The earliest known observations of sunspots appear in Chinese records from the fourth century BC, when unusually large spots were probably seen through haze or smoke. A ninth century Benedictine monk observed a large spot for over a week, noting that it moved from day to day. The invention of the telescope in the early 17th century allowed Galileo to correctly deduce the sun’s rotation from sunspot motion.
As telescopes improved, so did sunspot science. The fact that sunspot numbers waxed and waned periodically was discovered in 1843. Soon after it was noticed that sunspots appeared closer to the sun’s equator as its 11-year cycle progressed. A plot of sunspot latitude as a function of time resembles butterflies flying in a horizontal line.
The “butterfly diagram” provided a clue to the nature of sunspots, along with the early 20th century discovery that sunspots were magnetized.
Sunspots form because the sun’s equator rotates faster than its poles.
Over time, its magnetic field gets twisted up until loops and knots begin to protrude from the surface, like twisted rubber bands. The concentrated magnetic field in these twisted loops slows the rising of hot gas from below, allowing the spot to cool and darken. As the global field winds up, the field lines twist from a north-south to an east-west orientation, which is also why sunspots migrate to lower latitudes later in the cycle.
Eventually, something has to give: The magnetic field can only take so much tension before field lines break and the global field collapses.
After that it begins to rebuild, but with north and south magnetic poles reversed. Currently, sunspot numbers are expected to peak in early 2013 and drop off afterwards (with new spots forming ever closer to the equator). The field should collapse around 2021 or 2022, before the new cycle begins.
Next column: The last total lunar eclipse in Idaho until 2014.
Chris Anderson manages the College of Southern Idaho’s Centennial Observatory in Twin Falls. He can be reached at 732-6663 orcanderson@csi.edu.
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