ScienceWeek

HISTORY OF THE MEASUREMENT OF TIME

Although a verbal definition of time (other than a purely operational definition) presents philosophical difficulties, from the standpoint of physics, time is the most accurately measured physical quantity. In general, there are two independent and fundamental time scales: a) the dynamical time scale, which is based on the regularities of the motions of the celestial bodies fixed in their orbits by gravitation; b) the atomic time scale, which is based on the characteristic frequency of electromagnetic radiation emitted or absorbed in quantum transitions between internal energy states of atoms or molecules.

The first known device for indicating the time of day was the "gnomon", which appeared in approximately 3500 BC. This instrument consisted of a vertical stick or pillar, the length of the shadow cast by the stick or pillar providing an indication of the time of day. By the 8th century BC, more precise devices were in use. The earliest known sundial still preserved is an Egyptian shadow clock dating at least from the 8th century BC, and which consists of a straight base with a raised crosspiece at one end. On the base is inscribed a scale of 6 time divisions. The base is placed in an east-west direction with the crosspiece at the east end in the morning and at the west end in the afternoon. The shadow of the crosspiece on the base indicates the time.

The Babylonian hemispherical sundial (hemicycle), apparently invented by the astronomer Barosus in approximately 300 BC, consisted of a cubical block into which was cut a hemispherical opening. To the opening was fixed a pointer whose end lay at the center of the hemispherical space. The path traveled by the shadow of the pointer was approximately a circular arc whose length and position varied according to the seasons. An appropriate number of arcs were inscribed on the internal surface of the hemisphere, each arc divided into 12 subdivisions. Each day, reckoned from sunrise to sunset, had 12 equal intervals or "hours". Since the length of the day varied according to the season, these hours were known as "temporary hours".

The Greeks developed and constructed sundials of considerable complexity in the 3rd and 2nd centuries BC, including instruments with either vertical, horizontal, or inclined dials, indicating time in temporary hours. The Romans also used sundials with temporary hours, and some of these Roman sundials were portable. The Arabs increased the variety of sundial designs, and at the beginning of the 13th century AD the Arabs wrote on the construction of sundials with cylindrical, conical, and other surfaces.

In general, a "clock" is a device that performs regular movements in equal intervals of time, the device linked to a counting mechanism that records the number of movements. The first public clock that struck the hours was made and erected in Milan (IT) in 1335. The oldest surviving clock is that at Salisbury Cathedral, which dates from 1386. In approximately 1500, small portable clocks driven by a spring appeared, the dials with an hour hand only. The pendulum was applied as a time controller in clocks beginning in 1656, although Galileo had already suggested this in 1582.

The familiar subdivision of the day into 24 hours, the hour into 60 minutes, and the minute into 60 seconds is of ancient origin, but these subdivisions came into general use in approximately 1600 AD. When the increasing accuracy of clocks led to the adoption of the "mean solar day", which contained 86,400 seconds, the "mean solar second" became the basic unit of time.

The adoption of the International System (SI) second, defined on the basis of atomic phenomena, as the fundamental time unit, occurred provisionally in 1964 and finally in 1967. A second is now defined as 9,192,631,770 cycles of radiation associated with the transition between the two hyperfine levels of the ground state of the cesium-133 atom. The number of cycles of radiation was chosen to make the length of the defined second correspond as closely as possible to that of the previous standard, the astronomically determined second of "Ephemeris Time" (defined as 1/(86,400) of the mean solar day).

The following points are made by J.C.Bergquist et al (Physics Today 2001 March):

1) The authors point out that although a unit of time can be constructed from other physical constants, time is usually viewed as an arbitrary parameter to describe dynamics. The frequency of any periodic event, such as the mechanical oscillation of a pendulum, or the quantum oscillation of an atomic dipole, can be adopted to define the unit of time, the second.

2) For centuries, the mean solar day served as the unit of time, but Earth's period of rotation is irregular and slowly increasing. In 1956, the International Astronomical Union and the International Committee on Weights and Measures recommended adopting Ephemeris Time, based on Earth's orbital motion around the Sun, as a more accurate and stable basis for the definition of time. This recommendation was formally ratified in 1960 by the General Conference on Weights and Measures.

3) Until the definition of the second in terms of atomic time in 1967, most work in standards laboratories was devoted to developing secondary standards, such as lumped-element circuits and quartz crystals, whose resonant frequencies could be calibrated relative to Ephemeris Time. But frequencies derived from resonant transitions in atoms or molecules offer important advantages over macroscopic oscillators. Any unperturbed atomic transition is identical from atom to atom, so two clocks based on such a transition should generate the same time. Also, unlike macroscopic devices, atoms do not wear out, and as far we know they do not change their properties over time.

4) The basic idea of most atomic clocks is straightforward: a) First, identify a transition between two non-degenerate energy states of an atom. b) Then, create an ensemble of these atoms (e.g., in an atomic beam or storage device). c) Next, illuminate the atom with radiation from a tunable source that operates near the transition frequency. d) Sense and control the frequency where the atoms absorb maximally. e) When maximal absorption is achieved, count the cycles of the oscillator: a certain number of elapsed cycles generates a standard interval of time. But although the general idea of an atomic clock is straightforward, in practice there are a number of experimental difficulties that limit accuracy. The latest atomic clocks use a single ion to measure time with an anticipated precision of one part in 10^(18).