The Year the Sun Was Found to Have Blemishes

In late February 1610, a twenty-three-year-old German theology student named Johannes Fabricius, returning to the University of Wittenberg from a Christmas visit to his father at the small Frisian village of Osteel, pointed a recently-acquired Dutch refracting telescope at the early morning Sun through the partial overcast of a North Sea winter sky and saw, on the disk of the Sun, several small dark spots. He showed the spots to his father, David Fabricius — a Lutheran pastor and amateur astronomer of some reputation — over the following days. The two of them tracked the spots across the Sun’s face for several weeks during March and April 1610, using a series of pinhole projections to avoid damaging their eyes. They confirmed that the spots were moving — slowly, regularly, in a manner consistent with the Sun rotating on its axis. The spots, in other words, were on the Sun itself.

Johannes Fabricius wrote up the observations in a short treatise titled De Maculis in Sole Observatis (On Spots Observed on the Sun), published at Wittenberg in June 1611. The treatise was the first published European account of sunspot observations through a telescope. It was also the first published European account to argue, on the basis of direct observation, that the Sun was not the unblemished perfect sphere of Aristotelian cosmology.

The treatise had a modest distribution. It was read by professional astronomers across central and western Europe within a few months. It was not read by Galileo.

This is the beginning of one of the most contested priority disputes in the history of European science.

The four observers

By the summer of 1611, four different European astronomers had independently observed sunspots through early telescopes. Beyond Johannes and David Fabricius in Frisia (Germany), the four were:

1. Galileo Galilei in Padua, who began observing sunspots in approximately July or August 1610 (after he had observed and reported the moons of Jupiter and the phases of Venus). His observations were extensive and methodologically careful — he developed a projection technique that allowed accurate daily recording without eye damage, and he tracked specific sunspot groups across the solar disk for weeks at a time. He did not, however, publish on sunspots until 1613.

2. Thomas Harriot in England, the brilliant and difficult mathematician working under the patronage of the Earl of Northumberland at Syon House outside London. Harriot observed sunspots beginning approximately 8 December 1610. His observations were also methodologically careful. He never published them. The notebooks survive in the Petworth House archives and were not made public until the late nineteenth century. Harriot is the great non-publisher of early-seventeenth-century English science; he saw an enormous amount before anyone else and chose, for reasons that have never been satisfactorily explained, to keep almost all of it private.

3. Christoph Scheiner, the Jesuit mathematics professor at Ingolstadt in Bavaria, who began observing sunspots in approximately March 1611 — after Fabricius’s observations but before his publication, and almost simultaneously with Galileo’s continued observations. Scheiner published his first observations in October 1611 in a series of letters under the pseudonym Apelles latens post tabulam (Apelles hiding behind the painting), a reference to a Greek legend about a master painter who concealed himself behind his own work to overhear customer reactions.

Each of the four made the observation independently. Each had access to the new telescope technology (perfected by Dutch lensmakers in 1608-1609 and rapidly disseminated across European scientific networks). Each was using projection or pinhole techniques to observe the Sun safely. None of them, in early 1611, knew that any of the others were doing the same work.

By late 1611, with both Fabricius’s and Scheiner’s publications in print, the situation had become a priority dispute.

Why it mattered

The discovery of sunspots in 1610-1611 was substantially more consequential than it might appear. The Aristotelian cosmology that European universities had taught for nearly two thousand years held that the celestial bodies above the sphere of the Moon were eternal, perfect, and unchanging. The Sun, in particular, was the most perfect of the celestial bodies — a flawless luminous sphere whose only proper attributes were perfect light and perfect circular motion. The Sun could not have blemishes. The blemishes had already been challenged by Tycho Brahe’s 1572 supernova and his parallax measurements of the 1577 Great Comet, but sunspots were a more direct refutation: they were on the most prominent celestial body of all, visible to anyone with adequate equipment.

For the Catholic Church, the discovery was a particular problem. The Aristotelian cosmology had been integrated, over the previous three centuries, into the standard theological reading of scripture; the perfection of the Sun was associated with Christian theological claims about divine perfection. The Jesuit astronomers, including Scheiner, were therefore institutionally committed to interpretations of the sunspots that preserved as much of the traditional Aristotelian framework as possible. Scheiner’s first published explanation in 1611 was that the sunspots were not on the Sun itself but were the silhouettes of small satellites orbiting close to the solar surface — an interpretation that, while clearly difficult to maintain, allowed the Sun to remain unblemished in its essential nature.

Galileo’s interpretation, when he published his Letters on Sunspots in 1613, was that the spots were physical features on the Sun itself, that they rotated with the solar surface, and that the Sun therefore had a rotation period of approximately twenty-five to twenty-seven days. This interpretation was, by every later measurement, correct. It was also a direct rejection of the Aristotelian cosmology and an indirect endorsement of the Copernican heliocentric model.

The dispute between Galileo and Scheiner over both the priority and the interpretation of the sunspot observations would run for the rest of both men’s lives. It was one of the contributing factors in Galileo’s increasingly difficult relationship with the Jesuit scientific establishment. By the mid-1610s the dispute had become personal and hostile. Scheiner’s eventual major treatise on solar observation, Rosa Ursina sive Sol (Bracciano, 1626-1630), is a substantial astronomical work that is also, in significant portions, a sustained attack on Galileo’s priority claims.

What survived

The sunspot observations were the first detailed measurement of the modern solar cycle. By the 1840s, when continuous European observatory data had accumulated for over two centuries, the German astronomer Heinrich Schwabe was able to identify the approximately 11-year solar activity cycle by looking at the long-term sunspot count. By the 1890s, E. W. Maunder at Greenwich had identified the 1645-1715 sunspot gap — a discovery that depended entirely on having continuous sunspot records going back to the Fabricius-Galileo-Scheiner generation.

The original observation notebooks of all four 1610-1611 observers survive. Johannes Fabricius’s are at the University of Wittenberg library. Galileo’s are in the Biblioteca Nazionale Centrale in Florence. Harriot’s are at Petworth House. Scheiner’s are in the Vatican Apostolic Archive. They have all been digitized and are accessible through the relevant institutional websites.

Johannes Fabricius — the youngest of the four observers, the first to publish, and the most thoroughly forgotten — died in 1616, aged twenty-nine, of what his father’s correspondence describes as “wasting illness” (probably tuberculosis). His grave is at the parish church in Resterhafe, the small Frisian village where his father had been pastor. The headstone is original. It gives his dates and his name. It does not mention sunspots.