is that of a Persian polymath, Al-Biruni. He mentioned the term around the year 1000 in a treatise called The Chronology of Ancient Nations.
- The method of measuring time in hours, first used by the Babylonians, expanded in the Middle Ages when the first mechanical clocks were invented, with the addition of the minute (the 60th part of an hour) and the second (the 60th part of a minute).
- The concept of a very small cycle, such as the second, came to life in pendulums. What is now known as 'the seconds pendulum' was incorporated in the imposing grandfather clocks, where each swing takes one second. Movement translates as well into a rather hypnotic sound.
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 still 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.
Three hundred 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.
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