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Astronomic  Fundamental  Principles

2  Astronomic  Fundamental  Principles

The  chronological  sequence  of  marine  tides  has  been  associated  with  the
Moon’s  and  the  Sun’s  relative  positions  to  Earth  already  since  ancient  times.
A  characteristic  of  North  Sea  tides  is  the  fact  that  to  each  upper  culmination  of
the  Moon  can  be  assigned  a  high  water  and  the  ensuing  low  water.  On  average
12  h  25  min  later,  after  the  lower  culmination,  follows  another  high  water  and
subsequent  low  water.  The  average  time  passing  between  the  Moon’s  transit
over  the  prime  meridian  in  upper  culmination  (or  over  180°  W  in  lower  culmination) ­
  and  the  onset  of  the  next  following  high  water  is  called  mean  high  water
interval  (MHWI).  In  the  German  Bight,  intervals  between  9  h  45  min  (Borkum)
and  16  h  27  min  (Geesthacht)  are  observed;  in  Cuxhaven,  the  interval  is  11  h
50  min  (cf.  Table  3).
In  contrast  to  wide  parts  of  the  global  ocean  with  its  fringing  and  marginal  seas,
the  North  Sea  tides  are  very  regular,  in  particular  within  the  German  Bight;
mostly  these  are  semi-diurnal  with  tidal  ranges  of  2  to  4  m.  The  astronomically
explicable  deviations  of  high  and  low  water  intervals  from  their  mean  values  are
less  than  1  h.  The  correspondent  height  deviations  vary  in  the  amount  of  less
than  0,5  m.  Because  this  is  so,  temporal  mean  values  of  levels  and  intervals
already  provide  good  approximations  for  each  individual  case.
A  detailed  illustration  of  the  celestial  rudiments  of  tidal  theory  shall  be  dispensed ­
  with  here.  Commendable  illustrations  of  tides  as  a  function  of  time  have
been  published  already  by  Horn  (1948  and  1960).