  Any model of tides is based on a specific hypothesis of how lagging depends
on the tidal-flexure frequency. For example, Gerstenkorn (1955), MacDonald
(1964), and Kaula (1964) assumed constancy of the geometric lag angle, while
Singer (1968) and Mignard (1979, 1980) asserted constancy of the time lag.
Thus, each of these two models was based on a certain law of scaling of the
geometric lag.
  The actual dependence of the geometric lag on the frequency is more
complicated and is determined by the rheology of the planet. Besides, each
particular functional form of this dependence will unambiguously fix the
appropriate form of the frequency dependence of the tidal quality factor, Q.
Since at present we know the shape of the dependence of Q upon the frequency,
we can reverse our line of reasoning and single out the appropriate actual
frequency-dependence of the angular lag. This dependence turns out to be
different from those employed hitherto, and it entails considerable alterations
in the time scales of the tide-generated dynamical evolution. Phobos' fall on
Mars is an example we consider.
