Recalibrating historical solar observations may lead to a greater understanding of the sunspot cycle. | By Dr. Kristine Larsen and Rodney Howe
While the AAVSO Solar Section traces its existence back only eighty years (approved by the Council on October 6, 1944), solar observing has a long and storied history. English astronomer Thomas Harriot was the first to observe sunspots telescopically in 1610, and skywatchers have been recording sunspots back to antiquity.
The Solar Section provides individuals with the knowledge necessary to (safely) make scientifically useful observations of our nearest star’s ever-changing face and submit them to its database, for access by solar scientists from around the world.
The goal of the section is to continue to contribute to the understanding of solar activity through maintaining a reliable archive of sunspot and sunspot group counts, collected using uniform observing techniques that allow researchers to compare “apples to apples” – from one sunspot cycle to another.
The individual that takes care to observe the Sun safely (whether using the projection or a solar filter methods described in the section’s observing manual) is rewarded with a new vista each and every time. As seen in two sketches from November 1 and 4, 2024, the Sun never looks the same twice. Not only does the roughly month-long period of rotation of the Sun change what an observer sees from day to day (carrying sunspots into and out of our line of sight), but individual spots and groups pop into existence, grow, and then melt away, over the course of days to weeks. Some persistent groups can even be seen through more than one rotation cycle!
The Future of Solar Physics
Members of the AAVSO Solar Section are looking toward the future development of solar physics, to a time when we understand the sunspot cycle well enough to be able to predict the timing and level of activity of solar maximum with much greater precision than we can today. Intriguingly, it is sunspot counting history that might offer an important clue.
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Solar physicist Dr. Leif Svalgaard of Stanford University is one of several solar scientists who are interested in using past solar activity to hone our understanding of our Sun’s cycles. Svalgaard has partnered with amateur astronomers around the world to better interpret a particularly important record of sunspot observations by 18th century German amateur astronomer Johann Caspar Staudach. Between 1749 and 1799, Staudach made 1,130 sunspot drawings—an impressive body of work. Solar scientists are particularly interested in such large data sets gathered by a single individual, because their consistency makes them uniquely useful for studying patterns. Since Staudach’s telescope was smaller than those used by most of today’s modern backyard solar observers, in order to understand the patterns seen in his work we should be making comparisons with similar instruments (that old “apples to apples” thing again). Svalgaard found out a way to do this without inventing a time machine, contacting the Antique Telescope Society in 2015 to recruit individuals to make solar observations using simple homemade instruments similar to what Staudach had used. One such reproduction is shown below:
John Briggs, member of the Antique Telescope Society, helped Svalgaard advertise this research project in a March 2021 article in the Astronomical League’s Reflector magazine, “A Funny Thing Happened in Solar Physics” [PDF]
Svalgaard collects scans of drawings (like those shown above) as part of his project, and is interested in recruiting more observers who want to combine history and astronomy for the benefit of future solar physics. As with all sunspot observing projects, safety is the number one priority, so please be sure to research safe solar observing techniques before beginning and always follow all provided instructions to the letter. If you are interested in learning more about Dr. Svalgaard’s research project, his personal website includes numerous articles and presentations on the project (and others). 