Early observations of sunspots

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Seasonal changes were all-important to early agrarian societies and so they naturally worshipped the Sun. Indeed, the sun-god headed the pantheon in many cultures, ranging from Egypt to Peru. Since astronomy was also practised in these cultures, it seems likely that sunspots must occasionally have been detected with the naked eye, which is possible when the Sun is low on the horizon and partially obscured by dust storms, volcanic dust or smoke. Thus Needham (1959) conjectured that the traditional Chinese image of a red sun with a black crow superimposed upon it was derived from early sunspot observations. The first recorded mention of sunspots comes, however, from Greece: around 325 BC, Theophras-tus of Athens, who was a student of Aristotle and succeeded him as leader of the Lyceum, referred in a meteorological treatise to black spots on the Sun as indicators of rain.2 His casual references to spots suggest that they were already well known. Written records of non-telescopic observations of sunspots made in China date back to 165 BC (Wittmann and

1 From Letters on Sunspots, translated by Stillman Drake (1957).

2 The brief comments in Theophrastus's De Signis were amplified in the following century by Aratus in his Phaenomena, a didactic poem that was later translated into Latin by Cicero and Germanicus (Sider and Brunsch├Ân 2007). Virgil and Pliny the Elder both mention dark spots on the Sun as portents of rain, as does Bede much later.

Xu 1987; Yau and Stephenson 1988). These accounts range from straightforward ("Within the Sun there was a black spot") to fanciful ("The Sun was orange in colour. Within it there was a black vapour like a flying magpie. After several months it dispersed."). The series of naked-eye observations continues until the early twentieth century, overlapping the telescopic records that begin in 1611. However, the total number of recorded sunspot sightings before 1611 is less than 200, fewer than one sunspot per decade. This is only a small fraction (less than 1%) of the total number of sunspots that should have been visible to the naked eye during this period (Eddy, Stephenson and Yau 1989; Mossman 1989), and the observations were clearly sporadic, depending on phases of the Moon and seasonal dust storms (Yau 1988), as well as on political tact and revolutionary upheavals (Stephenson 1990).

Aristotle had taught that the Sun was perfect and immaculate, and the few sunspots observed by Arab astronomers were interpreted as transits of Mercury or Venus, as was the earliest European observation in AD 807 (Wittmann and Xu 1987), which Einhard regarded as a portent in his Life of Charlemagne. Indeed, Kepler himself thought that he had observed a transit of Mercury when he detected a sunspot in 1607. The oldest known drawing of sunspots appears in the manuscript chronicle of John of Worcester, depicting two spots observed in December 1128, when the Sun was low in the English winter sky (Stephen-son and Willis 1999). Two centuries later, when the sky was obscured by smoke from forest fires in Russia, chroniclers there reported, "there were dark spots on the Sun as if nails were driven into it" (Wittmann and Xu 1987).

Progress since then has relied on technological advances. The invention of the telescope in the Netherlands in 1608 opened the possibility of detailed astronomical observations. News of the invention spread rapidly and reached Galileo Galilei (1564-1642) at Padua in June 1609. He used his improved instrument to observe the Moon and the Milky Way and to discover the satellites of Jupiter. In May 1612 he stated that he had been observing sunspots for 18 months (i.e. since November 1610) and there is no reason to doubt either this or his later claim that he had noticed them earlier that year, before he moved from Padua to Florence. The Sun was active at that time: Galileo may have been the first to see spots through a telescope but he was rapidly followed by others. Thomas Harriot (1560-1621), in England, was the first to record his observations though his manuscripts, with drawings made in December 1610, lay undiscovered at Alnwick Castle until 1786. The credit for publishing the first account goes to Johann Fabricius (latinized from Goldsmid, 1587-1616) who came from East Friesland. In his book An Account of Spots Observed on the Sun and their Apparent Rotation with the Sun, published at Wittenberg in June 1611, he describes how he and his father saw several spots in March 1611, first through a telescope and then using a camera obscura. They followed the spots as they moved across the solar disc and recognized one when it reappeared again; noticing that the spots were foreshortened at the limb, Fabricius concluded that they lay on the surface of a rotating Sun (Casanovas 1997; Hoyt and Schatten 1997).

A few days before Fabricius's first observation, Christoph Scheiner (1575-1659), a Jesuit professor at Ingolstadt in Bavaria, had also noticed some sunspots, through a smoke-filled sky; he made more systematic observations during the last few months of 1611 but was persuaded to publish them under a pseudonym, in the form of three letters addressed to Mark Welser, a wealthy patrician in Augsburg. In these letters, Scheiner asserted that the dark spots were caused by small bodies orbiting around the Sun and blocking its light; thus he was able to avoid any contradiction with the Aristotelian notion of a perfect Sun.

Galileo Drawing Sunspots
Fig. 2.1. Sunspot drawing by Galileo.

Welser forwarded these letters, published early in 1612, to Galileo in Florence and sought his comments. This set Galileo making his own systematic observations; he too recognized that the spots were foreshortened as they approached the limb and rapidly concluded that they were on the surface of the Sun and probably produced by clouds. Since he had just written a treatise on hydrostatics that contradicted Aristotelian notions, he eagerly inserted a paragraph on sunspots into the second edition. Then he went on to make a prolonged series of observations in the summer of 1612, using a projection technique developed by his colleague and former student, Benedetto Castelli. Galileo described his findings in three letters, addressed to Welser but written eloquently in Italian; they were published in 1613, as Istoria e Dimostrazioni intorno alle Macchie Solari3 by the Accademia dei Lincei in Rome. His new observations (see Fig. 2.1) confirmed that the spots rotated with the Sun. He noticed that sunspots always lie near the solar equator, "in a narrow zone of the solar globe corresponding to the space in the celestial sphere that lies within the tropics", and he also realized that sunspots are dark only in a relative sense and by themselves are "at least as bright as the brightest parts of the Moon". He also noted the existence of bright patches near sunspots (later to be named faculae). After mentioning his discovery of the satellites of Jupiter and the phases of Venus, he concluded the book with a firm statement of support for Copernicus's heliocentric system.

3 For an English translation, see Drake (1957), from which the quotations in this paragraph are taken.

Sunspot Rotation
Fig. 2.2. Sunspot drawing from Scheiner's Rosa Ursina.

Later, there was a dispute between Galileo and Scheiner (who moved to the Jesuit College in Rome in 1624) over priority in the discovery of sunspots. Scheiner observed sunspots meticulously from 1625 to 1627, again projecting the telescopic image on to a screen or sheet of paper. By this time he had discarded some of Aristotle's teaching and had come to accept that the spots were on the solar surface and rotated with the Sun. From their apparent motion he deduced that the Sun's axis of rotation is not quite perpendicular to the plane of the ecliptic, but is inclined at about 7^┬░ to the normal. In 1630 he published his results in a sumptuous volume, entitled Rosa Ursina sive Sol and dedicated to the Orsini family whose emblem was a rose, that remained the standard text on sunspots for a century and more. The first drawings by Scheiner and Galileo had already shown some sunspots with dark cores but Scheiner was now able to emphasize the distinction between the dark nucleus of a sunspot and the shadowy ring that surrounds it (which he confusingly referred to as the "umbra"), as shown in Figure 2.2. Scheiner continued to support a geocentric cosmology, though in Tycho's variant form (which allowed the other planets to rotate about the Sun) rather than Ptolemy's original version, and he remained a firm opponent of the Copernican system until the end of his life.

Galileo, in his Dialogo sopra i Due Massimi Sistemi del Mondo, ignored Kepler's discoveries but brought in the motion of sunspots as an argument in favour of a heliocentric system. He realized that there would be two occasions in the year, separated by six months, when the line of sight to the Sun was perpendicular to the plane through the solar centre containing the Sun's rotation axis and the normal to the ecliptic plane. At those times sunspots would appear to move in a straight line as the Sun rotated; in between, their paths would appear convex, pointing alternately up and down. This was confirmed by observations. In a geocentric system, on the other hand, Scheiner had to invoke an additional precession of the Sun's axis of rotation, with a period that just happened to be one year, which seemed less plausible. Galileo's disputes with Scheiner and Grassi, both professors at the Jesuit Collegio Romano, contributed to the events that led to his trial and sentence by the Inquisition.

Sunspots continued to be observed throughout the rest of the seventeenth century (Hoyt and Schatten 1997), though the non-achromatic refractors that were used had spatial resolution no better than 10 arcseconds. Among the most active observers was Johannes Hevelius (1611-1687) in Gdansk: in his Selenographia (1647) he included observations made in 1642-5, when sunspots were plentiful, but 20 years later, in his Cometographia (1667), he complained, "For a good many years recently, ten and more, I am certain that absolutely nothing of great significance (apart from some rather unimportant and small spots) has been observed either by us or by others. On the other hand, in former times (as Rosa Ursina and Selenographia confirm) a great many spots, remarkable for their size and density of distribution, appeared within a single year" (Weiss and Weiss 1979). He was not the only one to notice this dearth of sunspots, now referred to as the Maunder Minimum (Eddy 1976), which lasted from 1645 to 1715 and coincided with the reign of Louis XIV, the Roi Soleil.4 Boyle (in 1660) and Fogelius (in 1661), as well as Picard and Cassini in Paris (in 1671) all reported excitedly when new spots occasionally appeared. That the seventeenth-century observers were both assiduous and competent is amply demonstrated by the records of the Paris Observatory, where systematic observations were carried out from 1667 on a daily basis, whenever the skies were clear. Ribes and Nesme-Ribes (1993) found that there were generally at least 15 days of observation each month. Between 1670 and 1700 there were never more than eight spots visible and long intervals when none were seen at all; indeed, only a single, short-lived spot was seen during the last decade of the century. Moreover, almost all the spots that did appear after 1660 were in the southern hemisphere only, and it was not until 1715 that spots were once more detected in both hemispheres, as they had been at the time of Galileo.

The next step forward came in 1769, when Alexander Wilson (1714-1786), professor of astronomy at Glasgow, discovered that, as a sunspot approaches the solar limb, the width of the penumbra on the side farthest from the limb decreases faster than the width of the penumbra on the side nearest the limb. From this result, now called the Wilson effect, he deduced that a sunspot corresponds to a saucer-shaped depression of the visible surface of the Sun or, as he put it, "a vast excavation in the luminous matter of the sun". This so-called Wilson depression is now understood to be a consequence of the fact that in the cooler

4 By 1667 the change was sufficiently well known to be referred to by the poet Andrew Marvell in his satire Last Instructions to a Painter (Weiss and Weiss 1979):

So his bold Tube, Man to the Sun apply'd, And Spots unknown to the bright Star descry'd; Show'd they obscure him, while too near they please, And seem his courtiers, are but his disease. Through Optick trunk the Planet seem'd to hear, And hurls them off, e'er since, in his Career.

and less dense (and hence more transparent) atmosphere in the penumbra, and especially in the umbra, the emergent radiation comes from a deeper geometric level.5 Wilson also conjectured that the "excavations" might actually be revealing the dark interior of the Sun. This notion was taken up by William Herschel (1738-1822), who suggested that the Sun is a cool body covered in bright clouds, and that sunspots are holes in these clouds ("Places where the luminous Clouds of the Sun are Removed") revealing the cooler surface beneath, which he further suggested could be "richly stored with inhabitants" (Herschel 1795, 1801)! (This picture of the Sun as a cool body enveloped in layers of hot clouds persisted for a surprisingly long time, still appearing in the 1860s in John Herschel's standard textbook, but gradually gave way in the face of new results from spectroscopy.) With his 10-foot (focal length) reflecting telescope, William Herschel found that the sunspots, which he regarded as "openings" were surrounded by "shallows" which were "tufted": this is the first indication of fine structure in penumbrae. He also noted that sunspots had been scarce between 1795 and 1800 or, as he put it, "that our sun has for some time past been labouring under an indisposition, from which it is now in a fair way of recovering." He then went on to argue that there was a link between climate and the incidence of sunspots, supporting his case by citing variations in the price of wheat - conveniently gleaned from Adam Smith's Wealth of Nations - between 1650 and 1717; thus there were material advantages to be gained from solar observations.6

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