@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{DierckeArltDenker2015,
  author    = {Diercke, Andrea and Arlt, Rainer and Denker, Carsten},
  title     = {Digitization of sunspot drawings by Sporer made in 1861-1894},
  series = {Astronomische Nachrichten = Astronomical notes},
  volume    = {336},
  journal   = {Astronomische Nachrichten = Astronomical notes},
  number    = {1},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {0004-6337},
  doi       = {10.1002/asna.201412138},
  pages     = {53 -- 62},
  year      = {2015},
  abstract  = {Most of our knowledge about the Sun's activity cycle arises from sunspot observations over the last centuries since telescopes have been used for astronomy. The German astronomer Gustav Sporer observed almost daily the Sun from 1861 until the beginning of 1894 and assembled a 33-year collection of sunspot data covering a total of 445 solar rotation periods. These sunspot drawings were carefully placed on an equidistant grid of heliographic longitude and latitude for each rotation period, which were then copied to copper plates for a lithographic reproduction of the drawings in astronomical journals. In this article, we describe in detail the process of capturing these data as digital images, correcting for various effects of the aging print materials, and preparing the data for contemporary scientific analysis based on advanced image processing techniques. With the processed data we create a butterfly diagram aggregating sunspot areas, and we present methods to measure the size of sunspots (umbra and penumbra) and to determine tilt angles of active regions. A probability density function of the sunspot area is computed, which conforms to contemporary data after rescaling. (C) 2015 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim},
  language  = {en}
}