new_contents: """
      A tropical solar year is the length from spring equinox
      to the following spring equinox. 
      
      This recipe was implemented to reply to a topic opened
      at http://skyscript.co.uk/forums/viewtopic.php?t=8563
      and shows that the solar year has an amplitude of more
      than 25 minutes, considering the average year of 365.2425
      days. 
      
      To plot the graphics you must have matplotlib installed .
  
  """
  
  from flatlib.datetime import Datetime
  from flatlib.ephem import ephem
  
  
  def plot(hdiff, title):
      """ Plots the tropical solar length
      by year.
      
      """
      import matplotlib.pyplot as plt
      years = [elem[0] for elem in hdiff]
      diffs = [elem[1] for elem in hdiff]
      plt.plot(years, diffs)
      plt.ylabel('Distance in minutes')
      plt.xlabel('Year')
      plt.title(title)
      plt.axhline(y=0, c='red')
      plt.show()
  
  
  # Set the starting year
  sYear = 1980
  
  # Get successive spring equinox dates
  equinoxes = []
  span = 100
  for year in range(sYear, sYear + span):
      
      # Get the spring equinox date for the year
      dt = Datetime('%s/01/01' % year, '00:00')
      sr = ephem.nextSolarReturn(dt, 0.00)
      equinoxes.append([year, sr.jd])
      
  # Compute successive differences
  diffs = []
  for i in range(len(equinoxes) - 1):
      year1, jd1 = equinoxes[i]
      year2, jd2 = equinoxes[i+1]
      diffs.append([year1, (jd2 - jd1 - 365.2425) * 24 * 60])
      
  print(diffs)
  title = 'Solar year length from %s to %s\n' % (sYear , sYear + span)
  title+= '(Compared to average year of 365.2425)'
  plot(diffs, title)
subject: Add solar years to recipes