Modern View of the Sun: Materials for an Experimental History Before the Telescope

1. Introduction: The Sun from an Ideal Disk to the First Telescopic Sights

The Sun appears as a bright disk, and its dazzling luminosity prevented, normally, observations of its surface. Since the antiquity, the Sun was worshipped as a god. The Egyptians (Section 2.) represented it either as a disk or as a sphere, with some elements suggesting direct observations of the corona and prominences during total eclipses. In Christianity (Section 3.) St. Francis of Assisi in 1225 created his Cantico di Frate Sole, where the Sun well represents God’s qualities. Galileo Galilei (Section 4.) with his telescope first recognized in 1611 sunspots as belonging to a rotating photosphere, proving the spherical nature of the Sun. The solar symbol was frequently used in the Catholic Church (Section 5.) appearing also in the coat of arms of the Jesuit pope Francis. The occasional observations of giant sunspots in the middle age (Section 6.) with naked eye or through pinholes in a camera obscura could have risen the question on the solar rotation before the telescope? The answer is suggested by the evolution of the largest sunspots of the XXV solar cycle observed by naked eyes (Section 7.) and with a camera obscura (Section 8.) in order to better understand the quality of the pre-telescopic observations. With the pinhole-meridian line of St. Maria degli Angeli in Rome the sunspots are visible (Section 9.), and the largest pinhole-meridian line, realized by Paolo Toscanelli in 1475 in the Dome of Florence, could have shown the largest sunspots, but the Sun was in the Spörer Minimum (1460-1550). The limb darkening (Section 10.) is another clue for the solar sphericity, but the pinhole introduces itself a limb darkening of the image. The invention of the telescope occurred just after the first map of the Moon drawn with the unaided eye (Section 11.) when the Sun restored its sunspots activity (Section 12.).

2. The Representation of the Sun in Ancient Egypt

The Sun is there, it has been always there, but only in the last four centuries, the question about its physical nature has become meaningful.

The solar mythology in Egypt is very complex, and its discussion would exceed largely the scope of this work. The representations of the solar disk or sphere are well known. In the Figure 1 the god falcon Ra-Haratki, is represented with the solar disk on top.

There is a physical correspondence between the traditional representation of the Sun in Egypt, and the phenomena visible during a total eclipse of the Sun. The uraeus, in particular, are the red prominences departing from the solar disk. The rays are straight as in modern geometrical optics, firstly accounted by the arab polymath Alhacen in 1028-1038 (Mark Smith 2010) while he was working in Egypt. In Figure 2 another colored example is from the Egyptian Museum of Turin.

The winged Sun include the wings (of the falcon Horus) and one or two cobra (uraeus): their position starting always from the solar limb, recall the prominences visible (without modern H-alpha filters) only during total eclipses. The wings fully stretched, recall the streamers, occurring during the minimum phases of the solar cycles (Figure 3).

The eclipse of 2009 was so long (5 m 42 s) that in the middle no prominences appeared, because the Moon was angularly wider than the Sun. The one of 2024 is captured at the moment of the diamond ring (occurring just before or just after totality, when the photosphere appears. Four prominences are well visible and in red color (belonging to the Chromosphere). These are the natural correspondences to the Uraeis.

Despite of the faint luminosity of the solar corona, the memory of this feature, along with the red prominences visible at the beginning and at the end of the totality, can be unforgettable for an observer. The red prominences at the start of the totality and toward the end attracted my sight during the total solar eclipse of 1999 in Riedering, Bayern, Germany. Since then I started to reconsider the Egyptian solar symbology.

A great prominence on 14 July 2025 offered (Figure 4) the possibility to see its extension out the solar disk, to explain the iconic appearances of the Uraeis.

The hypothesis of identifying the wings with the solar corona during eclipses is not a novelty: other scholars did it in the past (Belmonte and Lull 2023).3 Several total eclipses occurred on the Egypt along the millennia of its history, but the eclipses are completely under reported in the ancient Egypt, probably because they were considered as bad omens (Belmonte and Lull 2023). The hypothesis that an eclipse influenced Amenothep IV to change his name and the capital city in Egypt in his 4^th year of reign: this eclipse was total in the place where the new capital Akhetaten was created (Belmonte and Lull 2023). The identification of the Uraei with the solar prominences visible during the totality is new. The Uraeus4 is the symbol of the goddess Wadget, a cobra who was considered as the patroness and protectress of Egypt. The colors and the representations of the figures of the solar disk (fig. 1, fig. 2 and fig. 3) in general, reflect symbolic models, possibly with a real observation made in the beginning and transmitted as an iconic tradition. Real observations did not improve these symbolic representations, even if the exception of the solar halo in the tomb of Meryre in Amarna, is an interesting reported case.5 Finally in the Egyptian representation of the Sun, there are no sunspots imaged.6

3. The Sun of Christianity

Christ is called the “Sun rising from on hig” (Luke 1:68-79), and to see the great brightness of the Sun eagle eyes are needed.7 This legend and the text of Revelation (4:6-8) is at the basis of representing St. John the Evangelist with an Eagle, because he was the author of the most theological gospel. His sight was able to penetrate the secrets of God, as the eagle can aim fixedly to the Sun. Examples of Christ-Sun are in Figure 5.

St. Francis of Assisi in 1225 composed the Cantico di Frate Sole expressing in poetry the representative role of God played by the Sun.

Laudato Si’ mi Signore per frate Sole

Lo quale è iorno et allumini noi per lui

Et ellu è bellu et radiante cum grande splendore

Di te porta significatione8

Among the Christian symbols, the Sun was used to represent Charity,9 Wisdom10 and Jesus (figure 4). Representing God with the Sun is based on the Psalm 18, 6 “In Sole posuit tabernaculum Suum” (In the Sun He posed his tent). The representations of the solar symbols are flat, both before and after the evidences of a spherical Sun, not only in painting and mosaics, but also in sculptures.

St. Bernardin of Siena (1380-1444), a Franciscan friar, created the symbol of the radiant Sun with the acronym IHS Iesus Hominum Salvator inscripted inside (Figure 6).

He used them to give benedictions with this table, after his preachings.11

The possibility that the twisted rays recall prominences seen during total eclipses has been inspected (Zawilski 2021). During the life of st. Bernardin there were two total eclipses occurring near him: 1386 and 1431. He could have observed them.12 The difference between thick twisted and thin linear rays can be only instrumental to the number of 12, symbol of fullness in the Bible, because so many prominences cannot be visible at once.

There is an evolution of the rays are found from the original table of St. Bernardin (Figure 6) and in the following representations (Figure 7) up until the coat of arms of Pope Francis, where the number of 12 and the differences between rays is disappeared.

4. The Sun of Galileo and His Contemporaries: Planet or Star?

Galileo (1613) following the motion of the spots, interpreted them as located on the solar surface. Scheiner's orbiting planet or cloud interpretation dates from 1611 (the Apelles letters). In the Rosa Ursina (1630) he actually gave up that notion and even determined the Sun's rotational elements based on the spots, accepted to be on the solar surface or close to it. In the Borbonia Sidera (Tarde, 1620 see Figure 8), the spots were considered as moving stars.15 Jean Tarde (1561-1636) was a French prelate, historian and astronomer,16 who met personally Galileo in 1614.

Giordano Bruno18 (1548-1600) considered the stars as very far suns (Bruno, 1584), but his speculation did not pose on specific measures. A similar idea, supported by computations and observations was the one of Christiaan Huygens (1629-1695): the Dutch scientist Christian Huygens made an image of the Sun through a pinhole in a darkened room. He varied the size of the pinhole until the image seemed equal in brightness to an image of Sirius, the brightest star. Since the pinhole admitted 1/27,000 of the light of the Sun, Huygens concluded that Sirius was 27,000 times farther away than the Sun (it is actually 543,900 times farther away and substantially more luminous than the Sun, see Figure 9).19

From a physical point of view the Copernican hypothesis gave to the Sun21 the central role of the Universe, but in the view of Bruno the Sun become a star in the modern sense. Galileo, instead, when not dealing with “fixae” or fixed (stars), was still considering the possibility of the motion of stars. In this respect, the Earth orbiting around the Sun was a (moving) star (Galileo, Saggiatore, 1624). The Greek word for moving star is “planétes”. The difference with the other stars was only kinematic, while for Bruno started to be a physical difference: the stars were far suns. The clear difference between this view and the modern one required near three centuries to mature. Still at the end of XIX century, Camille Flammarion considered the possibility that the Sun could be inhabited (Astronomie Populaire, 1875) as all other planets.

5. The Sun and the Jesuits

The radiant Sun (figures 6-7) is the symbol characterizing the Company of Jesus, founded by St. Ignatius of Loyola in 1540. It has been also a main task of the scientific studies of the Company along the centuries.

The contemplation of the Nature is part of the spirituality of St. Ignatius,22 as Nature was created by the Word of God. A strong development of observational and theoretical studies occurred at the headquarter of Jesuits, the Collegio Romano. The astronomical tradition started with Christopher Clavius (1535-1612) continued spreading all over the World. Matteo Ricci (1552-1610), Giovanni Paolo Lembo (1570-1618), Orazio Grassi (1583-1654), Christoph Scheiner (1573-1650), Christoph Grienberger (1561-1636), Giovanni Battista Riccioli (1598-1671), Francesco Maria Grimaldi (1618-1663), Rudjer J. Boscovich (1711-1787) Francesco De Vico (1805-1848), Angelo Secchi (1818-1878) are part of a largely incomplete list, even limited to solar astronomy.

The idea that the sunspots were bodies extraneous to the nature of the Sun was functional to preserve the perfection of the celestial body used to represent the perfection of God. The first Jesuits who observed the sunspots were also willing to find a coherence with this paradigm.

No spot could be imagined in a divine symbol, and this has been a major concern in the early debate between scientists and theologians. The Jesuits, before accepting the Copernican system adopted the Tychonic system to preserve the agreement between the Scriptures and the physics of the Cosmos (Figure 10, the famous Sol ne movearis (Sun, stand thou still) of Joshua 10, 12), through father Scheiner, were oriented toward a spotless Sun, with opaque bodies orbiting it.

The representations of the Sun in Figure 8 and in Figure 11 (whose original is probably the image in figure 12, published by (Kircher 1665) are separated by 15 years at least: the solar activity appear very intense. Many spots in figure 8, and a volcanic activity with many clouds in Figure 11. It is relevant that this image of the Sun, among the scholars, as the Cardinal De Zelada, remained iconic up to the end of 18^th century, for at least 163 year, notwithstanding the improvement of the optics during that century.

The interpretation given to the sunspots is of clouds of smoke ejected by volcanoes on the solar surface. On the solar limb there are flames that recall the spiculae, firstly described by father Angelo Secchi (1818-1878) only two centuries after Scheiner and Kircher. Secchi observed the Sun with a very good refractor of Merz, fully exploiting the achromatic doublet, patented in 1758 by John Dollond;24 the quality of the solar image seen by him was superior to the one of Scheiner and Kircher. About the plumes of smoke out of the limb, drawn by Kircher, they might include the memory of some observations of prominences made during total eclipses or some exceptional observation at sunset/sunrise. Secchi (1884)25 reported such an observation of Pietro Tacchini at sunset on the 8 August 1865 in the Mediterranean sea onboard a steam ship. The white spots are the faculae, that the artist reported all over the Sun, but in reality are visible only at the royal zones of the limb (±50° from the solar equator).

The original figure, copied on the wooden window, is the following Figure 12.

6. Naked Eye Sunspots Since the Middle Age

The solar activity and the solar rotation have been observed occasionally and reported in the medieval chronicles, but they have not been recognized as sunspots on a rotating photosphere. It is noteworthy that Einhard, in the Vita Karoli Magni27 mentioned a sunspot appeared for seven days in 813 AD.28 Another mention is in the Annales Regni Francorum (Royal Frankish Annals)29 for 17 march 807 AD, lasted eight days and interpreted as the planet Mercury passing in front of the Sun (Neuhäuser et al., 2024). Two big spots on the Sun were drawn by John of Worcester on 8 December 1128, and Averroes reported of two spots seen at the time of Ibn Mu’adh by Ibn Mu’adh’s nephew (Vaquero and Vasquez 2009).

Galileo himself was able to see a naked-eye sunspot in 1612. He added a postscript on the “Disegni della macchia grande solare, veduta con la semplice vista dal Sig. Galilei, e similmente mostrata a molti, nelli giorni 19, 20, 21 d’Agosto 1612” to say that while he was undertaking his observations, a sunspot appeared which was so large it could be seen with the naked eye between 19 and 21 August 1612; and this was shown to many people. This is included in his series of illustrations (Figure 13).

I repeated the experiment of the sunspot sight with the naked eye during the appearance of the sunspot AR 4079, one of the largest of the last decades (details in Section 7).31

The observations with naked eyes of sunspots through fog or atmospheric haze (Schaefer 1993) (Vaquero 2007) (Vaquero 2007b), though historically plausible, are not a scientifically reliable or safe method. Observing through smoked glasses was less safe than with modern mylar filter. The sunspots were observed by Galileo through its telescope at sunset or sunrise, but this method is also dangerous for the retina. There is no way to observe a sunspot on the Sun directly; only for a solar eclipse a very short glimpse (less than 0.1 s) may allow to see in the transient image left on the retina the lunar profile “biting” the Sun. In rare cases, as in Figure 14, the clouds allowed for some instants to see the eclipse directly but their disomogeneity would not permit to observe the sunspots.

The eclipse of Figure 12 was visible to the naked eye at 15.2% of eclipse through that cloud. The Moon profile was 294” inside the solar disk. A large sunspot has a lower contrast than the Moon’s profile on the Sun; the dimension of AR 4079 at its maximum reached about 150”x50” included the penumbra (as in Figure 15 and Figure 16).

The Sun is dazzling, and these experiments have been realized with great care, to avoid any damage on the retina. I want to stress the warming that, without protecting filters, staring for a minute at the Sun with naked eyes, may permanently damage the retina, where the Sun is focused, namely the macula lutea, the zone where the colors are detected.

7. Naked Eye Sunspots in 2025: Experiencing the Eye Resolution

A sunspot’s umbra is 10⁴ times as intense as the atmospheric halo of the Sun, when the Sun is high over the horizon and the sky is clear.32

The luminosity of the photosphere can be reduced to 10^-4 through a Density 4 mylar filter or, much better, through projection (Section 5).

The angular dimension of the sunspot has to be around 0.5’, the angular resolution of the eye in daylight, even if the eye is not in direct sunlight. The diameter of the pupil in such conditions is 3 mm, and the Rayleigh criterion gives about 33” of resolution, but a sunspot has to be bigger than that to emerge from the still bright photosphere. Moreover, to be distinguished, the sunspot has also to be distant from the limb, which is darker than the central part of the Sun. This explains why a big and steady sunspot on the Earthside photosphere for 14 days, is actually visible to the naked eye only for 8 days, as at the time of Charlemagne.33

We can say –experimentally with AR 4079 in May 2025- that the longitude of a great sunspot with respect to the central solar meridian has to be comprised within 60°E and 60°W to be enough separated from the darker limb. To get visible to the (alerted) naked eye the dimension of the umbra has to be larger than 1’ at the center of the disk. Its visibility, if the spot is stable, can last for 8 days. The sunspot at Charlemagne epoch had to be at least 3’ wide to be evident under the fog, homogenous thin clouds or at sunrise or sunset, when the Sun appears dimmer.

The present observations were made knowing that there was a big sunspot, while a true discovery has not to be alerted.

For these observation I used: mylar filter D4, grey filter 13%, a pinhole of 1.75 mm to compensate the eye defects and another orange filer, because the Sun was at 1.5 airmasses, and it was too bright to permit to see the sunspots on it without reducing the whole luminosity.

Consequently, the conditions of visibility of a sunspot to normal eyes without refractive defects are:

1. Dimension of the umbra larger than one arcminute;

2. Distance from the limb at least 3 arcminutes.

We have to remark that modern populations have significantly degraded eyesight, particularly in urban and East Asian contexts (Holden 2016). The use of the pinhole –always along with the Sun filter- reduces the effects of eye’s refractive defects in the direct sight.34

8. The Camera Obscura for Pinholes and Telescopes

These iconographic and historical premises describe the long period before the telescope’s invention, when the observations of sunspots were casual and not understood.

The architects in Florence obtained representations in perspective using a Camera Obscura (King, 2009) illuminated by a pinhole. The largest pinhole-meridian line to study the obliquity of the ecliptic and shift of the Julian calendar around the summer solstice was realized by Paolo Toscanelli in 1475 in the Dome of Florence.35 Ulugh Begh in Samarcand realized a great gnomon with solar projection in 1435, along with a stellar catalogue. Giacomo Della Porta (1590) described the principles of the Camera Obscura and Kepler 1571-1630 utilized them in astronomy (Kepler, 1604; Sigismondi and Fraschetti, 2001). The visibility of sunspots with a pinhole, at 2 meters of distance is possible for sunspots as small as 13” and 3’ from the limb. The pinhole can be 2 mm wide,36 and the camera does not require a perfect darkening.

Christoph Scheiner (1570-1654) used a parallactic machine to follow the projected solar image in the Camera Obscura (Scheiner, 1630) and he made the best drawings of his time (Secchi, 1884).

Here we use the Camera Obscura to assess the details observable and the physical phenomena that can be followed in the days after, in order to understand how that technique has contributed to increase the knowledge of the Sun.

Scheiner was the first to notice in 1612 that the Sun’s disc is brighter at the centre than at the edges, an effect we now call limb darkening. In a letter to his friend Federico Cesi in 1613, Galileo denied that the effect exists, although he may have changed his mind later (Engvold and Zirker 2016).

The sunpots and the limb darkening are clearly visible in the whole image of the Sun in figure 16.39 The further details of umbra and penumbra and the faculae are well visible. The images here presented are obtained in a camera obscura, with the telescope introducing the light, to reproduce the observations described by Christoph Scheiner (1573-1650) in the Rosa Ursina (Scheiner, 1630).

The projecting telescope, used in these experiments, has a 60 mm-doublet,40 and a prismatic mirror before the eyepiece, to deviate the light so that the Sun is projected on the wall, and not on the floor as Scheiner (1630) did. The observation on the wall, at 540 cm of distance, is very comfortable: there is enough time to detect the faculae and the tiniest sunspots. The estimate of R,41 the daily sunspot number, is always within a 10% range from the official averages.42 The intensity of the image, is about 3/1000 of the direct sunlight,43 and it is bright enough to show also the sky background within 1° of the field of view of the telescope.44

The faculae are visible in white light only near the limbs (Figure 17), to about 1/10 of the solar diameter, although Scheiner and Kircher represented them even at the center (figure 11 and figure 12).

Once the rotation of the Sun was established, the presence of the faculae at the center could have been imagined, while the explanation of their invisibility at the solar center requires the knowledge of modern atomic physics,45 as well as their increasing temperature with the quote above the photosphere (Smith 1963).

Variations in the luminosity of the faculae, with occasional white light flare (Figure 18), or with flares at the limb, are possible,46 even if their occurrences are documented only after the Carrington event (Carrington 1859) on a great sunspot, where the contrast is much larger.47 Scheiner and Kircher (Figure 11 and Figure 12), pictured many “light wells”, bright spots, they may have occasionally observed a flare in white light near the limb.

9. The Sunspots in the Churches

V. S. vedendo in chiesa da qualche vetro rotto e lontano cader il lume del Sole nel pavimento, vi accorra con un foglio bianco e disteso, che vi scorgerà sopra le macchie. [Galileo, Istoria e Dimostrazioni sopra le macchie solari, Lettera 2] If your Excellency see the light of the Sun falling on the floor of a church, through some broken glass, go there with a white and plain paper, and you will see the spots.

Already thousands years ago the Nature could have given the possibility to see the spots, even not sharply defined as through a telescope. Galileo in the same Letter mentioned above expressed this thesis. An accidental pinhole can be found in windows or through the leaves of a tree, as already described by the pseudo-Aristotle in the third book of the Problems (Aristotele 2002). The solar eclipses were seen also in this way during the partial phases, since the antiquity.

Nevertheless accidental pinholes through the leaves of the trees (Figure 19), or through the glasses of a church’s window, did not permit to anticipate the discovery of sunpots. While projection is physically valid, contrast and sharpness limitations likely made these occurrences ineffective for meaningful observation.

In 1475 Paolo Toscanelli realized a pinhole in the dome of St. Maria del Fiore, at 90 meters of height, to study the position of the giant solar image (1 meter) formed on the Northern nave of that Cathedral, the largest church in the World until the completion of St. Peter’s in Vatican (1612). The idea of Toscanelli anticipated, with a stable instrument, the consideration of Galileo of more than a century, but the sunspots were not observed.49

Another city which could have been hosted the discovery of the sunspots is Bologna. There Egnazio Danti (1536-1586) realized a great pinhole-meridian line in the Basilica of St. Petronio in 1577. Cassini reshaping in 1655 the same instrument and creating his Heliometer, fixed the pinhole width as 1/1000 of its height (Heilbron, 2001). Probably the same proportion was made by Danti, but he did not have time to use the instrument, because he was called in Rome for the Reformation of the Calendar (Gregorius XIII 1582) and for the decoration of the Gallery of the Maps, now part of the Vatican Museums by the Pope Gregorius XIII. The Pope created him bishop of Alatri. The last historical instrument that could have anticipated the discovery of the sunspots is the pinhole camera of the Torre dei Venti in Vatican made by Danti in 1580 (Sigismondi, 2014). The ratio pinhole-height for the meridian of the Tower of Winds is 14:5180 or 1/370 (Table 1).

The limit of fair visibility of the sunspots with a 1/1000 diameter/height of the pinhole is around 100 MH and it has been verified in Santa Maria degli Angeli50 and in San Petronio51 meridian lines.

The observations with pinhole instruments may have not been systematic, and the contrast of the sunspots observed with the pinholes is shallow, but it is enough for following them for some days, during the solar rotation. There existed at least three instruments in Italy where the sunpots could have been seen, before their discovery with the telescope.

The lack of observations of the sunspots in the end of 15^th and during the 16^th century is a consequence of the duration of the minimum of Spörer, with the Sun without big sunspots (1460-1550).

Finally it is noteworthy that Kepler observed two big sunpots in 1607 (Kepler 1609) with a pinhole camera, normally used to measure the magnitudes of solar eclipses: he drawn sunspots on May 18/28 and made verifications with naked eye (Hayakawa et al., 2024). He believed firstly to have seen Mercury on the Sun, but the conditions of visibility di not last enough to anticipate the discovery of sunspots, even of only a few years before Galileo.

The case of Kepler, known as a very keen observer, not because his sight but for his intelligence, that even after observing spots did not recognize them beyond accidents, is also significant. Even if the spots were there, the low resolution of the instrument and their rapid variability of their appearance did not help to recognize them.

10. Limb Darkening and Rotation: The Proofs of the Spherical Sun

The Sun by analogy with the Moon (Section 11.) was considered a sphere, but the observing proof could come either from the rotation of the sunspots, either from the limb darkening. There exists an instrumental limb darkening of the image, due to the geometrical optics of rays from a disk uniformly luminous through the pinhole-objective.

The solar disk presents a limb darkening, evident in the first regions near the limb, but this effect appear entangled with the instrumental limb darkening produced by a pinhole (Figure 20). Moreover when the pinhole is small the effects of geometrical optics are entangled with wave optics due to the diffraction occurring in a narrow opening.

Is it possible to distinguish the solar limb darkening from the pinhole limb darkening? The latter is dependent on the pinhole dimension, and it enlarges the geometrical image D_¤=f·tan(θ_¤) of the Sun by the width d of the pinhole itself.

Therefore the observed image is D==f·tan(θ_¤)+d where f is the focal length of the pinhole (distance pinhole-image) d is the diameter of the pinhole, and θ_¤ is the angular diameter of the Sun. The pinhole d<<D allows better measurements, below d/D<30.52

The Limb Darkening contrast in white light is shallow (~16%) (Rogerson 1959) and atmospheric seeing (turbulence) also reduces limb sharpness in historical projection systems.

The Limb Darkening of the Sun has to be observed with a telescope (Figure 21) able to show the required detail to disentangle it from the pinhole’s shadowing of geometrical optics, or from the diffraction spread of the solar limb through the tiny pinhole. The main characteristics of the solar Limb Darkening is its steeper rise to the inflection point with respect to the pinhole’s limb darkening which is shallower in dependence of the pinhole’s diameter.

The limb darkening without focusing optics (Figure 22, St. Maria degli Angeli pinhole meridian line) could not help to perceive the spherical nature of the Sun.

11. The Moon Was Already Spherical and with Spots

After considering the similar nature between the Sun and the fixed stars, we have to address its sphericity. The three dimensional representations in Egypt (Figure 1) show already a sphere, but the definitive proof arrived in 1610 with the telescope. The analogy with the Moon, also regarding its spots (Figure 23), could have contributed to the idea of a spherical Sun with spots, which is not evident by itself.

Antares 20x600 telescope, Xiao-Mi11 smartphone.

The motion of the terminator and its shape contributes to evidence the spherical nature of the Moon. The contrast of the moonspots is rather shallow, and their distance from the brighter limb introduces a problem of visibility.53

The presence of permanent spots is a characteristic of our natural satellite, they are known since the most antique ages but strangely the Moonspots did not have names until a few years before the invention of the telescope, when William Gilbert (1544-1603) physician to Queen Elizabeth I and the discoverer of terrestrial magnetism drafted the one reported in Figure 24.

The resolution at naked eye allowed understanding the permanent nature of the moonspots, while only the changing terminator showed the spherical nature of the Moon. The geocentric orbit, producing the lunar phases, compensates the absence of visible rotation: the lunar rotation is synchronized with the orbit 1:1. But the solar rotation was discovered by Galileo after the observation of the sunspots. The solar limb darkening helped to perceive the three-dimensionality of the Sun.

The telescope of Galileo gave immediately the boost to understand the nature of the Moon, thanks to the amount of details visible. The uncertainty of the ancient theories on the lunar spots (e.g. Dante in Divine Comedy, Paradise 255 or Gilbert with continents floating on a vaste ocean), do vanish at the first sight with the telescope, and the modern planetography as well as celestial mechanics started in a modern way.

12. Conclusions: The Right Instrument at the Right Moment

The Sun had always spots during the human history, but the possibility to see them did not coincide with the actual discovery, even if great pinhole’s meridian lines were operating since 1475 in Florence, 1577 in Bologna and 1580 in the Vatican.

It is reliable that the Spörer minimum of the solar activity (1460-1550) contributed with a long lasting “blank Sun” completely spotless or with small spots, without the largest groups that become achievable to the unaided and unbleached eye, as it occurred in AD 807 and 813 when occasional great spots were observed up to 8 consecutive days.

The solar activity over millennia has been reconstructed (Usoskin, 2023) through some proxies, because the observation of great sunspots is too scattered before the invention of the telescope. This paper may suggest an indirect proof that the recovery of the sunspots’ solar cycle after the Spörer minimum was rather smooth and, if not completely spotless, without very great sunspots, that would have been noticed through many pinhole-instruments.

The improvements of the technique in building pinhole cameras for observing the Sun did permit Kepler to observe the sunpots in 1607, but he did not recognized their solar nature because he did not continue these observations or because the spots rapidly evolved loosing umbra and gaining penumbra, that is not visible through a pinhole.56 After the invention of the telescope and the discovery of the sunpots, Kepler observed them through pinholes cameras (Sigismondi and Fraschetti, 2001), before having the possibility to use the telescope of Galileo.

The discovery of the sunspots occurred both with the telescope and after the end of the Spörer minimum. The theory of the instruments as trigger57 of a scientific revolution is confirmed in this work, but also the Nature’s conspiracy of a long-term solar activity minimum phase contributed to set the date to 1610 and not a century before. The cultural environment was not ready to accept a Sun with spots. The Dominican Tommaso Caccini in 1614 with the homily on “Viri Galilaei” accused Galileo58 to move the Earth around a spotted Sun. Caccini’s disposition was based on a symbolic idea of the Sun as representative of God (Section 1. and .), and it would not change quickly in any case. The same cultural environment, one century before Galileo, may not have been the decisive factor for the discovery of the sunspots. It was the lack of evidence: no sunspots since 1460 for the Spörer minimum, and the lower resolution of a giant pinhole camera, with respect to a telescope, these two concomitants factors prevented the discovery of the sunspots, not a fixed paradigm of thinking.