def calc_dt_before_500(y)
      t = y / 100.0
      dt  = 10583.6         + \
           (-1014.41        + \
           (   33.78311     + \
           (   -5.952053    + \
           (   -0.1798452   + \
           (    0.022174192 + \
           (    0.0090316521) \
           * t) * t) * t) * t) * t) * t
      return dt
    rescue => e
      raise
    end
def calc_dt_before_500(y)
      t = y / 100.0
      dt  = 10583.6         + \
           (-1014.41        + \
           (   33.78311     + \
           (   -5.952053    + \
           (   -0.1798452   + \
           (    0.022174192 + \
           (    0.0090316521) \
           * t) * t) * t) * t) * t) * t
      return dt
    rescue => e
      raise
    end
=========================================================================
 deltaT (-500 <= year && year < 500)

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