Astronomy Principles and Practice Fourth Edition.pdf

Time 31
to house radio transmitters and direct measurements made on the Faraday rotation effect have been
used to explore the properties of the ionosphere.
5.5 Time
If measurements of the positions, brightnesses and polarization of astronomical sources are repeated,
the passage of time reveals that, in some cases, the position of the source or some property of its
radiation changes. These time variations of the measured values are of great importance in determining
many of the physical properties of the radiating sources. It is, therefore, very necessary that the times
of all observations and measurements must be recorded. The accuracy to which time must be recorded
obviously depends on the type of observation which is being attempted.
It would be out of place here to enter into a philosophical discussion on the nature of time.
However, it might be said that some concept which is called time is necessary to enable the physical
and mechanical descriptions of any body in the Universe and its interactions with other bodies to be
related. One of the properties of any time scale which would be appealing from certain philosophical
standpoints is that time should flow evenly. It is, therefore, the aim of any timekeeping system that
it should not show fluctuations in the rate at which the flow of time is recorded. If fluctuations are
present in any system, they can only be revealed by comparison with clocks which are superior in
accuracy and stability. Timekeeping systems have changed their form as clocks of increased accuracy
have been developed; early clocks depended on the flow of sand or water through an orifice, while the
most modern clocks depend on processes which are generated inside atoms.
About a century ago, the rotation of the Earth was taken as a standard interval of time which
could be divided first into 24 parts to obtain the unit of an hour. Each hour was then subdivided into
a further 60 parts to obtain the minute, each minute itself being subdivided into a further 60 parts to
obtain the second. This system of timekeeping is obtained directly from astronomical observation, and
is related to the interval between successive appearances of stars at particular positions in the sky. For
practical convenience, the north–south line, or meridian, passing through the observatory is taken as a
reference line and appearances of stars on this meridian are noted against some laboratory timekeeping
device. As laboratory pendulum clocks improved in timekeeping precision, it became apparent from
the meridian transit observations that the Earth suffered irregularities in the rate of its rotation. These
irregularities are more easily shown up nowadays by laboratory clocks which are superior in precision
to the now old-fashioned pendulum clock.
At best, a pendulum clock is capable of accuracy of a few hundredths of a second per day. A
quartz crystal clock, which relies on a basic frequency provided by the vibrations of the crystal in an
electronic circuit, can give an accuracy better than a millisecond per day, or of the order of one part in
10 8 ; and this is usually more than sufficient for the majority of astronomical observations. Even more
accurate sources of frequency can be obtained from atomic transitions. In particular, the clock which
relies on the frequency which can be generated by caesium atoms provides a source of time reference
which is accurate to one part in 10 11 . The caesium clock also provides the link between an extremely
accurate determination of time intervals and the constants of nature which are used to describe the
properties of atoms.
Armed with such high-precision clocks, the irregularities in the rotational period of the Earth can
be studied. Some of the short-term variations are shown to be a result of the movement of the observer’s
meridian due to motion of the rotational pole over the Earth’s surface. Other variations are seasonally
dependent and probably result in part from the constantly changing distribution of ice over the Earth’s
surface. Over the period of one year, a typical seasonal variation of the rotational period may be of the
order of two parts in 10 8 .
Over and above the minute changes, it is apparent that the Earth’s rotational speed is slowing
down progressively. The retardation, to a great extent, is produced by the friction which is generated