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Time
I
INTRODUCTION
Time, conscious experience of duration, the period during which an action or event
occurs. Time is also a dimension representing a succession of such actions or events.
Time is one of the fundamental quantities of the physical world, similar to length and
mass in this respect. The concept that time is a fourth dimension—on a par with the
three dimensions of space: length, width, and depth—is one of the foundations of
modern physics. Time measurement involves the establishment of a time scale in
order to refer to the occurrence of events. The precise determination of time rests on
astronomical and atomic definitions that scientists have established with the utmost
mathematical exactness.
Physicists agree that time is one of the most difficult properties of our universe to
understand. Although scientists are able to describe the past and the future and
demarcations such as seconds and minutes, they cannot define exactly what time is.
The scientific study of time began in the 16th century with the work of Italian physicist
and astronomer Galileo Galilei. In the 17th century English mathematician and
physicist Sir Isaac Newton continued the study of time. A comprehensive explanation
of time did not exist until the early 20th century, when German-born American
physicist Albert Einstein proposed his theories of relativity. These theories define time
as the fourth dimension of a four-dimensional world consisting not just of space but of
space and time.
Several ways to measure time are in use today. Solar time is based on the rotation of
Earth on its axis. It makes use of the Sun’s apparent motion across the sky to
measure the duration of a day. Sidereal time is also based on Earth’s rotation, but
uses the apparent motion of the “fixed” stars across the sky as Earth rotates as the
basis for time determination. Standard time, the familiar clock time most people use in
everyday life, is based on the division of Earth’s sphere into 24 equal time zones.
Dynamical time—formerly called ephemeris time—is the timescale of astronomy.
Astronomers use the orbit of Earth around the Sun, as well as the orbital motions of
the Moon and the other planets, to determine dynamical time. Atomic time is based on
the frequency of electromagnetic waves that are emitted or absorbed by certain atoms
or molecules under particular conditions. It is the most precise method for measuring
time.
II
MEASUREMENT AND DETERMINATION
The measurement of time passage probably began with the concepts of past, present,
and future. Throughout history humans have used various celestial bodies—that is, the
Sun, the Moon, the planets, and the stars—to measure the passage of time. Ancient
peoples used the apparent motion of these bodies through the sky to determine the
seasons, the length of the month, and the length of the year. See also Calendar.
Humans created the sundial and the hourglass to measure time. The first mechanical
clocks were invented in the 14th century. The use of the pendulum clock became
popular in the 1600s when Dutch astronomer Christiaan Huygens applied the
pendulum to regulate the movement of clocks (see Clocks and Watches). At this point,
clocks became accurate enough to record minutes as well as hours.
The use of chronometers (precision timepieces) for precise measurement of time
played an important role in navigation from the mid-18th century to the 1920s by
helping to determine longitude. Prior to the invention of an accurate chronometer in
the mid-18th century, navigators could easily determine their latitude, but determining
longitude was more difficult. If a reading of the Sun’s position was not made at
precisely the noon hour, great errors in longitude could result. For example, an error
of a second of longitude, for a ship at Earth’s equator, produces an error in longitude
position of about 400 m (about 1,300 ft). Precise time measurement gained further
importance with the evolution of modern industrial societies. During the late 18th
century, the Industrial Revolution prompted factory work to start and stop at
appointed times, thus changing the tempo of life. The growth of railroads and the use
of train schedules in the mid-19th century further emphasized the need for precise
timekeeping.
A
Solar Time
The apparent motion of the Sun across the sky has long been used as a basis for
measuring time. Under solar time, at any given locality it is noon—twelve o’clock in
the daytime, or midday—when the Sun reaches the highest point in the sky. Noon at
any place on the surface of Earth is when the Sun's direct rays pass over the meridian
of that particular place (See also Prime Meridian). A meridian is an imaginary line that
stretches from pole to pole on Earth's surface. A meridian is also known as a line of
longitude (see Latitude and Longitude). The interval between successive passages of
the Sun across the same meridian is one day, and this day is by custom divided into
24 hours. The amount of daylight in a day varies throughout the year, based on the
tilt of Earth’s axis and its orientation to the Sun as the seasons change (see Season).
For the same reasons, a day in solar time is not always 24 hours long. The difference
in the length of the 24-hour day during different seasons of the year can amount to as
much as 16 minutes. With the invention of accurate timepieces in the 17th century,
this difference in the length of the day became significant. To overcome this problem
scientists invented mean solar time, which is based on the motion of a hypothetical
sun traveling at an even rate throughout the year.
Universal time is simply the mean solar time measured at the Greenwich meridian,
which is designated 0° longitude and from which the longitude of all points on the
surface of Earth are measured. The meridian passing through the original site of the
Royal Greenwich Observatory in Greenwich, England, has been recognized by
international agreement since 1884 as the prime meridian. Universal time was
originally called Greenwich Mean Time (GMT) but replaced that designation in 1928.
Universal time is used to denote solar time when an accuracy to about one second
suffices.
Because the basis of mean solar time relates to the motion of a hypothetical sun,
scientists established a base position from which the mean time is calculated. This
base position is the vernal, or spring, equinox (see Ecliptic), an imaginary point in the
sky that is, nevertheless, calculated with great accuracy by astronomers (see
Astronomy). Practically, scientists define the location of the vernal equinox by
reference to the position of the “fixed” stars.
B
Sidereal Time
Sidereal time is based on the apparent motion of the distant, “fixed” stars across the
sky. It has various astronomical purposes, such as predicting locations of objects in
outer space. The primary unit of sidereal time is the sidereal day, which is subdivided
into 24 sidereal hours. Each sidereal hour is subdivided into 60 minutes, and each
minute into 60 seconds. Astronomers rely on sidereal clocks because any given star
will cross the same meridian, or line of longitude, at the same sidereal time
throughout the year.
According to convention, each sidereal day begins at the instant the vernal equinox
crosses the prime meridian. The vernal equinox is the point on the celestial sphere at
which the sun crosses the plane of the equator, moving from south to north. The
celestial sphere is the apparent surface of the heavens, on which the stars appear to
be fixed.
The U.S. Naval Observatory in Washington, D.C., uses mathematical tables to
calculate mean solar time from mean sidereal time. The sidereal day is almost four
minutes shorter than the mean solar day, so a discrepancy exists between the total
number of hours in a mean solar year and in a mean sidereal year. This discrepancy
arises because Earth rotates on its axis at the same time that it revolves around the
Sun. According to mean sidereal time, Earth returns to the vernal equinox every 365
days 6 hours 9 minutes 9.54 seconds. According to mean solar time, Earth returns to
the vernal equinox every 365 days 5 hours 48 minutes 45.5 seconds. The difference
between the two is 20 minutes 24.04 seconds.
C
Standard Time
In 1883 an international agreement introduced the concept of standard time. Standard
time was adopted to avoid the complications of adhering to railroad time schedules
when each community used its own local solar time. The base position for standard
time is the prime meridian. The distance east or west of Greenwich determines the
standard time zone and, thus, the standard time of a particular location.
D
Dynamical Time
Astronomers use dynamical time for the precise study of the motion of celestial
bodies. Dynamical time replaced ephemeris time in 1984, when the International
Astronomical Union (IAU) updated the Astronomical Almanac. Scientists introduced
ephemeris time in 1940 and selected the orbital position of Earth around the Sun as
the standard by which to define the numerical measure of ephemeris time. In the
1950s the IAU decided that ephemeris time could be based on the orbital position of
any planet or satellite. Time would be determined by comparing the orbital position of
a particular planet or satellite (natural or artificial) at a particular time to an
ephemeride. An ephemeride is a table of orbital positions of a planet or a satellite
mapped over a period of time.
The annual revolution of Earth around the Sun is the basis for dynamical time, and the
base position of measure (as in sidereal time) is the vernal equinox. When the
greatest degree of accuracy is required in computing the positions of a planet or star,
astronomers use dynamical time, because neither mean solar time nor mean sidereal
time is sufficiently accurate, as the motion of Earth on its axis is not regular and even.
Variations in the rate of Earth’s rotation amount to 1 or 2 seconds per year.
E
Atomic Time
Atomic time is the time scale of physics. Scientists use atomic time when they require
exceptionally precise measurements of time intervals relating to physical phenomena.
Clocks became more accurate and precise through the centuries, and with the
introduction of atomic clocks—specifically, the construction of a high-precision cesium
atomic clock in 1955—extremely accurate measurement of time became possible.
Early mechanical clocks varied by several minutes each day. In the 1920s, vibrating
quartz crystals were accurate to a few ten-thousandths of a second per day. The
cesium atom clocks used in the 1980s lost less than a second in 3,000 years. In the
1990s the National Institute of Standards and Technology (NIST) in the United States
established an atomic clock—the NIST-7, also a cesium clock—that is accurate to a
single second over 3 million years. The electronic components of atomic clocks are
regulated by the frequency of radiation emitted or absorbed by a particular atom or
molecule.
III
SCIENTIFIC STANDARD OF TIME
Until 1955 astronomers and scientists calculated the scientific standard of time—the
second—based on Earth's period of rotation. They defined the second as 1/86,400 of a
mean solar day. When scientists realized that Earth's rate of rotation is irregular, a
redefinition of the second became necessary. In 1955 the IAU defined the second as
1/31,556,925.9747 of the solar year that was in progress at noon on December 31,
1899. The International Committee on Weights and Measures adopted this definition in
1956. Since 1967 the official length of a second in the International System of Units
(SI) has been defined by atomic standards: a second is equal to 9,192,631,770
oscillations, or periods, of the radiation corresponding to the transition between two
hyperfine (closely spaced) energy states of the cesium-133 atom.
IV
TIME ZONES
For the purposes of standard time, Earth is divided into 24 standard time zones. The
time zones extend from the North Pole to the South Pole, and within each zone the
time is the same throughout. Within each time zone, local noon corresponds
approximately to the time at which the Sun crosses the central meridian, or longitude,
of that zone.
The distance east or west of the Greenwich meridian determines different time zones.
According to the scientific model of standard time, each standard time zone spans 15°
of longitude. In fact, the borders of time zones are bent to conform to state and
country boundaries, as well as to facilitate commercial activities. In 1966 the U.S.
Congress passed the Uniform Time Act, which established eight standard time zones
for the United States and its outlying regions. In 1983 several time zone boundaries
were altered so that most of Alaska, which formerly spanned four zones, could be
unified under one time zone. The U.S. standard time zones are the Atlantic, Eastern,
Central, Mountain, Pacific, Alaska, Hawaii-Aleutian, and Samoa zones.
There are five standard time zones in Canada. From east to west these are the
Atlantic, Eastern, Central, Mountain, and Pacific time zones. Newfoundland has its own
time zone, which is not a standard time zone. Newfoundland time is 30 minutes ahead
of Atlantic time.
A
International Date Line
The International Date Line is a time zone boundary. It is an imaginary line extending
from the North Pole to the South Pole and separating one calendar day from the next.
Along most of its length, the International Date Line corresponds to the 180th
meridian of longitude. A traveler moving eastward across the line sets his or her
calendar back one day, and one traveling westward sets the calendar a day ahead.
V
STUDY OF TIME AND TIMESCALES
Several areas of science and the humanities—including physics, geology, biology, and
philosophy—overlap with the scientific study of time. Time scales and the concept of
time are integral to our understanding of the universe, Earth, and the organisms that
live on Earth.
A
Physics of Time
Einstein’s first major contribution to the study of time occurred in 1905, when he
introduced his special theory of relativity and showed how time changes with motion.
The word relativity derives from the fact that the appearance of the world depends on
the observer’s state of motion and is relative to the observer. Today scientists do not
see problems of time or motion as absolute with single correct answers. Because time
is relative to the speed an observer is traveling, there can never be a clock at the
center of the universe to which everyone can set his or her watch. Einstein’s special
theory of relativity tell us that an object traveling at high speeds ages more slowly
than an object that is not traveling as fast. This means that if a person from Earth
were to travel in outer space at a speed close to the speed of light (about 300,000 km
per sec or about 186,000 mi per sec), that person could return to Earth thousands of
years into Earth’s future.
Time is distorted in regions of large masses, such as stars and black holes. In
Einstein’s general theory of relativity, which was introduced in 1916, the very
existence of time depends on the presence of space. Einstein’s general theory explains
how gravity warps and slows time and why time moves very slightly slower in regions
of high gravity, such as near stars, compared to regions of lesser gravity, such as on
planets. This time-slowing effect becomes pronounced in regions of extremely high
gravity, such as near black holes.
B
Geologic Time
Geologists—scientists who study Earth—use the geologic time scale to measure spans
of time in the 4.5-billion-year history of Earth. This time scale measures blocks of time
and is important for understanding the biological and geologic history—and evolution—
of Earth. The longest blocks of time, eons, are divided into shorter blocks called eras.
Eras are divided into periods, which are made up of epochs.
C
Biological Clocks
Many organisms exhibit biological rhythms. These are periodic biological fluctuations—
changes in sleep patterns or hibernation patterns, for example—that occur in response
to periodic environmental changes such as the cycles of night and day, darkness and
light, and winter and summer. Organisms use biological clocks—such as circadian, or
daily, rhythms—to remain in harmony with the cycles of day and night and the
seasons.
D
Philosophy of Time
Philosophers have long argued about the nature of time. Some philosophers, notably
German philosopher Immanuel Kant, have proposed that newborn babies may
experience the passage of time. Others have proposed that the human mind must
learn to construct time. For example, French philosopher Henri Bergson thought of
time as something entirely derived from experience. In Bergson's doctoral
dissertation, Time and Free Will (1889; translated 1910), he proposed that time is a
matter of subjective experience. According to Bergson, an infant would not experience
time directly but rather would have to learn how to experience it.
VI
TIME DILATION
Time is not a physical constant. Motion and gravity effect time by dilating (slowing) it
or by expanding its duration. In 1905 Albert Einstein described the effect of motion on
time in his special theory of relativity. In 1916 he described the effect of gravity on
time in his general theory of relativity.
Time dilation effects due to motion were experimentally observed in the early 1970s.
Researchers placed atomic clocks on commercial airliners and observed the expected
changes in time as measured by those clocks relative to similar clocks on the ground.
In particular, when the planes traveled east, in the direction of Earth’s rotation, the
clocks on the airliners were 59 nanoseconds (59 billionths of a second) slow relative to
the atomic clocks on the ground. When the airplanes traveled west, the clocks were
273 nanoseconds fast. This discrepancy is caused by the rotation of Earth, which
causes an additional time dilation. If the effect of Earth's rotation is removed, the time
dilation produced by the motion of the airliners confirms Einstein's theory of how time
changes with motion, as the dilation is in agreement with predictions made by the
theory.
Time dilation effects due to gravity have been experimentally verified in many ways.
For example, time on the Sun's surface runs about two parts in a million slower than
on Earth because of the Sun's much higher gravity. In 1968 American physicist Irwin
Shapiro confirmed this effect when he showed that radar signals (see Radar
Astronomy) and their reflections from planets are delayed when the Sun is near the
path of the signals.
Contributed By:
Clifford A. Pickover
Microsoft ® Encarta ® Encyclopedia 2004. © 1993-2003 Microsoft Corporation. All
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