Kepler's 400‑year-old solar observations unlock new mysteries

Johannes Kepler, a German mathematician born in 1571, is best known for formulating the laws of planetary motion. However, not everyone knows he was also a keen observer of the Sun. In 1607, using a camera obscura, he made important observations of our closest star. Moreover, his precise drawings allowed astronomers years later to determine the position of the Sun in its 11-year activity cycle.

As reported by Science Alert, Kepler's precise drawings enabled astronomers to determine the Sun's position in its 11-year activity cycle. By examining the location of Kepler during the sketches and the sunspots' positions in his drawings, a group of researchers established that the Sun was nearing the end of its thirteenth solar cycle.

In May 1607, Kepler recorded what was initially believed to be the transit of Mercury, but it eventually turned out to be a group of sunspots. These spots are cooler and darker than their surroundings, with a temperature of about 3,500°C compared to the photosphere's average temperature of nearly 5,730°C. These temporary solar phenomena can be observed in the photosphere. They appear dark against the hotter and brighter surroundings.

A team of researchers led by Hisashi Hayakawa from Nagoya University used innovative techniques to analyze Kepler's drawings and discovered new information concerning solar activity during that period. Applying Spörer's law, the researchers located these observations at the end of a solar cycle, just before the start of the well-documented Maunder Minimum—a period of significantly reduced sunspot numbers between 1645 and 1715. These discoveries may contribute to a better understanding of significant periods of solar inactivity.

"If this is true, it would be really interesting. However, another reconstruction based on trees indicated a sequence of solar cycles with normal duration times," said study author Hisashi Hayakawa from Nagoya University. He added, "So, which reconstruction should we trust? It is extremely important to verify these reconstructions with independent – preferably observational – records."

It is worth remembering that the Sun, although seemingly unchanging, goes through 11-year cycles in which its activity increases and decreases. Although the central body of the Solar System is about 150 million kilometres from our planet, it does not remain indifferent to us. An example is the powerful Coronal Mass Ejections (CME), which can directly affect the upper layers of our planet's atmosphere and the technology we use.