@article{ValliappanArltDierckeetal.2016,
  author    = {Valliappan, Senthamizh Pavai and Arlt, Rainer and Diercke, Andrea and Denker, Carsten and Vaquero, J. M.},
  title     = {Sunspot group tilt angle measurements from historical observations},
  series = {Advances in space research},
  volume    = {58},
  journal   = {Advances in space research},
  publisher = {Elsevier},
  address   = {Oxford},
  issn      = {0273-1177},
  doi       = {10.1016/j.asr.2016.03.002},
  pages     = {1468 -- 1474},
  year      = {2016},
  abstract  = {Sunspot positions from various historical sets of solar drawings are analyzed with respect to the tilt angles of bipolar sunspot groups. Data by Scheiner, Hevelius, Staudacher, Zucconi, Schwabe, and Sporer deliver a series of average tilt angles spanning a period of 270 years, additional to previously found values for 20th-century data obtained by other authors. We find that the average tilt angles before the Maunder minimum were not significantly different from the modem values. However, the average tilt angles of a period 50 years after the Maunder minimum, namely for cycles 0 and 1, were much lower and near zero. The normal tilt angles before the Maunder minimum suggest that it was not abnormally low tilt angles which drove the solar cycle into a grand minimum. (C) 2016 COSPAR. Published by Elsevier Ltd. All rights reserved.},
  language  = {en}
}
@article{ArltValliappanSchmieletal.2016,
  author    = {Arlt, Rainer and Valliappan, Senthamizh Pavai and Schmiel, C. and Spada, F.},
  title     = {Sunspot positions, areas, and group tilt angles for 1611-1631 from observations by Christoph Scheiner},
  series = {Mountain research and development},
  volume    = {595},
  journal   = {Mountain research and development},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {1432-0746},
  doi       = {10.1051/0004-6361/201629000},
  pages     = {10},
  year      = {2016},
  abstract  = {Methods. In most cases, the given orientation of the ecliptic is used to set up the heliographic coordinate system for the drawings. Positions and sizes are measured manually on screen. Very early drawings have no indication of their orientation. A rotational matching using common spots of adjacent days is used in some cases, while in other cases, the assumption that images were aligned with a zenith-horizon coordinate system appeared to be the most probable.},
  language  = {en}
}