On April 11, 1875, German astronomer Samuel Heinrich Schwabe died, leaving behind him a discovery that continues to both intrigue scientists and plunge them into controversy—the eleven-year solar cycle.
Schwabe had no such intent when he began observations in 1826. In fact, the sun wasn’t really what he was interested in at all. Schwabe had hopes of discovering the planet Vulcan, believed to exist somewhere within the orbit of Mercury, and the most likely way to find it seemed to be to watch the dark spots that crossed the sun.
Every clear day, Schwabe faithfully recorded the sunspots he observed, hoping one of them would turn out to be Vulcan. After 17 years of work, however, he was forced to abandon his quest for the elusive planet. As he looked over his records, Schwabe realized that instead of finding Vulcan he had discovered something else, something, in hindsight, significantly more valuable.
Those sunspots he had tracked for so many years were not mere vagrants meandering across the sun’s surface at random. They seemed to fall into distinct patterns of activity lasting about ten years each.
Schwabe hastened to inform the scientific community of his findings in an article titled “Solar Observations During 1843.” Very few were interested in Schwabe’s work at first. Only Rudolf Wolf, director of the observatory in Bern, Switzerland, was intrigued. Using Schwabe’s principle, he was able to trace the solar cycle back to the earliest sunspot observations of Galileo around 1610. Eureka!
How the Schwabe Cycle Works
The Schwabe cycle is now common knowledge among scientists, although they have since determined that the average length of the cycle is closer to 11 years than 10 (10.66, to be exact), with a range of 9 to 14 years.
What actually takes place during a solar cycle?
To start with, sunspots are cool, dark areas of intense magnetic activity on the sun’s surface. Sometimes there are many sunspots; sometimes there are very few. When the number of sunspots is at its lowest in the 11-year cycle, it is called a solar minimum. When the number is at its highest in the cycle, it is called a solar maximum. Cycles are counted from minimum to minimum.
Balancing the cooling, energy-blocking action of the sunspots are bright areas known as faculae, which tend to form around the sunspots as the flow of energy resumes. Although faculae increase solar radiation less than sunspots decrease it (a 0.05% increase versus a 0.3% decrease), they tend to be larger, and they tend to last longer, as well.
Interestingly enough, even the location of sunspots is not random. Sunspots typically occur along flowing bands of solar material called torsional oscillations, thought to generate the sun’s magnetic field. Just after a solar minimum occurs, these bands and their accompanying sunspots appear at about 25 to 30 degrees of latitude, both north and south of the sun’s equator. As the cycle progresses, the magnetic activity slowly makes its way toward the equator, usually ending up at about 5 to 10 degrees of latitude.
While the torsional oscillations are moving toward the equator, another phenomenon takes place which scientists call the “rush to the poles.” While the torsional oscillations encircling the sun work their way toward the middle, magnetic features in the upper atmosphere climb toward the extremities, reaching about 76 degrees of latitude (north and south) at about the same time as the solar maximum occurs. This interesting development is not fully understood yet, but it is believed that the rush to the poles has some connection to the way the sun discards its old magnetic field at the end of every Schwabe cycle.
Significance of the Schwabe Cycle
The Schwabe cycle definitely does have an effect on us earthlings, although scientists haven’t figured out all the particulars yet. Extreme solar events associated with high numbers of sunspots have the potential to interfere with or even damage satellites. Solar activity can affect also radio communications in a number of ways. And of course, the eerily beautiful aurora is more prevalent during the solar maximum.
One of the more controversial associations between the earth and the sun is the possible correlation between the Schwabe cycle and the weather. People have been trying to connect the two since at least the early 1800s, and the study was extremely popular for a time, until inaccurate forecasts led to discouragement after the early 1930s. Since then, the theory that solar activity affects weather and climate has been hotly debated.
How could the solar activity affect the weather? There are currently three major theories:
- Changing levels of solar irradiation heat and cool the earth.
- Variations in ultraviolet light levels affect both winds and temperatures in the earth’s stratosphere.
- In periods of low solar activity, cosmic rays fail to be dispersed by ejections of solar material and produce aerosol particles in the earth’s atmosphere, which in turn become condensation nuclei for clouds. (Clouds with large numbers of condensation nuclei tend to last longer than clouds with small numbers of nuclei, but they also produce less precipitation.)
So are one or more of these theories correct? Does the Schwabe cycle really affect the earth’s weather? Much evidence and many arguments have been accumulated on both sides of the question. Some have provided a compromise by suggesting that perhaps the effects tend to be regional in nature. Climate modeling suggests that periods of low solar activity might make winters colder in the United States but warmer in Canada, for example.
Climate Change Controversy
With the ongoing debate regarding the Schwabe cycle and the weather, it was only natural for the question to arise of what the sun’s role in global climate change might be. Are man-made pollutants the primary cause of changing climates worldwide, or is climate change really just a natural phenomenon?
In 1991, Eigil Friis-Christensen and Knud Lassen of the Danish Meteorological Institute in Copenhagen claimed to have the answer to this question. After careful study of sunspot and temperature records between 1861 and 1989, they found a strong correlation between the Schwabe cycle and temperatures in the northern hemisphere—so strong, in fact, it could account for nearly 80% of the temperature changes. However, Lassen and astrophysicist Peter Thejil updated their research in 2000, and weren’t so sure that there was a correlation after 1980. On the contrary, they were so impressed by the data, that Thejil unequivocally declared, “It has the fingerprints of the greenhouse effect.”
In 2000, another researcher from the United Kingdom, Peter Stott, reported that greenhouse gases were largely responsible for the warming trends being observed, and that the sun was producing no significant effect on climate change. In 2003, however, he revised his assessment and admitted that the solar contribution was actually quite significant.
So what does it all mean? Scientists are far from agreeing, and accusations of improper data handling continue to fly. Meanwhile, the Schwabe cycle has proven to be another discovery that still baffles the world.
- Plot the solar cycle data (use the up-to-date information found in the Solar Activity Report below). Identify the solar cycles.
- Research the various ways the solar cycle is used. The solar cycle is used to predict drought, predict the weather, and even predict the price of commodities, for example. What correlations are likely? What are other factors that may influence the effectiveness of the method?
- Many times raw data is fitted to a predetermined algorithm (or “curve”) before being utilized or released, the purpose being to eliminate any bad data (or anomalous data points outside prescribed ranges). What types of data error would this prevent? What types of data error would this process introduce?
- Interpret and explain the butterfly sunspot diagram above.
Schwabe, Samuel Heinrich (1789 – 1875)
What is the Solar Cycle
For students from NASA.
The Sun — History
Background information on the discovery and plotting of sun spots, flares, and other sun activity.
Discovery of the Sunspot Cycle
Simple excerpts and data from Schawbe’s scientific journal.
Solar Cycle — Video from NASA
(You may want to install an ad blocker before viewing.)
Solar Cycle Primer
Photos, charts, video and lots to see.
Solar Activity Report
Stats updated frequently.
Summer Fun Activity: Build a Sunspot Viewer
View sunspots with this complete guide from National Geographic.
Another method of viewing for the very interested.
Our Very Own Star: The Sun
Sun booklet from NASA with basic information for younger students.
Unit Studies & Lesson Plans
The Solar Cycle
Great lesson plan that guides students through plotting solar cycle numbers and analyzing the data. Includes data, handouts, instructions and sample spreadsheet.
Solar Storms and You
24-page NASA download exploring sunspots and solar activity cycles in three math- and science-oriented lessons.
Data Analysis and Measurement: Having a Solar Blast
16-page NASA download with lesson plans covering science, math, and technology dealing with solar flares.
Extensive lesson plans for four activities using the discuss, explore, apply, and reflect procedure dealing with sunspots and the sun’s rotation.
The Solar Cycle Notebooking Pages
Simple notebooking pages for science/history notebook, copywork, narrations, or wrapping up.