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Todd Arsenault

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On the first Sunday of November, most people in Canada and the United States set their clocks back one hour (except for those in Saskatchewan, Hawaii, most of Arizona and other places where Daylight Saving Time is not observed).

This twice-yearly ritual of adjusting timepieces is so routine that we hardly think about it.

But have you ever stopped to consider who decides exactly what time it is or how they do it?

The matter of whether or not Daylight Saving Time is used, and when it starts and stops, is mainly a concern of law rather than science, as are the boundaries of time zones.

However, what the actual time is anywhere on the face of the Earth at any given moment is very much a matter of science.

Some very powerful scientific instrumentation is required in order to keep the entire world "on time."

Historically, time measurement began with the calendar.

Various ancient civilizations noticed that the same event in astronomy (such as the solstices or the appearance of certain stars) or natural events (such as the annual flooding of the Nile River) repeated every 365 days or so.

As a result, the basic units of time were historically the calendar year and the day.

Smaller units of time (hours, minutes and seconds) were later defined as fractions of the day or of each other, as timepieces were gradually invented to keep track of them.

Precise measurement of time of day became an obsession during the age of exploration, as it was essential to determining longitude, and therefore, accurate navigation.

The Royal Observatory in Greenwich, England, eventually became the worldwide reference site for both longitude and time (called Greenwich Mean Time or GMT).

For hundreds of years, the second was just a derived fraction of the base units of time (one 86,400th of a mean solar day), something that had no fundamental meaning in its own right.

However in 1967, the Systeme International (SI, commonly known as the metric system) completely reversed the concept of what was the basic unit and which units were derived when it defined the second as the new base unit of time.

Minutes, hours, days and even years were now just multiples of the second.

One of the reasons for this change was the communications revolution, which was demanding improved measurements of fractions of seconds in order to develop new technologies.

New words like milliseconds, microseconds and nanoseconds started to enter everyday language, and more accurate ways to measure time on this scale were needed.

SI therefore defined the second based on the most precise time-keeping instrument ever developed, the cesium atomic clock.

Atomic clocks aren't, despite their name, radioactive. A cesium atomic clock (also called a cesium oscillator) uses a vapour cloud or beam of cesium metal atoms in a high vacuum chamber as a frequency reference source.

SI defined the second as the period equal to exactly 9,192,631,770 cycles of the radiation required to cause a certain absorption of energy by the cesium atoms.

Absorption of microwave energy of exactly this frequency (about 9.19 gigahertz) will cause a change that can be detected by the instrument.

Since frequency and time are the inverse of one another (think "per second" versus "second"), precise measurement of the microwave frequency allows one to calculate the second to high precision.

The most precise cesium atomic clocks in the world are reproducible to within one second in 20 million years.

About 250 cesium atomic clocks in national laboratories around the world are used to define a time standard called UTC (co-ordinated universal time) that replaced GMT as the official time standard in 1972.

Now that we can always set our clocks right to the nanosecond, you really have no excuse to be late!

To learn more, visit www.scienceeast.nb.ca.

Todd Arsenault is a volunteer member of the executive committee of Science East. He holds a Ph.D. in chemistry. His column appears every fourth Wednesday.