The long road to understanding our star

by Jay R. Thompson
Monday, December 3, 2012

The earliest written records of sunspots date back to 165 B.C. in China, but human understanding of the sun didn’t begin making leaps forward until the early 1600s, shortly after the invention of the telescope. That’s when Galileo Galilei, Thomas Harriot and others began drawing sunspots in detail and tracking how they moved and changed.

Danish astronomer Christian Horrebow was probably the first to suggest the sun’s activity was cyclical. He and a group of researchers in Copenhagen observed sunspots starting in 1761. In a diary entry from the year of his death in 1776, Horrebow wrote that the presence of sunspots must be periodic but that he couldn’t determine the period in the years of his observations, according to a 2010 review by David Hathaway, a solar physicist at NASA’s Marshall Space Flight Center in Huntsville, Ala.

German pharmacist and amateur astronomer Samuel Heinrich Schwabe is credited with discovering the cycle that Horrebow suspected. Initially looking for a planet inside the orbit of Mercury, Schwabe observed sunspots for 18 years. Publishing his findings in 1844 in the German journal Astronomical Notes, Schwabe concluded that sunspot activity had a cycle of about 10 years. Swiss astronomer Rudolf Wolf took the research further, combining his own observations with those from previous decades. Wolf’s research, published in 1861 in Monthly Notices of the Royal Astronomical Society, concluded that the solar cycle was 11.1 years.

About the same time as Wolf’s study, the most famous solar event in history changed human understanding of the sun for good.

On Sept. 1, 1859, Richard Carrington was, as usual, at his private observatory in Surrey, England, using a telescope to collect light from the sun and project its image on a painted panel of glass, from which he then recorded sunspots. It was a Thursday and not quite half past 11 in the morning. Carrington had finished his drawings and was counting the sunspots on the 28-centimeter-diameter projection when, rather suddenly, he saw two spots of light that were so bright against his projection screen that he first thought a ray of light had burned a hole in it. When he realized that wasn’t the case, Carrington scrambled to find someone to observe the event with him. By the time he returned, the two white spots had faded somewhat and, in a few minutes, were gone.

The next morning, before sunrise, an immense cloud of charged particles — a coronal mass ejection —¬†bulldozed and energized Earth’s magnetic field. The northern and southern auroras, normally only visible in polar regions, were observed in such tropical locations as the Caribbean and Hawaii, and the fluttering geomagnetic field induced currents in telegraph cables, famously creating sparks and even fires at telegraph stations.

According to NASA, nothing like the Carrington Event has been seen since. Researchers studying ice cores have determined that the geomagnetic storm was nearly twice as severe as the next most severe storm in the last 500 years.

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