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 10⁸; 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 10¹¹. 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 themovement 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 10⁸.
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 by the tidal movement of the oceans and seas and is thus connected to the motion of the Moon. The effects of the retardation show up well in the apparent motions of the bodies of the Solar System.
After the orbit of a planet has been determined, its positions at future times may be predicted. The methods employed make use of laws which assume that time is flowing evenly. The predictions, or ephemeris positions, can later be checked by observation as time goes by. If an observer uses the rotation of the Earth to measure the passage of time between the time when the predictions are made and the time of the observation, and unknowingly assumes the Earth’s rotational period to be constant, it is found that the planets creep ahead of their ephemeris positions at rates which are proportional to
their mean motions. The phenomenon is most pronounced in the case of the Moon.
Suppose that a time interval elapses between the time the calculations are performed and the time that the ephemeris positions are checked by observation. The time interval measured by the rotation of the Earth might be counted as a certain number of units. However, as the Earth’s rotation is continuously slowing down and the length of the time unit is progressively increasing in comparison with the unit of an evenly-flowing scale, the time interval corresponds to a larger number of units on an evenly-flowing scale. Unknown to the observer who takes the unit of time from the Earth’s rotation, the real time interval is actually longer than he/she has measured it to be and the planets, therefore, progress further along their orbits than is anticipated. Thus, the once unexplained ‘additional motions’ of the planets and the Moon are now known to be caused by the fact that the Earth’s rotational period slows down during the interval between the times of prediction and of observation. It is now practice to relate astronomical predictions to a time scale which is flowing evenly, at least to the accuracy of the best clocks available. This scale is known as Dynamical Time (DT)