code: def cos_zen(utc_time, lon, lat):
      
      lon = np.deg2rad(lon)
      lat = np.deg2rad(lat)
  
      r_a, dec = sun_ra_dec(utc_time)
      h__ = _local_hour_angle(utc_time, lon, r_a)
      return (np.sin(lat) * np.sin(dec) + np.cos(lat) * np.cos(dec) * np.cos(h__))
docs: Cosine of the sun-zenith angle for *lon*, *lat* at *utc_time*.
      utc_time: datetime.datetime instance of the UTC time
      lon and lat in degrees.