208 Chapter 8 Time and Time-Related Parameters
Europe. Later, during the 16th century, came spring-loaded clocks. The spring mechanism
design eventually led to smaller clocks and to watches. The oscillation of a pendulum was
the next advancement in the design of clocks.
Quartz clocks eventually replaced the mechanical clocks around the middle of the 20th cen-
tury. A quartz clock or watch makes use of the piezoelectric property of quartz crystal. A quartz
crystal, when subjected to a mechanical pressure, creates an electric field. The inverse is also
true — that is to say, the shape of the crystal changes when it is subjected to an electric field.
These principles are used to design clocks that make the crystal vibrate and generate an electric
signal of constant frequency.
As we stated in Chapter 6, the natural frequency of the cesium atom was adopted as the
new standard unit of time. The unit of a second is now formally defined as the duration of
9,192,631,770 periods of the radiation corresponding to the transition between the two
hyperfine levels of the ground state of the cesium 133 atom.The Need for Time Zones
You know that the earth rotates about an axis that runs from the South Pole to the North Pole,
and it takes the earth 24 hours to complete one revolution about this axis. Moreover, from
studying globes and maps, you may have noticed that the earth is divided into 360 circular arcs
that are equally spaced from east to west; these arcs are calledlongitudes. The zero longitude was
arbitrarily assigned to the arc that passes through Greenwich, England. Because it takes the
earth 24 hours to complete one revolution about its axis, every 15 degrees longitude corre-
sponds to 1 hour (360 degrees /24 hours 15 degrees per one hour). For example, someone
exactly 15 degrees west of Chicago will see the sun in the same exact position one hour later as
it was observed by another person in Chicago one hour before. The earth is also divided into
latitudes, which measure the angle formed by the line connecting the center of the earth to the
specific location on the surface of the earth and the equatorial plane, as shown in Figure 8.2.N
S
Equator
Longitude
Latitude
■Figure 8.2
The longitudes and latitudes
of the earth.Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀