Astronomy Principles and Practice Fourth Edition.pdf

32 The astronomer’s measurements
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).