Q:  What is the frequency on the date marked "1000"?

          f1  =   approx 11.2085 Hz  

              =          11.2085  rev/s


          P1  =   1/f1   =   1 / (11.2085 rev/s)

              =   0.089218  s


  Q:  What is the frequency on the dark marked "5000"?

          f2  =   approx 11.2030 Hz  

              =          11.2030  rev/s


          P2  =   1/f2   =   1 / (11.2030 rev/s)

              =   0.089262  s



  Q:  What is the change in period over these 4000 days?

          P2 - P1    =   0.089218  -  0.089262

                     =   0.0000438  s

                     =   4.38 x 10^(-5) s