# Time

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Time

Time has two aspects. For civil and some scientific purposes, we-want to know the time of day so that we can order events in sequence. In much scientific work, we want to know how long an event lasts. Thus, any time standard must be able to answer two questions: When did it happen?” and What is its duration? Table 1-4 shows some time intervals.

Any phenomenon that repeats itself is a possible time standard. Earth’s rotation, which determines the length of the day, has been used in this way for centuries.Shows one novel example of a watch based on that rotation. A quartz clock, in which a quartz ring is made to vibrate continuously, can be calibrated against Earth’s rotation via astronomical observations and used to measure time intervals in the laboratory. However, the calibration cannot be carried out with the accuracy called for by moderr. scientific and engineering technology.

To meet the need for a better time standard, atomic clocks have been developed. An atomic clock at the National Institute of Standards and Technology (NIST) in Boulder, Colorado, is the standard for Coordinated Universal Time (UTC) in the United States. Its time signals are available by shortwave radio (stations WWV and WWVH) and by telephone (~03-499-7111). (To set a clock extremely accurately at your particular location, you would have to account for the travel time that is required for these signals to reach you.)

Sshows variations in the length of one day on Earth over a 4-year period, as determined by comparison with a cesium (atomic) clock. Because the variation displayed by. is seasonal and repetitious, we suspect the rotating Earth when there is a difference between Earth and atom as timekeepers. The variation is probably due to tidal effects caused by the Moon and to large-scale winds. The 13th General Conference on Weights and Measures in 1967 adopted a standard second based on the cesium clock:

One second is the time taken by 9 192 631 770 oscillations of the light (of a specified wavelength) emitted by a cesium-133 atoms.

Atomic clocks are so consistent that, in principle, two cesium clocks would have to run for 6000 years before their readings would differ by more than I s. Even such accuracy pales in comparison to that of clocks currently being developed; their precision may be I part in 10  that is, 1 in 1 X 1018 s (about 3 X 1010 y).

Variations in the length of the day over a
4-year period. Note that the entire vertical scale
amounts to only 3 ms (3 milliseconds =
0.003 s).

some approximate time intervals

When the metric system was
proposed in 1792. the hour was redefined
to provide a IO-hour day. The
idea did not catch on. The maker of
this 10-hour watch wisely provided a
small dial that kept conventional 12-
hour time. Do the two dials indicate
the same time?

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