STD Time Frames

The measurement of time has two aspects. For civil and for some scientific purposes we want to know the time of day so that we can order events in sequence. In most scientific work we want to know how long an event lasts (the time interval). Thus any time standard must be able to answer the questions "At what time does it occur?" and "How long does it last?" In the below table shows the range of time intervals that can be measured. They vary by a factor of about 10⁶³.

We can use any phenomenon that repeats itself as a measure of time. The measurement consists of counting the repetitions, including the fractions thereof. We could use an oscillating pendulum, a mass - spring system, or a quartz crystal, for example. Of the many repetitive phenomena in nature the rotation of the Earth on its axis, which determines the length of the day, was used as a time standard for centuries. One (mean solar) second was defined to be 1/86,400 of a (mean solar) day.

Approximate Values

Quartz crystal clocks based on the electrically sustained periodic vibrations of a quartz crystal serve well as secondary time standards. A quartz clock can be calibrated against the rotating Earth by astronomical observations and used to measure time in the laboratory. The best of these have kept time for a year with a maximum accumulated error of 5 µs, but even this precision is not sufficient for modern science and technology.

To meet the need for a better time standard, atomic clocks have been developed in several countries. Figure 1 shows such a clock, based on a characteristic frequency of the microwave radiation emitted by atoms of the element cesium. This clock, maintained at the National Institute of Standards and Technology, forms the basis in this country for Coordinated universal Time (UTC), for which time signals are available by shortwave radio (stations WWV and WWVH) and by telephone.

Figure 2 shows, by comparison with a cesium clock, variations in the rate of rotation of the Earth over a 4-year period. These data show what a poor time standard the Earth's rotation provides for precise work. The variations that we see in Fig 2 can be ascribed to tidal effects caused by the Moon and seasonal variations in the atmospheric winds.

The second based on the cesium clock was adopted as the international standard by the 13th General Conference on Weights and Measures in 1967. The following definition was given. 

Figure 1: Cesium atomic frequency standard No. NBS-6 at the

 National Institute of Standards and Technology in Boulder, 

Colorado. This is the primary standard for the unit of time in the

United States. Dial (303) 499-7111 to calibrate your watch 

against the standard. Dial (900) 410.8463 for 

Naval Observatory time signals

One second is the time occupied by 9,192,631,770 vibrations of the radiation (of a specified wavelength) emitted by a cesium atom.

Figure 2: The variation in the length of the day over a 4 year period.

Note that the vertical scale is only 3 ms = 0.003 s. See "The Earth's Rotation Rate," 

by John Wahr, American Scientist. January-February 1985.

Two modern cesium clocks could run for 300,000 years before their readings would differ by more than 1 s. Hydrogen maser clocks have achieved the incredible precision of 1 s in 3,000,000 years. Clocks based on a single as much as 3 orders of magnitude. Figure 3 shows the impressive record of improvements in timekeeping that have occurred over the past 300 years or so, starting with the pendulum clock; invented by Christian Huygens in 1656, and ending with today's hydrogen maser.