TY  - JOUR
A1  - Valliappan, Senthamizh Pavai
A1  - Arlt, Rainer
A1  - Diercke, Andrea
A1  - Denker, Carsten
A1  - Vaquero, J. M.
T1  - Sunspot group tilt angle measurements from historical observations
JF  - Advances in space research
N2  - 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.
KW  - Sun: sunspots
KW  - Tilt angles
KW  - Cycle-averaged tilt angle
Y1  - 2016
U6  - https://doi.org/10.1016/j.asr.2016.03.002
SN  - 0273-1177
SN  - 1879-1948
VL  - 58
SP  - 1468
EP  - 1474
PB  - Elsevier
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Arlt, Rainer
A1  - Valliappan, Senthamizh Pavai
A1  - Schmiel, C.
A1  - Spada, F.
T1  - Sunspot positions, areas, and group tilt angles for 1611-1631 from observations by Christoph Scheiner
JF  - Mountain research and development
N2  - 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.
KW  - Sun: activity
KW  - sunspots
KW  - history and philosophy of astronomy
Y1  - 2016
U6  - https://doi.org/10.1051/0004-6361/201629000
SN  - 1432-0746
VL  - 595
PB  - EDP Sciences
CY  - Les Ulis
ER  -