@article{ValliappanArltDierckeetal.2016,
  author    = {Senthamizh Pavai Valliappan and Rainer Arlt and Andrea Diercke and Carsten Denker and J. M. Vaquero},
  title     = {Sunspot group tilt angle measurements from historical observations},
  journal   = {Advances in space research},
  volume    = {58},
  pages     = {1468 -- 1474},
  doi       = {10.1016/j.asr.2016.03.002},
  year      = {2016},
}