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The Changing Sun
Plato and Aristotle from a segment of Raphael's School of Athens
Ancient Greek scholars from Thales to Aristotle developed a complex and rich vision of the universe. Proof depended upon philosophical and logical rigor in this culture. The Ancient Greeks did not use detailed observation and experimentation, which is integral to modern science.
Aristotle's explanation of the physical universe was well developed and well defended according to the ancient Greeks. His views were consistent with the ancient Greek culture. Aristotle described an Earth-centered (geo-centered) universe with the Moon, Sun and all heavenly bodies moving around the Earth. Aristotle taught that the world below the moon was changing and imperfect. It was composed of the elements; earth, water, air and fire. These elements should not be confused with the modern substances of the same names.
Aristotle taught that above the moon the heavens were permanent and perfect. The heavenly bodies must, therefore, be made of a perfect element that Aristotle called aether (from which we get the word ethereal). Later scholars, using Latin, called this fifth element quinta essentia or 5th essential (from which we get the word quintessential). He believed that aether or quinta essentia was the most perfect element. The motion in the heavens was always circular; a shape that Pythagoras had taught was perfect. The Sun, therefore, was perfect and unchanging and moved around the Earth in a perfect circle.
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Christian Aristotelian Cosmos. From Peter Apian, Cosmographia (1524)
These were the dominant ideas of educated persons in the Western world until late in the seventeenth century. In the thirteenth century, they became part of the doctrine of the Holy Roman Catholic Church.
In the mid-1500's Copernicus (ref 1, ref 2)offered an alternative to Aristotle's (and the Church's) structure of the universe. He proposed a Sun-centered (helio-centered) universe rather than an Earth-centered universe. This drawing from his famous book, De revolutionibus orbium coelestium, shows the difference between his system and Aristotle's.
In the 1600s Kepler (1, 2)and Galileo challenged many of the teachings of Aristotle. They used careful experimentation, rather than logic alone, as proof. In 1609 Galileo built a telescope to allow him to observe more detail in the heavens. The methods and tools used by Kepler, Galileo and others were revolutionary.
Credit and Copyright IMSS - Firenze Galileo's Telescope. Wood, paper. Length: 1360 mm
Credit and Copyright IMSS - Firenze Galileo's Telescope. Wood, paper. Length: 920 mm
Galileo observed dark spots on the Sun that moved. He concluded that the Sun was spinning. With his simple telescope Galileo also observed moons around Jupiter, rings around Saturn and mountains and valleys on the Moon. With these observations and careful experiments measuring motion of objects Galileo was able to seriously challenge the Greek ideas that had dominated Western culture for 2000 years. One of the important changes in worldview was the nature of proof. Logic alone would not be sufficient. Experimentation and careful observation became essential to proof. Galileo attacked two of Aristotle's primary teachings about the heavens. Galileo showed that the heavens were neither perfect nor unchanging. This is the beginning of our understanding that the Sun changes. It is the beginning of our careful observation of the Sun. The image below shows a modern picture of the Sun in visible light. The small dark dots are Sunspots.
Source/Credits: Solar & Heliospheric Observatory (SOHO). SOHO is a project of international cooperation between ESA and NASA.
Many individuals contributed to early research of these strange spots on the Sun in the early 1600's. However, sunspot activity decreased from 1645 until 1715. With so few sunspots people lost interest. However, in 1843 Heinrich Schwabe discovered the number of sunspots increased and decreased in a cycle. The following graph shows the sunspot number from about 1610.
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The vertical axis is the number of sunspots counted and the horizontal axis shows the year in 10-year increments. The pattern shows peaks of high sunspot number about 11 years apart. The long period with few sunspots occurred during a period called the "Little Ice Age" in which temperatures decreased globally. This coincidence of the lack of sunspots with a decrease in global temperatures is not sufficient proof of a cause-effect relationship. It does raise interesting questions. More accurate measurements have found the sunspot cycle to be, on average, 11.1 years with ranges between 8 and 16 years.
Ground based telescopes and satellite-based observatories are contributing much to our understanding of the Universe. In particular, satellites launched under the Sun-Earth Connection (SEC) program have revealed much about our local neighborhood. We now know that the Sun changes on scales ranging from fractions of a second to billions of years and from kilometers to tens of Earth radii.
What was the sunspot activity when you were born or on any important date?. If you would like to find out more about sunspot number and how to compute them visit the NOAA website.
Become a Sunspotter in the Student Observation Network.
- NASA Official: Dr. James Thieman
- Project Manager: Don Robinson-Boonstra
- Website Manager: Bryan Stephenson