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The Maunder Minimum was a period with significantly reduced solar activity between 1645 and 1715, approximately. The transition between the low solar activity in the Maunder Minimum and the subsequent "normal" regime of solar activity was gradual. However, there are discrepancies in the solar activity level from sunspot number indices and solar activity proxies in that period. Among the contemporaneous observers, Johann L. Rost and Sebastian Alischer were two key sunspot observers to understand the solar activity in this transition just after the Maunder Minimum. We have revised all their sunspot records, counting the number of groups and individual sunspots to derive reliable data for the solar activity level for the period 1716–1726. We found significant misinterpretations of the sunspot group counting assigned to these astronomers in the existing group number databases. Our new group sunspot counting significantly reduces the number of groups for Rost and Alischer's observations compared to entries in existing databases. Furthermore, our sunspot number estimates (obtained from the active day fraction methodology) of the maximum amplitude of Solar Cycles −3 and −4 are significantly lower than the amplitudes according to the official sunspot number, but they are compatible with sunspot number values obtained from solar activity proxies such as radioisotopes. Our result would imply that solar activity after the Maunder Minimum recovered more gradually and with a lower intensity than previously considered. [ABSTRACT FROM AUTHOR]

In this work, we predict the maximum amplitude (using the 13-month smoothed Solar Influences Data Analysis Center (SILSO) Sunspot Number, version 2) of Solar Cycle 25 using as a predictor the slope of the inflection point during the ascending part of the cycle. After a description of the data and methodology employed in this work, we obtain a value of 131 ± 32 for the maximum amplitude of Solar Cycle 25. Finally, we discuss this result in the context of the current debate on the prediction of solar activity and compare it with other predictions of Solar Cycle 25 obtained by other methods. [ABSTRACT FROM AUTHOR]

Long-term solar activity can be studied using several parameters. Some of the most used are based on the sunspot counting. The active day fraction (ADF) is the simplest index derived from this counting. It is reliable in periods of low solar activity such as the Maunder minimum (MM). In this work, we study the relationship between the ADF and the sunspot number. We have obtained that the optimal fit of that relationship is an exponential function whose exponent is a degree 3 polynomial including all data except those with ADF equal to 100%. Then, we use that fit to estimate the sunspot number during the MM from the ADF calculated from the most recent sunspot group number database. Our estimations of the annual sunspot numbers are below 15, except that for 1656, which is 40.8, whereas our estimations of the triennial sunspot numbers are below 10 from 1648 to 1714. We have found peaks of the solar cycle in the middle of the 1650s, 1670s, 1680s, and 1700s but no clear evidence of solar cycle in the 1660s and 1690s, likely due to the scarcity of the available data. Our results agree with previous works obtaining values significantly higher than those of the group sunspot number derived by Hoyt and Schatten in 1998 but still fully compatible with a grand minimum period. [ABSTRACT FROM AUTHOR]

The Maunder Minimum (MM) was a period of prolonged solar activity minimum between 1645 and 1715. Several works have identified a significant number of problematic spotless days in the MM included in existing data bases. We have found a list of exact spotless (in the second half of 1709) and spot days (January and August 1709) provided by Johann Heinrich Müller. We computed the most probable value and upper/lower limits of the active day fraction (ADF) from Müller's data using the hypergeometrical probability distribution. Our sample is not strictly random because Müller recorded observations in consecutive days when he observed sunspots. Therefore, our result represents an upper threshold of solar activity for 1709. We compared this result with annual values of the ADF calculated for the Dalton Minimum and the most recent solar cycles. We concluded that, although 1709 is one of the most active years in the MM, it was less active than most years both in the Dalton Minimum and in the most recent solar cycles. Therefore, the solar activity level estimated in this work for 1709 represents robust evidence of low solar activity levels in the MM. [ABSTRACT FROM AUTHOR]

A series of measurements of the solar diameter taken in the meridian line of the Basilica of San Petronio (Bologna, Italy) between 1655 and 1736 has been analyzed. This series is of interest because the measurement period includes the Maunder Minimum (1645–1715; hereafter MM) when solar activity was abnormally low. Some authors have suggested an increase of the solar diameter during the MM. Trying to detect these changes, statistical analyses comparing measurements taken in San Petronio during the MM (1655–1715) and other ones taken in a subsequent period (1716–1736) have been performed. Mann–Whitney U tests and Student's t-tests indicate that there is no statistically significant difference in the medians and averages of the solar diameter in both periods. In fact, we have found differences around 0.″6 in the medians and the averages, which are below the mean accuracy of the instrument. Therefore, we conclude that there is no difference between the solar diameter value measured during the MM (1655–1715) and that for the subsequent period 1716–1736. This implies that there has not been an increase in the solar diameter of several arcseconds during the MM as has been speculated by some authors. [ABSTRACT FROM AUTHOR]

Pierre Gassendi (1592 – 1655) carried out sunspot observations during the 1630s. This period is particularly interesting because it occurred a few years before the Maunder Minimum. Gassendi was the second most active sunspot observer in that decade, surpassed only by Christoph Scheiner. Moreover, the sunspot observations made by Gassendi are of interest because most of them were made in days when no other sunspot observations were available. We have analyzed the sunspot drawings included in his Complete Works (Opera Omnia) and the textual reports with sunspot information after translating them from the original Latin. Thus, we have detected mistakes in previous works that studied these observations, such as incorrect dates and incorrect daily number of groups assigned to Gassendi. In addition, we found some observation days recorded by Gassendi not included in the current sunspot-group-number database. In this work, we provide a new account of the number of groups recorded by Gassendi in addition to the translations of his relevant texts on observed sunspots. Our main objective is to include these new recounts into the current sunspot-group-number database, which is the basis of the group-number index, to have a more accurate knowledge of solar activity of the third solar cycle observed with the aid of the telescope. [ABSTRACT FROM AUTHOR]

Naked-eye sunspot observations (NESOs) have been recorded for the past two millennia, approximately. These kinds of records were made around the world, mainly in Asian civilizations, and they are compiled in several catalogs. In this work, we analyze solar activity on days of the 19th century when NESOs were recorded. We found that only more than five sunspot groups were recorded in 39% of days corresponding to these NESO events. Furthermore, regarding the largest groups observed on days when NESOs were reported, we show that the uncorrected areas of these groups were below 200 millionths of solar disk (msd) in 3.2% of total cases, while it is 12.9% for areas between 200 and 499 msd. Thus, NESO records do not necessarily imply high solar activity and big sunspot groups. Therefore, these results contradict the interpretations of recent works that, using the same NESO set, suggest the solar activity level during the Maunder Minimum is still an open question. NESO records support the Maunder Minimum as a very low solar activity period. [ABSTRACT FROM AUTHOR]

We present the sunspot observations made by Barnaba Oriani in 1778 and 1779 at the Brera Observatory (Milan, Italy). We have computed the number of sunspot groups and individual sunspots and extracted the positions of all individual sunspots recorded by Oriani. It must be highlighted that the observations made by Oriani in 1779 are not included in the current sunspot-group number database. The observations made in 1778 were already included, but we have found important deficiencies in these values that underestimated the true level of solar activity. The highest daily number of groups recorded by Oriani was 12 (24 August 1778). Only Horrebow in Solar Cycle 2 observed a higher daily number of groups (up to 16 groups in 1769) in the second half of the 18th century. We have compared the sunspot observations made by Oriani and Staudacher for the common dates. In general, Oriani observed a few more groups than Staudacher, while Staudacher recorded more individual sunspots than Oriani. Furthermore, the sunspot positions recorded by both observers are similar, but some significant differences can be found. Finally, we want to highlight that the sunspot records made in the 18th century will help us to understand better the behavior of the long-term solar activity. Therefore, we need to improve the observational coverage of the sunspot records carried out in that century. [ABSTRACT FROM AUTHOR]

An analysis of the sunspot observations made by Hevelius during 1642–1645 is presented. These records are the only systematic sunspot observations just before the Maunder Minimum (MM). We have studied different phenomena meticulously recorded by Hevelius after translating the original Latin texts. We reevaluate the observations of sunspot groups by Hevelius during this period and obtain an average value 7% greater than that calculated from his observations given in the current group database. Furthermore, the average of the active day fraction obtained in this work from Hevelius's records previous to the MM is significantly greater than the solar activity level obtained from Hevelius's sunspot observations made during the MM (70% versus 30%). We also present the butterfly diagram obtained from the sunspot positions recorded by Hevelius for the period 1642–1645. It can be seen that no hemispheric asymmetry exists during this interval, in contrast with the MM. Hevelius noted a ∼3-month period that appeared to lack sunspots in early 1645 that gave the first hint of the impending MM. Recent studies claim that the MM was not a grand minimum period, speculating that astronomers of that time, due to the Aristotelian ideas, did not record all sunspots that they observed, producing thus an underestimation of the solar activity level. However, we show that the good quality of the sunspot records made by Hevelius indicates that his reports of sunspots were true to the observations. [ABSTRACT FROM AUTHOR]

A revision is presented of the sunspot observations made by Charles Malapert from 1618 to 1626, studying several documentary sources that include those observations. The revised accounting of the group numbers recorded by Malapert for that period shows new information unavailable in the current sunspot group data base. The average solar activity level calculated from these revised records of Malapert is by almost one-third greater than that calculated from his records included in the current group data base. Comparison of the sunspot observations made by Malapert and by other astronomers of that time with regard to the number of recorded groups and sunspot positions on the solar disc shows good agreement. Malapert reported that he only recorded one sunspot group in each sunspot drawing presented in Austriaca Sidera Heliocyclia (the documentary source that includes most of the sunspot records made by Malapert), although he sometimes observed several groups. Therefore, the sunspot counts obtained in this work on Malapert's sunspot observations represent the lower limit of the solar activity level corresponding to those records. [ABSTRACT FROM AUTHOR]

Eric H. Strach (1914‐2011) studied medicine at University of Prague and graduated in 1938. Strach dedicated a great part of his life to astronomy becoming a consistent and meticulous observer. He joined the Liverpool Astronomical Association and British Astronomical Association during the 1960s and obtained two recognitions as proof of his great work in solar physics: the BAA's Merlin Medal and Gift in 1999 and Walter Goodacre Medal and Gift, 10 years later. Strach recorded four decades (1969‐2008) of systematic solar records in his observation notebooks although he started his observations from the late 1950s. In this work, we document the valuable effort made by Strach in getting four decades of solar records and the importance of this kind of long observation series for studies of space weather and climate. We present the sunspot group number series according to Strach's data and a long observation series of prominences recorded by Strach. Key Points: Eric H. Strach recorded solar observations (sunspots, prominences, plages, filaments, and solar flares) during the period 1969–2008Strach joined the Liverpool Astronomical Association and British Astronomical Association and was a meticulous observerStrach's data can be consulted at HASO website (http://haso.unex.es/) [ABSTRACT FROM AUTHOR]

We perform case studies of the sunspot observations by Charles Malapert and William Derham from the point of view of the reconstruction of the solar activity. They made sunspot observations prior and after the deep Maunder Minimum, respectively, being two of the most active sunspot observers of their time. The currently available sunspot group data bases assign one sunspot group for most of observations by Malapert and Derham. However, with documental evidence, we attest that these observers would have sometimes recorded several sunspot groups as only one representative group. Therefore, we conclude that their sunspot group counting should be used with caution for the reconstruction of the solar activity level around the Maunder Minimum. [ABSTRACT FROM AUTHOR]

Keywords: errata; addenda; Sun: activity; Sun: general; sunspots EN errata addenda Sun: activity Sun: general sunspots 5715 5716 2 05/09/22 20220601 NES 220601 This is an erratum for our article Carrasco et al. ([1]): 'Strong evidence of low levels of solar activity during the Maunder Minimum'. Sun: activity, Sun: general, sunspots, errata, addenda The annual values of the international sunspot number index (version 2) are available on the website of the Sunspot Index and Long-term Solar Observations (http://www.sidc.be/silso/). [Extracted from the article]

The use of spotless days to predict future solar activity is revised here based on the new version of the sunspot number index with a 24-month filter. Data from Solar Cycle (SC) 10 are considered because the temporal coverage of the records is 100% for this solar cycle. The interrelationships of the timing characteristics of spotless days and their comparison with sunspot cycle parameters are explored; in some cases, we find very strong correlations. Such is the case for the relationship between the minimum time between spotless days either side of a given solar maximum and the maximum time between spotless days either side of the previous solar minimum, with $r = -0.91$ and a $p\mbox{-value} < 0.001$ . However, the predictions for SCs 24 or 23 made by other authors in previous works using spotless days as a predictor of solar activity are not correct since the predictions have not been fulfilled. Although there seems to be a pattern of strong correlation for some relationships between the parameters that have been studied, a prediction of future solar cycles from these parameters defined as functions of spotless days should be made with caution because the estimated values are sometimes far from the observed ones. Furthermore, SC 23 seems to show a mode change, a break with respect to the behaviour of previous solar cycles and more similar to SCs 10 - 15. [ABSTRACT FROM AUTHOR]

Solar activity affects geophysical and heliophysical processes. Long-term changes in solar activity are closely related to climate change. Solar physicists and earth science researchers need longer observations of solar activity. Current continuous observations of solar activity are only about 400 years. Some scholars have compiled naked-eye observations of sunspots from 200 BC to 1918 AD from historical documents. In this present work, the authors use the weighted wavelet transform to study the observations series. The results show the Suess - de Vries cycle with a period from 195- to 235-year existing in the discontinuous sunspot series. Meanwhile, the cycle signal changes with time. Especially, the Suess/de Vries cycle is relatively obvious from 200 BC to 400 AD, 800 AD to 1340 AD, 1610 AD to 1918 AD, with a period of about 211-year, about 195-year, and about 235-year, respectively.

The Madrid Astronomical Observatory implemented a solar observation program from 1876 to 1986. In addition to sunspots, the observers at this observatory recorded other solar features such as prominences. In this work, we have consulted the documentary sources of the Madrid Astronomical Observatory (the information is not digitally available), digitized the records of the observers on the annual number of prominences, and constructed a homogeneous series of the total and hemispheric annual number of prominences with heights of 25″ and more for the period 1906–1957. To evaluate the quality of the data and assess their potential, we have compared the Madrid prominence series with the number of prominences recorded by the Astronomical Observatory of the University of Coimbra and other time series such as the sunspot number index, solar radio flux at 10.7 cm, and sunspot areas. We have also analyzed the hemispheric prominence numbers and the asymmetry index. We obtained the strongest correlation between Madrid and Coimbra prominence series (r = 0.7), whereas the correlations between the Madrid prominence series and the other solar activity time series are similar (r ≈ 0.6). In addition, we found that the correlation coefficient between the Madrid prominence series and the sunspot number is lower than that from the Coimbra prominence series and the sunspot number. We suggest that these differences are a consequence of the way prominences were counted in the Madrid Astronomical Observatory.

A new normalized sunspot-area series has been reconstructed from the series obtained by the Royal Greenwich Observatory and other contemporary institutions for the period 1874 - 2008 and the area series compiled by De la Rue, Stewart, and Loewy from 1832 to 1868. Since the two sets of series do not overlap in time, we used the new version of sunspot index number (Version 2) published by Sunspot Index and Long-term Solar Observations (SILSO) as a link between them. We also present a spectral analysis of the normalized-area series in search of periodicities beyond the well-known solar cycle of 11 years and a study of the Waldmeier effect in the new version of sunspot number and the sunspot-area series presented in this study. We conclude that while this effect is significant in the new series of sunspot number, it has a weak relationship with the sunspot-area series. [ABSTRACT FROM AUTHOR]

We describe a revised collection of the number of sunspot groups from 1610 to the present. This new collection is based on the work of Hoyt and Schatten ( Solar Phys. 179, 189, 1998). The main changes are the elimination of a considerable number of observations during the Maunder Minimum (hereafter, MM) and the inclusion of several long series of observations. Numerous minor changes are also described. Moreover, we have calculated the active-day percentage during the MM from this new collection as a reliable index of the solar activity. Thus, the level of solar activity obtained in this work is greater than the level obtained using the original Hoyt and Schatten data, although it remains compatible with a grand minimum of solar activity. The new collection is available in digital format. [ABSTRACT FROM AUTHOR]

We revisited the sunspot observations published by Johannes Hevelius in his book Machina Coelestis () corresponding to the period of 1653 - 1675 (just in the middle of the Maunder Minimum). We show detailed translations of the original Latin texts describing the sunspot records and provide the general context of these sunspot observations. From this source, we present an estimate of the annual values of the group sunspot number based only on the records that explicitly inform us of the presence or absence of sunspots. Although we obtain very low values of the group sunspot number, in accordance with a grand minimum of solar activity, these values are significantly higher in general than the values provided by Hoyt and Schatten ( Solar Phys. 179, 189, ) for the same period. [ABSTRACT FROM AUTHOR]

Aims. The Maunder minimum (MM) of greatly reduced solar activity took place in 1645-1715, but the exact level of sunspot activity is uncertain because it is based, to a large extent, on historical generic statements of the absence of spots on the Sun. Using a conservative approach, we aim to assess the level and length of solar cycle during the MM on the basis of direct historical records by astronomers of that time. Methods. A database of the active and inactive days (days with and without recorded sunspots on the solar disc) is constructed for three models of different levels of conservatism (loose, optimum, and strict models) regarding generic no-spot records. We used the active day fraction to estimate the group sunspot number during the MM. Results. A clear cyclic variability is found throughout the MM with peaks at around 1655-1657, 1675, 1684, 1705, and possibly 1666, with the active-day fraction not exceeding 0.2, 0.3, or 0.4 during the core MM, for the three models. Estimated sunspot numbers are found to be very low in accordance with a grand minimum of solar activity. Conclusions. For the core MM (1650--1700), we have found that (1) A large portion of no-spot records, which correspond to the solar meridian observations, may be unreliable in the conventional database. (2) The active-day fraction remained low (below 0.3--0.4) throughout the MM, indicating the low level of sunspot activity. (3) The solar cycle appears clearly during the core MM. (4) The length of the solar cycle during the core MM appears for 9 ± 1 years, but this is uncertain. (5) The magnitude of the sunspot cycle during MM is assessed to be below 5-10 in sunspot numbers. A hypothesis of the high solar cycles during the MM is not confirmed. [ABSTRACT FROM AUTHOR]

In the 19th century, several astronomers made observations of sunspots, recording their positions and sometimes their areas. These observations were published in the form of extensive tables, but have been unhelpful until now. Three of these observers were Richard C. Carrington, Christian H.F. Peters and Warren de la Rue (and their respective collaborators). They published, in various articles, the data corresponding to 26 641 sunspot positions (Carrington, Peters and de la Rue registered 4900, 14040 and 7701 sunspot positions, respectively). In this paper we present a translation of more than 400 pages of their printed numerical tables into a machine readable format, including an initial analysis targeted at detecting possible mistakes in the reading or in the original transcription. The observations carried out by these three astronomers have been made available at the Centre de Donées Astronomiques de Strasbourg ( http://cdsarc.u-strasbg.fr/cgi-bin/VizieR?-source=VI/138 ). [ABSTRACT FROM AUTHOR]

Solar rotation rates can be measured from changes in sunspot positions. In this paper, we propose using digital camera images taken with small telescopes to determine the solar rotation rate at different heliographic latitudes. This lowcost practical experiment can be performed easily by undergraduate students, introducing them to topics related to solar physics. The data obtained in this work from the analysis of sunspots observed in 2010, 2011 and 2012 were compared with current models of solar rotation, showing good agreement. [ABSTRACT FROM AUTHOR]

A simple method to detect inconsistencies in low annual sunspot numbers based on the relationship between these values and the annual number of active days is described. The analysis allowed for the detection of problems in the annual sunspot number series clustered in a few explicit periods, namely: i) before Maunder minimum, ii) the year 1652 during the Maunder minimum, iii) the year 1741 in Solar Cycle −1, and iv) the so-called 'lost' solar cycle in the 1790s and the subsequent onset of the Dalton Minimum. [ABSTRACT FROM AUTHOR]

Solar rotation rate has been measured using the sunspot positions recorded by W.C. Bond during the period 1847 – 1849 at the Harvard College Observatory. From the drawings carried out by Bond we have selected the sunspots and groups of sunspots with more reliable positions presented in three or more drawings on successive days. We have calculated from the positions of the selected sunspots (41 in total) a synodic rotation rate of ω=[(12.92±0.08)−(1.5±1.0)sin 2 φ] degrees/day, where φ is the heliographic latitude. This rate, although slightly lower, is similar to the actual solar rotation rate, confirming no important changes in the solar rotation during the last 160 years. [ABSTRACT FROM AUTHOR]

We present the sunspot ideas and observations of the 18th century Portuguese scholar Teodoro de Almeida (1722 – 1804) and Mexican scientist José Antonio Alzate (1737 – 1799). We describe the implications of dating a single sunspot observation performed by Almeida in the early 1760s, during the maximum of cycle number 1. A possible solar cycle peak in 1760 (instead of 1761) is investigated. We present several observations of sunspots obtained by Alzate during 1769 (partially associated with the Venus and Mercury transits) and also on 20 July 1786. We estimate 100±34 as the Group Sunspot Number for this date. These records were unknown and, therefore, not included in the database compiled by Hoyt and Schatten (1998). [ABSTRACT FROM AUTHOR]

Observations of aurorae borealis at low latitudes are very rare and are clearly associated with strong geomagnetic storms. Morphologically, they are characterized by a diffuse red colour with no rapid motions. The main aim of this paper is to analyse two hitherto ignored aurorae that were observed at two low-latitude sites, Tenerife (28°N 18°W) and Mexico City (19°N 99°W), in 1770 and 1789, respectively. These observations can give supplementary information about the level of solar activity at those times where direct solar observations were rather scarce. Studying also the behaviour of the heliosphere during this period using different proxies, we find that the open magnetic field better describes auroral occurrences. The variation over time in geomagnetic latitude at the two sites is also calculated. [ABSTRACT FROM AUTHOR]

Recently, new estimates of the solar cycle length (SCL) have been calculated using the Zurich Sunspot Number ( R Z) and the Regression-Fourier-Calculus (RFC)-method, a mathematically rigorous method involving multiple regression, Fourier approximation, and analytical expressions for the first derivative. In this short contribution, we show estimates of the solar cycle length using the RFC-method and the Group Sunspot Number ( R G) instead the R Z. Several authors have showed the advantages of R G for the analysis of sunspot activity before 1850. The use of R G solves some doubtful solar cycle length estimates obtained around 1800 using R Z. [ABSTRACT FROM AUTHOR]

Sunspot drawings made by Galileo Galilei in 1612 are used to derive the law of differential rotation at that time. The main interest of the work is during the time of observations, just at the beginning of telescopic observations and some decades before the Maunder Minimum (1645 – 1715), a period where the sunspots almost disappeared from the solar surface. For this purpose we have carried out careful corrections of the different sources of errors derived from the observing technique. By comparing with other results of the same century, a significant difference is only detected by comparing with data corresponding to the deep Maunder Minimum (Paris Observatory drawings). The characteristics of the solar differential rotation, and extrapolating the behavior of solar activity, did not differ before or after the Maunder Minimum. We also include an analysis of hitherto ignored sunspot drawings by N. Bion made in October and November 1672. [ABSTRACT FROM AUTHOR]

In this work, a procedure to elaborate a homogeneous sunspot area series using the Royal Greenwich Observatory/USAFINOAA data (from 1874 to the present) and the De la Rue and co-workers data (from 1832 to 1868) is presented. These two data series correspond to time intervals that do not overlap and a direct comparison between them could not be carried out. We used the International Sunspot Number (Ri) and the Group Sunspot Number (RG) as a link between the two original series. Thus, two homogeneous sunspot area series have been built using a simple mathematic procedure based on linear relations. [ABSTRACT FROM AUTHOR]

The solar observations performed by the Mexican astronomer J. A. Alzate during the year 1784 are analysed in this work. These observations are very valuable for the reconstruction of solar activity because Hoyt and Schatten (1998), who defined the Group Sunspot Number (RG), only found five observations during this year – all performed by J. C. Staudacher. Using conjointly the data provided by Alzate and Staudacher for 1784, one can determine a value of RG equal to 0.3±0.1 with eighty records for that year. [ABSTRACT FROM AUTHOR]

In the 19th century De la Rue, Stewart, and Loewy carried out a compilation of drawings and photographs of the solar sunspots corresponding to the interval 1832–1868. Using these drawings and photographs, they determined fortnightly values of the percentage of the solar photosphere covered by the sunspots. In this work, we have performed a spectral analysis of these data in order to determine possible periodic signals. In addition to the 11-year solar cycle, short cycles of about 330 days and 30–50 days have been recovered, lacking the 150–160 days period discovered by other authors using several solar activity indicators. [ABSTRACT FROM AUTHOR]

The knowledge of historical solar activity is of great interest. The present work analyses descriptions of the solar corona observed during the eclipse of 24 June 1778. The corona was described as showing streamers at all solar latitudes. The descriptions are indicative of a high level of solar activity, in concordance with historical sunspot number data. [ABSTRACT FROM AUTHOR]

Angelo Secchi (1818–1878) was an Italian Jesuit who made relevant scientific contributions in geophysics, meteorology, and astrophysics. He was a well-known pioneer in solar physics due to his theories and observations. Secchi published Le Soleil (The Sun) a summary of knowledge about our star in that time. Moreover, he published in this book his sunspot and prominence observations made during the period 1871–1875. In this work, we present a machine-readable version of these observations and a preliminary analysis of them.

Newly discovered descriptions about the great aurora observed in March 1582 are presented in this work. These records were made by Portuguese observers from Lisbon. Both records described the aurora like a great fire in the northern part of the sky. It was observed during three consecutive nights, according to one of the sources. Thus, we present a discussion of these auroral records in order to complement other works that studied the aurora sighted in March 1582.

Two sunspot observations recorded by the Mexican Felipe de Zúñiga y Ontiveros have been revealed from a manuscript. One sunspot group was recorded on 9th January 1777 and four sunspot groups on 29th December 1777. Both records were taken during the observation of solar eclipses from Mexico City and their description also included sketches of the solar disk with sunspots. The sunspot group corresponding to 9th January was also observed by Erasmus Lievog. The observation on 29th December 1777 is the only record corresponding to this date.

A record is presented of the auroral observations made by Giuseppe Toaldo and his assistant Vincenzo Chiminello in Padua, Italy, in the second half of the 18th century. The historical sources consulted include the original manuscripts containing the meteorological records made by those two observers. A small catalogue is constructed with the 148 auroral observations. The characteristics of these observations are analysed. The main feature of this data set is that the annual number of auroral observations presents an intense peak at around 1779 and an abrupt decline at around 1790.

The ratio of penumbral to umbral area of sunspots is an important topic for solar and geophysical studies. Hathaway (Solar Phys.286, 347, 2013) found a curious behaviour in this parameter for small sunspot groups (areas smaller than 100 millionths of solar hemisphere, msh) using records from Royal Greenwich Observatory (RGO). Hathaway showed that the penumbra-umbra ratio decreased smoothly from more than 7 in 1905 to lower than 3 by 1930 and then increased to almost 8 in 1961. Thus, Hathaway proposed the existence of a secular variation in the penumbra-umbra area ratio. In order to confirm that secular variation, we employ data of the sunspot catalogue published by the Coimbra Astronomical Observatory (COI) for the period 1929 - 1941. Our results disagree with the penumbra-umbra ratio found by Hathaway for that period. However, the behaviour of this ratio for large (areas greater or equal than 100 msh) and small groups registered in COI during 1929 - 1941 is similar to data available from RGO for the periods 1874 - 1914 and 1950 - 1976. Nevertheless, while the average values and time evolution of the ratio in large groups are similar those for small groups according to the Coimbra data (1929 - 1941) it is not analogous for RGO data for the same period. We also found that the behaviour of the penumbra-umbra area ratio for smaller groups in both observatories is significantly different. The main difference between the area measurements made in Coimbra and RGO is associated with the umbra measurements. We would like to stress that the two observatories used different methods of observation and while in COI both methodology and instruments did not change during the study period, some changes were carried out in RGO that could have affected measurements of umbra and penumbra. These facts illustrate the importance of the careful recovery of past solar data. [ABSTRACT FROM AUTHOR]

We present and analyse the sunspot observations performed by Franz I.C. Hallaschka in 1814 and 1816. These solar observations were carried out during the so-called Dalton minimum, around the maximum phase of Solar Cycle 6. These records are very valuable because they allow us to complete observational gaps in the collection of sunspot group numbers, improving the coverage for this epoch. We have analysed and compared the observations made by Hallaschka with the records made by other contemporary observers. Unfortunately, the analysis of the sunspot areas and positions showed that they are too inaccurate for scientific use. We conclude, however, that the sunspot counts made by Hallaschka are similar to those made by other astronomers of that time. The observations by Hallaschka confirm a low level of solar activity during the Dalton minimum. [ABSTRACT FROM AUTHOR]

Recently, Svalgaard has proposed that Waldmeier introduced a discontinuity in the International Sunspot Number (ISN) around 1945. In this paper, we study some characteristics of the sunspot cycle using the classical ISN and the proposed version derived from the “Waldmeier discontinuity”. We conclude that this proposed version does not significantly improve the statistics of the characteristics of solar cycle.