def calc_dt_before_1600(y)
      t = (y - 1000) / 100.0
      dt  = 1574.2         + \
           (-556.01        + \
           (  71.23472     + \
           (   0.319781    + \
           (  -0.8503463   + \
           (  -0.005050998 + \
           (   0.0083572073) \
           * t) * t) * t) * t) * t) * t
      return dt
    rescue => e
      raise
    end
def calc_dt_before_1600(y)
      t = (y - 1000) / 100.0
      dt  = 1574.2         + \
           (-556.01        + \
           (  71.23472     + \
           (   0.319781    + \
           (  -0.8503463   + \
           (  -0.005050998 + \
           (   0.0083572073) \
           * t) * t) * t) * t) * t) * t
      return dt
    rescue => e
      raise
    end
=========================================================================
 deltaT (500 <= year && year < 1600)

 @param:  y   (Year coefficient for deltaT calculation)
 @return: dt  (sec)
=========================================================================